Conservation Materials COPYRIGHTED MATERIAL€¦ · Analytical techniques for studying culturally important objects have been developed using a wide range of established experimental

1Conservation Materials

1.1 INTRODUCTION

There is no doubting the cultural importance of museum artefacts. Avast array of objects that reflect heritage are collected, displayed andstored by museums. Objects including paintings, ceramics, textiles, stat-ues, glass, furniture, books, plastics and metals artefacts all find a homein museums. Many types of materials, organic and inorganic, naturaland synthetic, have been used to make such artefacts. As many mate-rials are susceptible to deterioration, an important issue for museumconservators and curators is to identify and understand the degradationprocesses occurring on objects. Such information can be vital in prevent-ing further deterioration and conservation processes, such as cleaning orconsolidation, can be applied. Also of interest in museums is the originof an object and identifying the chemical properties of a material canprovide vital information regarding provenance.

There are many analytical techniques used in laboratories that provideinformation about materials. Information regarding the elemental com-position, molecular structure and physical properties can be obtainedand used to characterise a material. There are special requirementsto be considered when choosing an analytical technique suitable forstudying a museum artefact. The goal is to minimise damage to theobject of interest, so non-destructive techniques are very much favoured.Where only a destructive technique is suitable to obtain the requiredinformation, it should require only a very small quantity of material.Analytical techniques for studying culturally important objects havebeen developed using a wide range of established experimental methods

Analytical Techniques in Materials Conservation Barbara H. Stuart 2007 John Wiley & Sons, Ltd

COPYRIG

HTED M

ATERIAL

2 CONSERVATION MATERIALS

and each of these are described in this book. Each chapter providesbackground on how each technique works, the sampling requirementsand what information is provided by the experiment. Examples ofthe application of each technique to the different heritage materialsencountered are provided to illustrate the information obtained by eachtechnique.

To determine how to protect the items found in museums, an under-standing of the chemical and physical properties of the material fromwhich the item has been produced is vital. The origin of an objectcan be determined through an examination of its chemical structure.This enables the material to be dated and can aid in identifyingforgeries. The properties of a material are also crucial to the taskof detecting and preventing deterioration of precious objects. Thus,in the first instance, the material that is being dealt with must becharacterised before embarking on any treatment. This chapter pro-vides an overview of the materials that are encountered in museummaterials.

Humans have always made use of materials naturally occurring aroundthem to produce items. Naturally occurring materials may contain pro-teins, lipids, carbohydrates, resins, colourants and/or minerals and areall found in heritage materials. In more recent times, with the develop-ment of synthetic chemistry, man-made materials, such as polymers andcolourants, have been used to produce objects. The component materialsof paintings, books and manuscripts, textiles, glass, ceramic, stone andmetal objects are also described.

1.2 PROTEINS

Proteins appear in many natural materials of animal origin in themuseum environment, such as paintings, manuscripts, textiles, adhesivesand many artefacts. Proteins are large molecules with a characteristicamide group (–NH–CO–) and consist of varying sequences of aminoacids [Creighton, 1993]. The structures of amino acids are shown inFigure 1.1 and these acids can be isolated by hydrolysis of the protein.There are many proteins in existence, but only those that may beencountered in works of cultural heritage are described here.

Collagen is a fibrous protein obtained from the connective tissues,such as skin, bone, hide or muscle, of animals [Creighton, 1993; Millsand White, 1994]. The amino acid composition of collagen is listedin Table 1.1 [Bowes et al., 1955; Mills and White, 1994]. Collagencontains repeating sequences of glycine–proline–hydroxylproline and

PROTEINS 3

HO CH2 CH CO2

_

NH3+

HS CH2 CH CO2

_

NH3+

L-Serine (ser)H2N CH2CH2C

O

CH CO2

_

NH3+

L-Glutamine (gln)CHCH3

OH

CH CO2

NH3+

L-Threonine (thr)

H2N CH2C

O +

CH CO2

_

NH3

L-Asparagine (asa)HO CH2

+

CH CO2

_

NH3

L-Tyrosine (tyr)L-Cysteine (cys)

CH2 CH CO2

_

NH3+

L-Histidine (his)

CH2 CH CO2

_

NH3+

L-Tryptophan (trp)

CH2

+

CH CO2

_

NH3

L-Phenylalanine (phe)

N NH

C

O

−O CH2 CH CO2

_

NH3+

L-Aspratic acid (asp)C

O

−O CH2 CH CO2

_NH3

+

CH2

L-Glutamic acid (glu)

S CH2 CH CO2

_

NH3+

CH2CH3

L-Methionine (met)

C CH2 CH CO2

_NH3

+NH2+

NH CH2H2N CH2

L-Arginine (arg)CH2 CH CO2

_NH3

+

+CH2CH2 CH2H2NL-Lysine (lys)

CH2 CHCH CO2

_

NH3+

H3C

H3C

CH3

L-Leucine (leu)

H CH CO2

_

NH3+

Glycine (gly)

N

H

CHCH CO2

_

NH3+

H3C H2C

H2CH3C

L-Valine (val)

CHCH CO2

_

NH3+

H3C

CH2L-Isoleucine (ile)

CH3

CH2

CH

CH CO2

_

_

NH3+

CO2+

L-Alanine (ala)

L-Proline (pro)

N

HH

Figure 1.1 Structures of amino acids

forms a structure of three separate molecules hydrogen-bonded andcoiled in a α–helical conformation (Figure 1.2) [Creighton, 1993; Fraseret al., 1979]. The protein gelatin is formed from the breakdown of theintermolecular bonding of collagen.

Keratin is also a fibrous protein and is contained in horn, hoof, nail,feathers, hair and wool [Mills and White, 1994; Timar-Balazsy andEastop, 1998; Ward and Lundgren, 1954]. The amino acid compositionof keratin from wool is listed in Table 1.1, but the composition variessomewhat depending on the source. Keratin has a high degree of S–Scross-linking between the cysteine residues and extensive hydrogen


Table 1.1 Amino acid compositions of some proteins (%)

Amino acid Collagen Keratin(wool)

Fibroin(silk)

Egg(white)

Egg(yolk)

Casein

Glycine 26.6 6.0 42.8 3.6 3.5 1.7Alanine 10.3 3.9 33.5 6.3 5.6 2.7Valine 2.5 5.5 3.3 8.3 6.4 7.2Leucine 3.7 7.9 0.9 10.3 9.2 9.0Isoleucine 1.9 3.8 1.1 6.2 5.1 6.0Proline 14.4 6.7 0.5 4.5 4.5 13.2Phenylalanine 2.3 3.7 1.3 5.2 3.9 5.1Tyrosine 1.0 5.2 11.9 1.4 2.8 5.5Tryptophan 0.0 1.9 0.9 0.0 0.0 0.0Serine 4.3 8.4 16.3 5.8 9.1 4.0Threonine 2.3 6.6 1.4 3.7 5.6 2.7Cystine 0.0 12.8 0.0 1.9 1.9 0.0Methionine 0.9 0.6 0.0 1.2 2.3 2.3Arginine 8.2 9.9 1.0 6.8 5.5 4.0Histidine 0.7 3.0 0.4 2.4 2.4 3.6Lysine 4.0 0.9 0.6 8.0 5.7 6.7Aspartic acid 6.9 6.9 2.2 10.5 11.5 6.1Glutamic acid 11.2 14.5 1.9 13.9 15.0 20.2Hydroxyproline 12.8 0.0 0.0 0.0 0.0 0.0Hydroxylysine 1.2 0.2 0.0 0.0 0.0 0.0

bonding and α–helical structures, and so is a strong and rigid protein.The structure of keratin is illustrated in Figure 1.3.

Silk fibroin is a fibrous structural protein produced by silkwormsand spiders [Creighton, 1993; Marsh et al., 1955; Timar-Balazsy andEastop, 1998]. The amino acid composition of silk fibroin is listed inTable 1.1 [Lucas et al., 1958; Mills and White, 1994] and it is observedthat the structure is predominantly composed of glycine, alanine andserine. The nature of the amino acid sequence results in a β –sheetpleated structure for fibroin as illustrated by Figure 1.4. This type ofsecondary structure results in a strong silk fibre.

Albumin refers to the proteins contained in eggs [Creighton, 1993;Mills and White, 1994]. Ovalbumin is the main protein of egg white,making up 50 % of the protein content, and conalbumin and lysozymeconstitute 15 % and 3 % of the proteins in egg white, respectively.These proteins form globular conformations via intramolecular hydro-gen bonding. The albumins are easily denatured using heat or certainchemicals. There are small quantitative differences in the amino acidcomposition of the proteins in egg white and egg yolk (Table 1.1) [Keckand Peters, 1969; Mills and White, 1994] and these may be used todistinguish the two components.

PROTEINS 5

Figure 1.2 Structure of collagen. Reprinted from Journal of Molecular Biology,Vol 129, Fraser et al., 463–481, 1979 with permission from Elsevier

Casein is a major protein in milk [Mills and White, 1994; Newman,1998]. Casein contains about 1 % phosphorus, mainly with phosphoricacid esterifying the hydroxyl groups of serine and the amino acidcomposition of casein is also listed in Table 1.1 [Keck and Peters, 1969;Mills and White, 1994].


H

H

H

H

H

H

H

H

HH

CCCO

O

O

O

O

O

O

O

O

O

O

C C

CCCC

CC

CC

CCCC

C C

N

N

NN

N

N

NN

N

N

Figure 1.3 Structure of keratin

= Hydrogen bonding

H NH N

H NH N

O C

O CO C

N HN H

N H

H C RH C R

R C H R C HR C H

R C H

H C RH C RH C R

C O

C O

H NC O

C O

C O

H NC O

Figure 1.4 The β –sheet structure of fibroin

1.3 LIPIDS

Lipids are a group of natural organic compounds which possessstructures that make them insoluble in water but very soluble in organic

LIPIDS 7

solvents. The types of lipids that may be encountered in materialsconservation are oils, fats and waxes [Gunstone, 2004; Mills and White,1994]. Fats and oils are both composed of triglycerides, but are classifiedbased on their physical state at normal temperatures. Oils are usuallyliquids, while fats are solid at normal temperatures. The general struc-ture of a triglyceride is shown in Figure 1.5. Hydrolysis of a triglycerideproduces glycerol and three molecules of fatty acid. Usually triglycerideshave a structure that results in different fatty acids; the structures of themajor fatty acids found in fats and oils are shown in Table 1.2.

Fatty acids can contain double bonds in their hydrocarbon chainand are referred to as unsaturated fatty acids. The presence of twoor more double bonds makes the molecule susceptible to oxidation,but this property can be exploited when an oil is being used as a

CH2 O

O

C R R

O

C OH

A triglyceride Glycerol Fatty acids

O

CH2 O C R

CH O

O

C R 3H2O+ +

CH2 OH

R

O

C OH

R

O

C OHCH2 OH

CH OH

Figure 1.5 General structure of triglycerides and fatty acids

Table 1.2 Structures of common fatty acids (%)

Carbonatoms

Commonname

Structure

Saturated12 Lauric CH3(CH2)10COOH14 Myristic CH3(CH2)12COOH16 Palmitic CH3(CH2)14COOH18 Stearic CH3(CH2)16COOHUnsaturated16 Palmitoleic CH3(CH2)5CH=CH(CH2)7COOH18 Oleic CH3(CH2)7CH=CH(CH2)7COOH18 Linoleic CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH18 Linolenic CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH18 Eleostearic CH3(CH2)3CH=CHCH=CHCH=CH(CH2)7COOH18 Licanic CH3(CH2)3CH=CHCH=CHCH=CH(CH2)4CO(CH2)2COOH


drying oil [Erhardt, 1998; Mills and White, 1994; Sward, 1972]. Theoil transforms from a liquid to a solid as a result of free radical chainreactions. The triglyceride molecules cross-link due to oxidative andthermal polymerisation reactions. The drying process is affected by thepresence of other substances such as pigments and chemical driers,including metallic salts. The oxidative process is usually modified insuch cases [Erhardt, 1998].

Oil films can yellow with age and oils with a high linolenic acidcontent tend to be susceptible to this effect. The yellowing effect is dueto the formation of oxidation products, the nature of which are complexdepending on other components present in an application such as apainting [Mills and White, 1994]. The ester and the double bonds inthe dried oil remain reactive in a dried oil film and so are susceptible tofurther oxidation. Continued oxidation may also lead to a weatheringof the oil film.

The oils that have been used in paints and varnishes are generallyvegetable oils extracted from seeds and have been utilised for hundredsof years [Erhardt, 1998; Sward, 1972; Turner, 1980]. Oils consist ofglycerol esters of higher fatty acids with even carbon numbers, withdifferent oils containing a mixture of different types and compositionsof fatty acids. They are classified according to their ability to dry to asolid film. Drying oils form a film at normal temperatures and containa substantial amount of fatty acids with three double bonds in theirstructure. Semi-drying oils, however, require heat to form a film andmainly contain fatty acids with two double bonds. The compositionsof some common oils are summarised in Table 1.3. Linseed oil is themost commonly used oil in the paint industry. It has been used for manycenturies as a constituent of oil paint and now in printing inks. Linseed

Table 1.3 Composition of some oils

Oil Fatty acids, (%)

Saturatedacids

Oleic Linoleic Linolenic Eleostearic

Linseed 10 22 17 51 –Tung 5 9 – 15 71Poppyseed 10 15 73 2 –Safflower 10 14 76 – –Soya 13 28 54 5 –Walnut 9 17 61 13 –Perilla 8 14 14 64 –Oiticica 12 7 – – –

CARBOHYDRATES 9

contains large percentages of linolenic and linoleic triglycerides and thehigh degree of unsaturation results in a relatively short drying time. Inthe past, the oil was obtained from seed by pressing, but now solventextraction is used. As well as being used as a drying oil, linseed oil isalso used as an ingredient in resins. Tung oil is also known as wood,Chinese wood or mu oil and it has a relatively high viscosity and driesquickly. It consists mainly of eleostearic acid and is used as a varnishand in alkyd paints. Walnut oil is one of the earliest oils used in paintingand was a common medium in the early days of oil painting, althoughit is little used today. It dries more slowly than linseed oil and has atendency to turn yellow. Poppyseed oil is used as a medium for artists’colours and it is semi-drying and resistant to yellowing. Poppyseed oil isless viscous than linseed and walnut oils and does not easily turn rancid.Safflower oil has a very low linolenic acid content, which means that ithas the advantageous property of low yellowing. It is a semi-drying oilused for paint and varnishes. Soybean oil is a semi-drying oil that is usedto prepare alkyd paints.

Waxes are naturally occurring esters of long chain carboxylic acidswith long chain alcohols [Mills and White, 1994; Newman, 1998]. Theyhave a characteristic smooth ‘waxy’ feel. Wax is used as an ingredientof polishes for paintings and wooden surfaces and in coatings for paper.Natural waxes of animal and vegetable origin such as beeswax, paraffin,carnauba and candelilla have all been used. Paraffin is obtained fromcrude petroleum and consists of long chain saturated hydrocarbons.Beeswax is a complex mixture of hydrocarbons, esters and free fattyacids that is obtained from the hives of honey bees [Horie, 1987; Millsand White, 1994]. It has been historically used in paintings: beeswaxwas used by the Egyptians to protect the surface of paintings in tombs.Beeswax is usually purified by melting and filtering and consists mainlyof myricyl palmitate ester (C15H31CO2H). Beeswax has been widely usedfor relining paintings, usually mixed with natural resins and fillers. Ithas also been used as a consolidant and in polish formulations. Beeswaxhas the advantage that it does not significantly deteriorate with time,but it can yellow due to oxidation.

1.4 CARBOHYDRATES

A range of heritage objects are made up of carbohydrates: paintings,adhesives, paper and wooden objects. The carbohydrates of interestare monosaccharides (simple sugars) and polysaccharides [Davis andFairbanks, 2002; Mills and White, 1994; Newman, 1998; Timar-Balazsy


CH3

H

H

H

H

H

H

H

H

HHOH2C

H

H H

H H

HHO

OHOHOH

OHOH

OH

OH OH

OHOH

OOO

H

HH

H

H

OH

OH

O

COOH

OH

OH

CH2OH

CH3H H

H

H

OH

O

OH

OH

OH

CH2OH

HH

H

H

H

OH

OH

OH

OH

O

Glucose Galactose Galacturonic acid

RhamnoseXyloseArabinose

Figure 1.6 Structures of some monosaccharides

and Eastop, 1998]. There is a large number of sugars in existence and thestructures of some natural monosaccharides are illustrated in Figure 1.6.The D- and L- nomenclature is used to represent the different enantiomersand the + and − in the name of an enantiomer indicates a positive ornegative sign of rotation. Monosaccharides can form bonds with oneanother to produce larger molecules known as oligosaccharides. Suchstructures can vary from two to about 10 sugar units. For example, acommon sugar sucrose is made up of two hexose units.

Large molecular weight sugars are known as polysaccharides andsome important polysaccharides are cellulose, starch and gums. Cellu-lose is a very common polysaccharide found in plant fibres [Mills andWhite, 1994]. It is a high molecular weight polymer of D-glucose andthe structure is illustrated in Figure 1.7. The cellulose molecules formhydrogen bonds between the hydroxyl groups of the chains, allow-ing fibrils to be formed. Cellulose fibres are chemically stable and are

O

OO

OHOH

OH

HOHO

HOCH2OH

O

OO

O

CH2OH

CH2OH

Figure 1.7 Structure of cellulose

NATURAL RESINS 11

strong, but can deteriorate due to oxidative or hydrolytic reactions. Thehydroxyl groups can be oxidised to carboxylic acid groups, especiallywhen exposed to light. Cellulose can also be hydrolysed with acids atthe links to break the structure down into smaller units.

Another commonly encountered polysaccharide is starch, which isfound in vegetable matter [Mills and White, 1994]. It contains glucose,as well as amylose and amylopectin units. The properties of starch varydepending upon the source due to the varying proportions of amyloseand amylopectin. Starch swells on exposure to water and if heated, athick viscous paste is formed.

Plant gums are exuded by certain plants when their barks are broken.Gums consist of complex branched polysaccharides [Mills and White,1994; Newman, 1998]. Despite their complex structures, it is possibleto identify the sugars of gums by determining the monosaccharidecomposition after cleavage of the glycosidic bonds. The possible sugarsin plant gums are: L-arabinose, D-galactose, D-mannose, L-rhamnose,D-xylose, L-fucose, D-glucose, D-galacturonic acid and D-glucuronicacid. Gum arabic (also known as gum acacia) is the most commonlyproduced and is a high molecular weight polysaccharide extracted fromthe Acacia species [Horie, 1987]. The gum is a complex mixture ofarabinogalactan oligosaccharides, polysaccharides and glycoproteins. Itis transparent, brittle, soluble in water at room temperature and canform viscous solutions. Gum arabic may be cross-linked and precipitatedby metal ions such as aluminium, iron, lead, mercury salts and gelatin.It is susceptible to biodeterioration. Gum arabic has long been usedin inks and water based paints, as well as an adhesive for paper andtextiles. Some other gums that can be encountered in objects of culturalsignificance are gum tragacanth, gum karaya and cherry gum.

1.5 NATURAL RESINS

Many natural resins are extracted from trees and plants and are basedon terpenoid structures [Horie, 1987; Mills and White, 1994; Millsand White, 1977; Newman, 1998]. Such resins have been used exten-sively because of their attractive properties including adhesion, waterinsolubility and glassiness. Although resins are complex mixtures ofterpenoid components, classification is aided by the fact that diterpenoidsand triterpenoids are not found together in the same resin. Oil ofturpentine is a common film forming resin derived from pine trees. Theresin is composed of monoterpenoids including pinene, but the exactcomposition depends on the source.


Some common triterpenoid resins include dammar and mastic.Dammar is composed of a mixture of molecules includingdammaradienol (Figure 1.8); it is hard and brittle and has been usedfor picture varnishes. Mastic resin is also a mixture of triterpenoids;Figure 1.8 illustrates a typical component, masticadienonic acid. Masticis yellow to green in colour and was used as a picture varnish for manyyears for its good working and film properties. However, the resin isbrittle and was superceded by dammar.

A number of diterpenoid resins have been used in heritage applications:rosin, sandarac and copals may be encountered. Rosin is a brittle, glassyresin and abietic acid is a major component (Figure 1.8). Rosin has beenused for painting materials since the 9th century. Sandarac is a yellowgum resin and is also known as gum juniper. Sandarac has been used invarnish preparations and is largely composed of polymerised communic

Dammaradienol

HO

Masticadienonic acid

O

98

7

H

COOH

COOH

COOH

COOHH

98

7

COOH

Abietic acid

COOH

Sandaracopimaric acid

COOH

‘Iso-communic acid’

Agathic acid Eperuic acid

Figure 1.8 Structures of some terpenoids found in natural resins

NATURAL MATERIALS 13

acid with sandaracopimaric acid also present (Figure 1.8). Copal is ageneral term used to describe some terpenoid resins. Like sandarac,polymerised communic acid makes an important contribution to thestructures of copals. Copal is sometimes referred to as immature amberand most copals originate from African, Asian or Central Americansources. Agathic acid (Figure 1.8) is found in a number of copal resinsand eperuic acid (Figure 1.8) in a number of African copals. Copals havebeen extensively used for artefacts and as a varnish for oil paintingswhen dissolved in solvents.

Amber is a fossilised resin and is mainly found in the Baltic region ofNorthern Europe. Amber is an attractive material as it is translucent andthe colour can vary from pale yellow to deep brown. It is easily workedand polished and has been widely used for jewellery. Another well-known feature of amber is the presence of prehistoric insect bodies – itprovides a good preservation medium. Amber can also be dispersed inoil to form a varnish. The chemical composition of amber is a complexmixture of abietic acid components and diterpenoids, with succinic acidbelieved to be present.

Shellac is a naturally derived polymeric resin obtained from thesecretions of insects [Katz, 1994; Horie, 1987; Quye and Williamson,1999]. The insect secretes a protective covering against predators ontotwigs and a brownish resin in extracted by melting. The resin may becooled in various shapes. Approximately 30–40 % of shellac is aleuriticacid, which is combined with sesquiterpene acids. The Egyptians usedshellac thousands of years ago to coat their mummies. Shellac has beenused for centuries as a lacquer for a protective and decorative finishon both metals and wood. When it was discovered that shellac couldbe compounded with fillers such as wood powder to produce a toughmouldable material, more applications were established in the 19thcentury. A particularly useful property was that shellac was capable ofshowing fine details and this led to the first pressing of sound records.

1.6 NATURAL MATERIALS

Animal glues are obtained from the skin, bone and other tissues ofvarious species and have been widely used as adhesives and paintbinders [Horie, 1987; Newman, 1998]. Gelatin is extracted by boilingand the extract is cooked to form a gelatin material. The gelatin canthen be reliquefied with heat, which provides quick setting properties.Connective tissue is composed of various proteins, mainly collagen, andmany other compounds. While collagen molecules are held together


with many hydrogen bonds, these molecules are partly hydrolysed onheating in water to produce a soluble product. Parchment glues arederived from the hides of cattle and sheep and have high strength, butmay contain contaminants from skin preservatives and tanning agents.Bone glues are derived from the bones of farm animals and fish glues aremade from skins or swim bladders. Animal glues were the most effectiveadhesives until the development of synthetic adhesives. Glue and starchpastes were introduced during the 18th century for the relining of canvaspaintings and were used on textiles. Glue is also used as an adhesive forconserving furniture.

The skins and hides of animals have long served as clothing orshelter for man [Lambert, 1997]. Skin is composed of collagen and hairsof keratin. Such materials require a preservation process for use andtanning is a common chemical procedure for preserving skin. A simpletanning process involves applying oils or vegetable extracts to makethe skin pliable and impermeable to water. Skins were also used forwritten records and a smooth surface was often created by the chemicalremoval of hair. After soaking in water, the skins could be treated withan alkaline agent, such as an aqueous lime solution.

Horn, hoof and tortoiseshell are chemically similar and are materialsbased on the protein keratin [Katz, 1994; Quye and Williamson, 1994].The thermoplastic nature of these keratotic materials has been exploitedsince prehistoric times. Horn was once very plentiful and the process ofmoulding was simple to carry out. This material has been used to producedrinking containers, shoe horns, combs, boxes, jewellery and buttons.In the 19th century a fashion for hair combs provided a demand forhorn, which was eventually superceded in this application by celluloidin the 1920s. Hoof can be moulded more easily than horn and wasused to produce buttons. Tortoiseshell produces an attractive mottledappearance when heated and pressed and was widely used to produceitems such as decorative boxes. The techniques developed to producedecorative pieces from horn, hoof and tortoiseshell were precursors tothose used in the modern plastics industry.

Ivory is the creamy white substance that forms the tusks or teeth ofmammals and has been used to produce many decorative and practicalobjects [Cronyn, 1990; Lambert, 1997]. Although the main source ofivory is elephant tusks from Africa and India, tusks or teeth of mammoth,whale, walrus, pig, warthog and hippopotamus have all been used anddescribed as ivory over the years. Ivory is composed of osteons, whichcontain lamellae of collagen and other proteins embedded in an inor-ganic matrix of hydroxyapatite [Ca10(PO4)6(CO3).H2O]. The collagen

NATURAL MATERIALS 15

provides elasticity and a high tensile strength, while hydroxyapatiteprovides hardness and rigidity. The properties of ivory made it a popu-lar material for carved objects including jewellery, statues, piano keys,cutlery handles and billiard balls.

Natural rubber was first brought to Europe during the 18th cen-tury from South America, but it was not until the mid-19th centurythat the material was chemically modified and became suitable formanufacture [Fenichell, 1997; Katz, 1994; Mossman, 1997; Quye andWilliamson, 1999]. Rubber is extracted from the latex of tropical treesand consists mainly of poly(cis-isoprene), with small amounts of theother components including proteins and lipids. Rubber in solution wasused in the early 19th century as a waterproof coating for cloth inMacintosh raincoats. An important breakthrough came when ThomasHancock discovered the vulcanisation process, the process by whichrubber is lightly cross-linked by heating it with sulfur to reduce plasticityand to develop elasticity. Natural rubber has been used in a wide rangeof applications including shoes and tyres. When rubber is cross-linkedwith a greater amount of sulfur, an extremely hard dark material knownas vulcanite or ebonite is produced. This process was developed in the1840s and vulcanite was widely used to produce a range of decorativehousehold items and its good electrical insulation properties led to use inthe emerging electrical industry of the late 19th century. Gutta percha isrelated to natural rubber in chemical structure, but shows quite differentphysical properties. Gutta percha is also a tree exudate, but is mainlycomposed of poly(trans-isoprene). The different isomer results in a hardbrittle material. Gutta percha was used from the mid-19th century untilthe 1930s to produce many moulded objects including containers, toysand tubing [Fenichell, 1997; Katz, 1994; Mossman, 1997; Quye andWilliamson, 1999]. As gutta percha oxidises and becomes brittle withtime, mouldings of this material are now scarce.

Casein is a major protein found in milk and is precipitated fromskimmed milk treated with acid [Katz, 1994; Mills and White, 1994;Newman, 1998; Quye and Williamson, 1999]. The curds are reactedwith formaldehyde to produce a hard material. Casein was first patentedin 1899 and was produced in Europe as galalith and erinoid. Theability of casein to be surface dyed meant that decorative products suchas buttons (the major product), fountain pens, jewellery, candlesticks,spoons and gaming chips could be produced in small batches. Caseinhas also been used as a consolidant or adhesive [Horie, 1987]. Calciumcaseinite is the most useful form for adhesives and has traditionally beenused for wood and plaster restoration.


Wood from plant sources has been used for the production of manyobjects, especially furniture, for many years. Wood is made of cellulose(40–45 %), hemicellulose (20–30 %) and lignin (20–30 %) [Hoadley,1998; Lambert, 1997]. Like cellulose, hemicellulose is a polysaccharide,but usually has a low molecular weight. Lignin is a complex poly-mer molecule made up of cross-linked phenolic annamyl alcohols. Thepolysaccharide components of wood tend to decay faster than lignin.Wood is protected using coatings such as oils and resins.

There are several bituminous materials encountered in artefacts thatare natural carbon-based products [Lambert, 1997; Mills and White,1994]. Bitumen is a tarry petroleum product that has been used as anadhesive and for moulded artefacts. It is considered one of the earliestmoulding materials and shows plastic behaviour [Mossman, 1997; Quyeand Williamson, 1999]. As a property of bitumen is electrical resistance,it found application in the electrical industry at the end of the 19thcentury. Asphalt is similar but usually has calcite (CaCO3), silica (SiO2)or gypsum (CaSO4) added. Jet, a form of brown coal, is a fossilised woodderived from an ancient species of tree [Lambert, 1997; Muller, 2003].It contains approximately 12 % mineral oil and traces of aluminium,silica and sulfur. Jet became very popular in Britain in Victorian times,particularly for mourning jewellery. Pyrolysis of carbon-based materialssuch as wood, coal and peat produces aromatic compounds such asphenol, heterocyclics and polynuclear aromatic hydrocarbons, whichare distilled as coal tar. The higher molecular weight residual moleculesform pitch. Tar and pitch have been used for adhesives, sealants andcoatings.

Bois durci is a moulding material based on cellulose and was patentedin Paris in 1855 [Katz, 1994; Mossman, 1997; Quye and Williamson,1999]. It was made from wood flour blended with albumen from egg orblood and when moulded produced a brown or black thermoset. Thesurface of bois durci gave it a metallic-like finish. Bois durci mouldingswere used for plaques, inkwells, desksets and picture frames.

The papier mache process was patented in the 18th century [Katz,1994; Mossman, 1997]. In the process, pulp made from finely groundwood flour or paper is mixed with animal glue or gum arabic, thenpressed into a mould and dried in an oven. The moulding was thensanded, polished and decorated. Papier mache was popular for theproduction of decorative items in the 19th century, such as trays,spectacle cases and snuff boxes. Pulp ware came along in the latter partof the 19th century [Katz, 1994]. The pulp was made from purifiedground wood and linseed oil and phenolic resin and melamine used to

SYNTHETIC POLYMERS 17

coat the surface. As pulp ware was impact and water resistant, it founduse for inexpensive kitchenware.

1.7 SYNTHETIC POLYMERS

Synthetic polymers are a 20th century phenomena and have been usedfor an enormous range of commodities. Despite their reputation as cheapand disposable, there are many polymeric products that are of interest toconservators. Polymers have been used to produce culturally importantmaterials held in museums, such as sculptures, paintings, toys, clothing,jewellery, furniture, household goods and cars [Fenichell, 1997; Katz,1994; Mossman, 1997; Quye and Williamson, 1999].

Synthetic polymers are synthesised from their constituent monomersvia a polymerisation process and most are based on carbon compounds.Polymers have been commonly referred to as ‘plastics’. Plastic refersto one class of polymers known as thermoplastics, which are polymersthat melt when heated and resolidify when cooled. Thermoplastics tendto be made up of linear or lightly branched molecules. Thermosets arepolymers which do not melt when heated, but decompose irreversibly athigh temperatures. Thermosets are cross-linked; the restrictive structurepreventing melting. Some cross-linked polymers may show rubber-likecharacteristics and these are known as elastomers. Elastomers can bestretched extensively but rapidly recover their original dimensions.

When examining a polymeric material it is important to be awarethat it may be composed of more than one component. Copolymers arecomprised of chains containing two or more different types of monomers.Polymer blends are mixtures of polymeric materials and consist of atleast two polymers or copolymers. Composites are materials composedof a mixture of two or more phases and polymers can constitute thefibre or matrix component of a composite.

The identification and characterisation of polymers is made morecomplex by the fact that they often contain additives to modify theirproperties. Fillers are added to polymers to improve their mechanicalproperties, such as strength and toughness, and usually consist of mate-rials such as calcium carbonate, glass, clay or fibres. Colourants suchas dyes or pigments are used to impart a specific colour. Polymers mayhave their flexibility improved by the addition of plasticisers, which tendto be low molecular weight liquids such as phthalate esters. Stabilisersare used to counteract degradation of polymers by exposure to lightand oxygen and include lead oxide, amines and carbon black. The


flammability may also be minimised by the addition of flame retardantssuch as antimony trioxide.

The means by which polymers are processed can also affect the prop-erties of the material produced [Osswald, 1998]. A common method forprocessing thermoplastics is injection moulding, where the polymer, inthe form of a powder or granules, is heated by a rotation screw untilsoft then forced through a nozzle into a cooler mould. Extrusion is alsocommon for thermoplastics, where the polymer is melted and formedinto a continuous flow of viscous fluid then forced through a die. Ther-moset polymers can be formed by transfer moulding, where the powderis heated and compressed in a chamber and enters a mould cavity in aflowing state. Compression moulding is also used to produce thermosetsand involves placing partially polymerised thermoset powder into atwo-part mould. The mould is closed, heat and pressure are applied, andthe polymer adopts the mould shape as it cross-links and hardens.

Cellulose nitrate (or nitrocellulose) (Figure 1.9) was an early syntheticplastic developed during the 1840s [Fenichell, 1997; Katz, 1994; Moss-man, 1997; Quye and Williamson, 1999]. Although initially recognisedas an explosive, it was soon realised that cellulose nitrate was also ahard elastic material which could be moulded into different shapes.In the 1860s, Alexander Parkes plasticised cellulose nitrate with oilsand camphor. However, his material, known as Parkesine and used formouldings, such as combs and knife handles, soon resulted in warpingand cracking. In the United States, the Hyatt brothers plasticised cellu-lose nitrate with camphor and patented celluloid in 1869. Celluloid wasa commercial success, leading to the development of photography andthe cinema industry. Celluloid was also used to produce toys, combs andother moulded household goods. Cellulose nitrate has been supersededin this type of application because of its flammability and degradabil-ity, but now finds use in the field of coatings as a lacquer. By thelate 19th century other modifications of cellulose had been developed.When cellulose is dissolved via a particular chemical reaction and thenreprecipitated as pure cellulose, the product is known as regeneratedcellulose. When regenerated cellulose is prepared as a fibre, it is knownas viscose or viscose rayon and has been widely used for textile fibres.When prepared as a film, regenerated cellulose is known as cellophane,a well known packaging and wrapping material. The development ofthe commercially important cellulose acetate early in the 20th centurywas a result of the esterification of cellulose [Fenichell, 1997; Katz,1994; Mossman, 1997; Quye and Williamson, 1999]. When completeacetylation is carried out, cellulose triacetate is formed (Figure 1.9).


NO2

NO2

CH2

HOO

O

O

O

CH3

CH3CC

OO

CH2

CH3

OO

O

O C

O

O

Cellulose nitrate Cellulose triacetate

Nylon 6

Nylon 6,6

Kevlar

Polystyrene

Poly(ethylene terephthalate) Polyurethane

Poly(methyl methacrylate) Poly(vinyl acetate)

O

OO

O

C

N

H

C

C C

N

N N

n

n

(CH2)5

(CH2)6 (CH2)4

H

H H

O

C

O

C

N

H

N

H

CH2 CH CH2

CH3

CH3

C

C O

O

CH2

CH3

CH

O

C O

COCH2CH2O

OO

CO

O O N

H

R R'C

O

C N

H

Figure 1.9 Structures of some common polymers

However, the acetylation reaction may also be reversed to a point wherecellulose diacetate is formed, which is more suitable for use as a fibre.Cellulose acetates are employed in a wide range of forms includingfibres, moulded products, films and packaging.

Phenol-formaldehyde or phenolic resins are thermosetting polymersmade by reacting phenol and formaldehyde and adding filler. Phenolicresins are better known by the tradename Bakelite [Fenichell, 1997;Katz, 1994; Mossman, 1997; Quye and Williamson, 1999]. At thebeginning of the 20th century, Leo Baekeland in the United States wasthe first to successfully commercialise the polymerisation of phenolicresin by incorporating wood flour into the phenol and formaldehyde


reaction process. The reaction is stopped before completion and the solidmixture ground, before being heated in a mould. Amber cast phenolicmouldings produced slowly without filler can be completely transparentand the incorporation of wood flour produces dark mouldings. Bakelitewas highly successful and became widely used for household goods andin the developing electronic and car industries. Other resins, includingurea-formaldehyde and melamine-formaldehyde resins, emerged in the1930s for use in the production of household items due to their lightercolour, enabling a wider range of colours to be produced.

Nylon was the result of a deliberate process by Carothers of theDuPont company in the United States to produce a material which couldreplace silk [Fenichell, 1997; Mossman, 1997; Quye and Williamson,1999]. World War II was responsible for the development of syntheticpolymers with war-time needs forcing the production of low-cost plas-tics. The first application of nylon was as fibres in fabric for parachutes,stockings and toothbrushes. There are a number of different types ofnylons, but all contain an amide linkage (Figure 1.9). Single numbernylons, such as nylon 6, nylon 11 and nylon 12, are so named becauseof the number of carbon atoms contained in the structural repeat unit.Double number nylons, nylon 6,6, nylon 6,10 and nylon 6,12, are namedby counting the number of carbon atoms in the N-H section and thenumber in the carbonyl section of the repeat unit. The more recentlydeveloped aromatic polyamides are known as polyaramids and there aretwo established fibres in this class, poly(m-phenylene terephthalamide)(Nomex) and poly(p-phenylene terephthalamide) (Kevlar) (Figure 1.9).Kevlar fibres show unusually high tensile properties and have beenused for applications including protective clothing, ropes, composites,sporting goods and aeronautical engineering.

Commercially important vinyl polymers were developed from the1930s onwards. Chemists at ICI in Britain, while experimenting withethylene at different temperatures and pressures, established polyethy-lene (PE) [–(–CH2–CH2–)–] [Fenichell, 1997; Mossman, 1997; Quyeand Williamson, 1999]. PE, often referred to as polythene, is themajor general purpose thermoplastic and is widely used for pack-aging, containers, tubing and household goods. There are two maintypes of mass produced PE: low density polyethylene (LDPE) has abranched chain structure and tends to be used for bags and pack-aging; high density polyethylene (HDPE) has a mostly linear struc-ture and finds uses in bottles and containers. Polypropylene (PP)shows a similar structure to PE, but with a substituted methyl group[–(–CH2–CH(CH3)–)–] [Quye and Williamson, 1999]. This polymer is


used for a wide range of applications such as chairs, bottles, carpets,casings and packaging.

Poly(vinyl chloride) (PVC), first commercialised in the 1940s, hasa structure containing a chlorine atom on alternate main chain car-bons [–(–CH2–CHCl–)–] [Fenichell, 1997; Mossman, 1997; Quye andWilliamson, 1999]. The lack of flexibility in PVC molecules means thatthis polymer is commonly processed with plasticisers. PVC has beenwidely used: toys were made from PVC from the 1940s and shoes andclothing were made of PVC from the 1960s.

Polystyrene (PS) is a clear, rigid and brittle material unless it is mod-ified with rubber (Figure 1.9) [Fenichell, 1997; Mossman, 1997; Quyeand Williamson, 1999]. PS is used widely in packaging and appliancehousings. In the 1940s, acrylonitrile–butadiene–styrene (ABS) blendswere developed to improve the impact resistance of PS and used fordecorative items and in cars. ABS blends are composed of two copoly-mers, with a matrix consisting of a styrene–acrylonitrile copolymer anda rubbery phase consisting of a styrene–butadiene copolymer.

Polyacrylates (or acrylics) are derived from acrylic acid[CH2=CHCOOH] or methacrylic acid [CH2=C(CH3)COOH] and arevaluable for use as varnishes and transparent plastics [Fenichell, 1997;Mossman, 1997; Mills and White, 1994; Quye and Williamson, 1999].Poly(methyl methacrylate) (PMMA) is a well known type of acrylicand was developed in the 1930s as an alternative to glass (Figure 1.9).Well known by the trade name Perspex, this polymer is a transparent,hard and rigid material, making this polymer particularly useful inglazing. A number of acrylic copolymers, such as methyl acrylate – ethylmethacrylate copolymers, have been employed in conservation asvarnishes due to their stability and clarity.

Polytetrafluoroethylene (PTFE) [–(–CF2–CF2–)–] is perhaps betterknown by its trade name of Teflon [Fenichell, 1997; Quye andWilliamson, 1999]. There is a common misconception that ‘the onlygood thing that came out of the space race was the non-stick frying pan’,referring to the emergence of PTFE as a common surface coating in the1960s. In fact, PTFE was originally discovered in the 1930s at DuPont,when tetrafluorine gas was accidentally polymerised; the polymer waslater commercialised in the 1950s. PTFE is a thermoplastic which showsremarkable chemical resistance, electrical insulating properties and alow friction coefficient. PTFE is used for non-stick coatings, electricalcomponents, bearings and tape.

Poly(vinyl acetate) (PVA) is commonly used in the form of an emulsion(Figure 1.9) [Mills and White, 1994]. PVA is tough and stable at room


temperature, but becomes sticky and flows at slightly elevated tem-peratures. PVA is a quick drying polymer and so can be used in theproduction of water-based paints and is also used commonly as anadhesive. Alcohols are added to PVA to produce poly(vinyl alcohol)(PVAl) [–(CH2–CH(OH)–)–], which is a water soluble polymer used forfibres, adhesives and as thickening agents.

Polyethylene oxide (PEO) [–(–CH2–CH2–O–)–] is a water solublepolymer. PEO has been used in conservation by impregnating thepolymer into water affected materials such as wood and leather [Millsand White, 1994]. Another polymer that has been used to impregnatefragile objects, such as paper and books, is poly-p-xylylene (Parylene).The p-xylylene monomer is diffused into a material of interest placedin a vacuum and the polymerisation occurs at room temperature. Thestrength of the object is improved with no change in appearance [Millsand White, 1994].

Poly(ethylene terephthalate) (PET) (Figure 1.9) is a polyester used toform fibres known by the trade names Dacron and Terylene [Fenichell,1997; Mills and White, 1994; Quye and Williamson, 1999]. PET iswidely used in film form for bottles and cinema film base. Another typeof polyester, alkyd resins, is widely used in paint and varnishes [Millsand White, 1994]. Alkyds are composed of a polyfunctional alcohol(e.g. glycerol), a polyfunctional acid (e.g. phthalic anhydride) and anunsaturated monoacid (e.g. drying oil). The presence of drying oilfatty acids means that further cross-linking can occur to produce across-linked network. Unsaturated polyesters can be cross-linked toform thermosets and are commonly used with glass fibres to formhigh strength composites. Linear polyesters are cross-linked with vinylmonomers, such as styrene, in the presence of a free radical curing agent.As polyester resins are low in viscosity, they can readily be mixed withglass fibres.

Epoxy resins contain epoxide groups and a common type of epoxyprepolymer is based on glycidyl ethers [Mills and White, 1994]. Theresins are cured using catalysts or cross-linking agents such as aminesand anhydrides. The epoxy and hydroxyl groups are the reaction sites forcross-linking and can undergo reactions which result in no by-products.This results in low shrinkage during hardening and they are used incoatings and composites.

Polyurethanes (PUs) are versatile thermosets which are used asfoams, elastomers, fibres, adhesives and coatings. There is a rangeof chemical compositions in PUs, but all contain the common urethanegroup (Figure 1.9). Urethanes are formed by a reaction between a

DYES AND PIGMENTS 23

hydroxyl containing molecule and a reactant containing an isocyanategroup. Another type of elastomer, silicone, is based on a silicon,rather than a carbon backbone. The structures of silicones are basedon silicon and oxygen and the most common silicone elastomer ispoly(dimethylsiloxane) (PDMS), which may be cross-linked to formSi–CH2 –CH2 –Si bridges. These elastomers are thermally stable andwater resistant and are used for medical applications and sealants.

Despite their reputation as materials that last forever, polymers dodeteriorate with time. Degradation affects the appearance and physicalproperties of polymers, with some common effects being discolorationand embrittlement [Allen and Edge, 1992; Lister and Renshaw, 2004;McNeill, 1992; Quye and Williamson, 1999]. The factors that can causepolymers to degrade include exposure to light, oxygen and moisture inthe atmosphere and the presence of additives, such as plasticisers andfillers.

There are various types of chemical change that may occur withina polymer that lead to decay. The polymer chains can be shorteneddue to the breakdown of bonds within the chains, which leads topoorer properties compared to the original polymer. Cross-linking ofpolymer chains may also occur. Reactions that produce bonds betweenthe polymer chains can produce a polymer that is brittle and less flexible.Reactions associated with the side groups in polymers can also lead todegradation. Such reactions may involve the release of small moleculessuch as water or acids. Not only do these reactions change the chemicalstructure, the molecules released can produce further changes or catalyseother reactions.

1.8 DYES AND PIGMENTS

Colourants are used in a broad range of museum objects includingpaintings, textiles, polymers, written works, inks and ceramics. Dyesand pigments are the compounds that are used to create an array ofcolours [Doerner, 1984; Feller, 1986; Lambert, 1997; Mills and White,1994; Needles, 1986; Roy, 1993; West Fitzhugh, 1997; Timar-Balazsyand Eastop, 1998]. Such materials must be reasonably stable to light.Pigments are coloured compounds that come in the form of solidparticles suspended in a medium that binds to a surface, such as thecanvas of a painting. Dyes are dissolved in a liquid and are usually bounddirectly to the surface, such as in the case of textile fibres. Pigments anddyes are obtained from naturally occurring minerals, extracted fromplants or insects or produced by chemical synthesis. Some organic dyes


can be converted to a pigment by incorporating them into kaoliniteor chalk and then powdering the mixture to produce a ‘lake’. Mostearly pigments were inorganic, but today both organic and inorganicpigments are used. Tables 1.4 and 1.5 show a summary of the namesand structures of a number of inorganic and organic colourants. Thechemical structures of a number of the organic molecules found in dyesand pigments are illustrated in Figure 1.10. The range of synthetic dyesand pigments is extensive. The synthetic colourants are standardisedby a CI (Colour Index) number, referenced by professional colourists’associations [Colour Index, 2002].

Table 1.4 Chemical compositions of inorganic pigments

Colour Common name ChemicalComposition

Origin

White Anatase TiO2 20th centuryAntimony white Sb2O3 SyntheticBone white Ca3(PO4)2 AntiquityChalk/Calcite/Whiting CaCO3 MineralGypsum CaSO4.2H2O MineralKaolin Al2(OH)4SiO5 MineralLead white 2PbCO3.Pb(OH)2 Synthetic, AntiquityLithopone ZnS, BaSO4 Synthetic, 19th centuryPermanent white BaSO4 Synthetic, 19th century/Barium white/Barite/ /MineralBarytes/Barium sulfateRutile TiO2 20th centuryTitanium white TiO2 Synthetic, 20th centuryZinc white ZnO Synthetic, 19th century

Yellow Barium yellow BaCrO4 Synthetic, 19th century/Lemon yellowCadmium yellow CdS Synthetic, 19th centuryChrome yellow 2PbSO4.PbCrO4 or PbCrO4 Synthetic, 19th centuryChrome yellow orange PbCrO4.PbO Synthetic/Chrome yellow deepCobalt yellow K3[Co(NO2)6] Synthetic, 19th centuryLead-tin yellow (type I) Pb2SnO4 Synthetic, 14th centuryLead-tin yellow (type II) Pb2Sn1−xSixO3 SyntheticLitharge/Massicot PbO 14th centuryNaples yellow Pb(SbO3)2, Pb3(SbO4)2 Synthetic, 1500–1300 BCOrpiment/Auripigmentum

As2S3 Mineral

Pararealgar As4S4 13th centuryStrontium yellow SrCrO4 Synthetic, early 19th centuryYellow ochre/Limonite Fe2O3. nH2O, Clay, Silica MineralZinc yellow ZnCrO4 Synthetic, early 19th century

Red Cadmium red CdSe, CdS Synthetic, 20th centuryChrome red PbCrO4.Pb(OH)2 Synthetic, 19th centuryHaematite/Mars red Fe2O3 Synthetic, 19th centuryLitharge PbO AntiquityMolybdate red 7PbCrO4 .2PbSO4.PbMoO4Realgar As4S4 Mineral


Table 1.4 (continued)

Colour Common name ChemicalComposition

Origin

Red lead/Minium Pb3O4 Synthetic, AntiquityRed ochre/Red earth Fe2O3. nH2O, Clay, Silica MineralVermillion/Cinnabar HgS Synthetic, 13th century

/Mineral

Blue Azurite 2CuCO3.Cu(OH)2 MineralCerulean blue CoO.SnO2 Synthetic, 19th centuryCobalt blue CoO.Al2O3 Synthetic, 18th centuryCobalt violet Co3(PO4)2 Synthetic, 19th centuryEgyptian blue CaO.CuO.4SiO2 Synthetic ca 3000 BCHan blue BaCuSi4O10 MineralHan purple BaCuSi2O6 MineralLazurite/Ultramarine/Lapis lazuli

Na8−10Al6Si6O24S2−4 Synthetic, 19th century/Mineral

Manganese blue BaSO4.Ba3(MnO4)2 20th centuryPosnjakite CuSO4.3Cu(OH)2.H2O MineralPrussian blue Fe4(Fe[CN]6)3 Synthetic, 18th centurySmalt K2O, SiO2, CoO 16th century

Green Atacamite CuCl2.3Cu(OH)2 MineralBasic copper sulfate CuSO4.nCu(OH)2 SyntheticBrochantite Cu4(OH)6SO4 MineralChromium oxide Cr2O3 Synthetic, early 19th centuryChrysocolla CuSiO3. nH2O Mineral, AntiquityCobalt green CoO.5H2O Synthetic, 18th centuryCopper chloride CuCl2 SyntheticEmerald green Cu(CH3COO).3Cu(AsO2)2 Synthetic, 19th centuryMalachite CuCO3.Cu(OH)2 Mineral, AntiquityScheele’s green Cu(AsO2)2 Synthetic, 18th centuryTerre-verte Variations on/Green earth K[(Al3+, Fe3+) (Fe2+, Mg2+)], Mineral

(AlSi3,Si4)O10(OH)2Veronese green Cu3(AsO4)2.4H2O Synthetic, 19th centuryVerdigris Cu(CH3COO)2.nCu(OH)2 Synthetic, AntiquityViridian/Guignet’s

greenCr2O3.2H2O Synthetic, 19th century

Black Antimony black Sb2S3 MineralCarbon black/Lampblack/Charcoal black

C Antiquity

Galena PbS MineralIvory black/Bone black C, Ca3(PO4)2 AntiquityIron black/Black iron

oxideFe3O4 Mineral/

/Mars black Synthetic, 19th centuryManganese oxide MnO, Mn2O3 Mineral/Manganese blackPlattnerite PbO2 Mineral

Orange Burnt sienna Fe2O3. nH2O, Al2O3 Mineral/Brown Cadmium orange Cd, S, Se 19th century

Mars orange Fe2O3 Synthetic, 19th centuryOchre/Goethite Fe2O3.H2O, Clay Mineral


Table 1.5 Chemical compositions of some organic dyes and pigments

Colour Common name Chemicalcomposition

Origin

Yellow Berberine Berberinium hydroxide Mahonia stems,19th century

Gamboge Gambogic acid Gum resin, 17th centuryHansa yellow Azo class Synthetic, 20th centuryIndian yellow Mg salt of euxanthic acid Cow urine, 15th centuryQuercitin Quercitin Quercus oak barkSaffron Crocetin Crocus flower, AntiquityTurmeric CurcuminWeld Luteolin Plant foliage, Stone age

Red Carmine, Carminic acid Scale insect, Cochineal,Cochineal AztecKermes Kermesic acid Scale insect, Kermes,

AntiquityMadder Anthraquinones including

Alizarin and purpurinPlant root 3000 BC,

Synthetic alizarin19th century

Permanent red Azo class Synthetic, 19th centuryToluidine red Azo class SyntheticBasic red Azo class SyntheticAcid red Azo class Synthetic

Blue Indigo/Woad Indigotin Plant leaf, AntiquityCopper

phthalocyaninePhthalocyanine class Synthetic, 20th century

Direct blue Azo class Synthetic

Purple Tyrian purple 6,6’-dibromoindigotin Mollusc 1400 BC,Synthetic, 20th century

Brown Sepia Melanin Cuttlefish ink, 19th centuryVan Dyck brown Humic acids, Allomelanins Lignite, 16th century

Various pigments and dyes have been introduced at different times inhistory, often enabling the age of an object to be estimated [Doerner,1984; Feller, 1986; Roy, 1993; West Fitzhugh, 1997]. In the ancientworld, pigments derived from clay or burnt stick, such as red earth,yellow earth, chalk and carbon black, were used. The technology ofcolour developed in ancient Egypt and Egyptian blue was the first syn-thetic colour. Other pigments used in ancient Egypt included malachite,azurite, cinnabar and orpiment. The Greeks and Romans developednew colours such as white lead and vermillion. Dyes such as indigo,madder and Tyrian purple were also used. Through the medieval andrenaissance periods new pigments were introduced as painting tech-niques developed. The number of pigments available to artists had


MeOOMe OH

N

O

O

OH O

O

O OH

OCOOH

Euxanthic acid

OH

OH

MeOOMe

Berberine

Gambogic acidCHO

HN

O

O

OO

HOOC

O

OH O

O

HO2CMe

O OH

OHHO

OHO

Kermesic acid

O OH

OHOHO

Purpurin

O

OROHO

R = H R = CH3 Alizarin 2-methyl ether

Alizarin

HO

OOH

OH

OHO

LuteolinCrocetin

HO

OOH

OH

OOH

OH

OCH3

OH

OCH3

CCH

CH2

O

CH

CCH

O

CH OH

Quercitin Curcumin

HO2C

OH

O OH

OHHO

HO

HO

CH2OH

OH

OO

Me

Carminic acid

Cl Acid Red 138 (18073)

Tyrian purple

Direct blue 2B

Indigotin

Cl Basic Red 18 (11085)

COOHHOOC

O

NaO3S SO3Na

N NNHCOCH3

H25C12

N N N X−+

O2NC2H5

C2H4N(CH3)3

Br

Br

HN

NH

C

M = Fe2+, Mn2+, Cu2+,etc

Skeleton of phthalocyanine dyes

O

O

C

OH OH

−O3S −O3SSO3− SO3

−

NH2 NH2

N N N N

N

N NN

NM

N NN

HN

NH

C

O

O

C

Figure 1.10 Structures of some organic dyes and pigments

considerably expanded by the time of the industrial revolution. Mostof the metal-based colours were developed by the 19th century. The20th century saw the development of pigments such as titanium white,Hansa yellows and cadmium red. Hundreds of synthetic colours arenow available.


1.9 TEXTILES

Fibres are used in many artefacts, especially textiles, and are made ofboth natural and synthetic materials [Ingamells, 1993; Joseph, 1986;Needles, 1986; Trotman, 1984; Timar-Balazsy and Eastop, 1998]. Cel-lulosic fibres such as cotton, flax (used to produce linen), hemp, jute andramie are derived from natural sources. There are also synthetic cellulose-based fibres such as viscose rayon and cellulose acetate. Protein-basedfibres from animal sources include wool and silk. There is a range of man-made polymer fibres: polyamides (e.g. nylon 6, nylon 6,6), polyaramids(e.g. Kevlar, Nomex), polyester (e.g. Dacron, Terylene), acrylics, poly-olefins (e.g. PE, PP), vinyl and urethanes (e.g. Lycra, Spandex). Metalthreads have been applied to textiles for many years [Timar-Balazsyand Eastop, 1998]. The most common metals have been gold, silver,copper and zinc, while today aluminium is the main metal used forthreads.

Dyes have been widely used to colour textile fibres for thousands ofyears [Ingamells, 1993; Mills and White, 1994; Needles, 1986; Timar-Balazsy and Eastop, 1998]. The early dyes were natural compounds,but today synthetic dyes are used. There are various methods usedto combine a dye with a fibre. Vat dyes involve the application of acolourless reduced solution of the dye (‘leuco’ form), which is thenoxidised by oxygen or an added oxidising agent. Mordant dyes areused in conjunction with a mordant, usually a metal salt, to form aninsoluble complex (or ‘lake’) with the dye. The dye is then applied tothe fibre which has been pre-treated with a metal salt. It is proposedthat a chelated complex is formed and the colour depends on the metalion used. Direct dyes can be applied directly to a fibre from an aqueoussolution and these dyes are useful for wool and silk. Disperse dyes areaqueous solutions of finely divided dyes or colloidal suspensions thatform solid solutions of the dye within the fibre. Disperse dyes are usefulfor synthetic polyester fibres.

Dyes may also be classified based on their chemical structure. Azo dyesare a major class and consist of a diazotised amine coupled to an amineor a phenol and have one or more azo linkages. An example of a diazodye, with two azo groups, is direct blue 2B, the structure of which isillustrated in Figure 1.10. Anthraquinone dyes are generally vat dyes andan example is alizarin. Many red dyes are quinones and are derivativesof naphthaquinone and anthraquinone (Figure 1.11). Indigoid dyes arealso vat dyes and indigo is an example. Triphenylmethane dyes arederivatives of a triphenylmethyl cation and are basic dyes for wool orsilk. An example is malachite green.

PAINTINGS 29

Naphthaquinone Anthraquinone

O

O

O

O

Figure 1.11 Structures of naphthaquinone and anthraquinone

1.10 PAINTINGS

Much of the world’s cultural heritage is contained in paintings. An arrayof materials have always been employed in paintings [Doerner, 1984;Gettens and Stout, 1966; Mayer, 1991; Wehlte, 1975]. The support, themain structural layer, of a painting has been principally canvas, woodor wall. Canvas is usually linen, although cotton, hemp and silk haveall been used. A typical cross section of the paint layers on the surfaceof a painting is illustrated in Figure 1.12. Generally a painted surfaceconsists of two paint layers on top of a ground layer on the support.The ground layer, also known as the primer or the preparation layer,is the layer on which a drawing is made before the paint is applied andacts as a barrier between the paint and the support. The primer is oftengypsum, chalk or a white paint in animal glue and is about 0.5–2 mm inthickness. A transparent varnish layer is used to protect the paint layers,and also provides gloss and colour improvement. Varnishes are naturalor synthetic resins and are applied from a solution using an organicsolvent. The solvent is volatile so after evaporation a glassy and hardfilm is formed on the surface.

Varnish

Paint layer 2

Paint layer 1

Ground layer

Support

Figure 1.12 Cross-section of the surface of a painting


Paint consists of pigment and a binding medium. Most paint pigmentsare inorganic compounds, as they tend to be stable and light-fast whencompared to pigments derived from organic compounds. Commonlyused pigments found in paints are listed in Tables 1.4 and 1.5. A rangeof materials has been used as paint binders over the years: egg white, eggyolk, casein, animal glue, gum arabic and oils. Oils were used in binders(in oil paints) from the 15th century until the 20th century and linseedoil has been the most widely employed. Tempera paints use an oil-wateremulsion as a binder. The emulsifying agent is usually gum arabic, glueor egg yolk. In watercolour paint, the pigment is finely ground andsuspended in water with a binder such as gum arabic. In fresco painting,the binder is applied first as damp plaster made from lime (CaO). Thepigments are mixed with lime and water and then applied to the dampsurface. The lime reacts with air to form calcium carbonate (CaCO3)and the pigments become encased.

The main components of 20th century paints are binders, pigmentsand extenders [Learner, 2004]. The main types of binders used are oils,alkyds, acrylics and PVA. Oils and acrylics are found in artists’ paints,while alkyds and PVA are used in commercial paints. Linseed oil isstill the most commonly used oil, but soya and safflower oils are morecommonly used in light colours to avoid the yellowing effect of linseedoil. Alkyds are often described as oil-modified polyester paints and havebeen used as the binding media in the majority of commercial paintssince the 1930s. Acrylics, in the form of an emulsion, were introducedin the 1950s. Early acrylic emulsions were based on copolymers of ethylacrylate and methyl methacrylate, but in recent times copolymers of butylacrylate and methyl methacrylate have been used. PVA-based paints werefirst used in the 1940s. Although PVA is now not used for artists’ paint,it is used in commercial household paints. The important pigments usedin 20th century paint are cadmium reds, oranges and yellows, ironoxide reds and ochres, naphthol reds and yellows, quinacridone redsand violets, phthalocyanine greens and blues, Prussian blue, ultramarineblue, cobalt blue, natural and synthetic iron oxides, titanium white,carbon black and iron oxide blacks. The extenders that are commonlyused in modern paints are calcium carbonate (CaCO3), barium sulfate(BaSO4), kaolinite, magnesium carbonate (MgCO3), calcium sulfate(CaSO4) and hydrated magnesium silicate.

1.11 WRITTEN MATERIAL

Manuscripts, books and other historical documents provide an importantrecord of cultural heritage. The types of materials used have evolved

WRITTEN MATERIAL 31

with time. Papyrus plants were one of the most important sources ofwriting material in ancient cultures and are first used over 5000 yearsago in the Nile Valley Kingdoms. The plants are swamp reeds supportedby a hollow fibrous stem. To create writing material two layers of stemare laid over each other at right angles, then pounded and smoothedinto a single sheet.

Parchment and vellum, produced from the dermis of animal skin, werewidely used after the 2nd century AD to produce ancient manuscriptsor book-bindings. Vellum is used to describe fine parchment made fromcalf skin. Parchment is prepared by cleaning the skin and removing thehair by a mechanical treatment that involves the scraping of fat from theskin [Covington et al., 1998; Vandenabeele and Moens, 2004]. The mainstructural component of parchment is collagen and an understandingof the state of this protein is required when developing a conservationtreatment for parchment. A choice of the optimum storage conditions forthe long term storage of documents and manuscripts made of parchmentis crucial. If parchment is stored in a high humidity environment, thecollagen component can denature to form gelatin and the parchment willbecome soft and sticky. There is also the possibility of microorganismsgrowing in a high humidity environment and attacking the material.On the other hand, if the humidity is too low, parchment can shrinkand deform and will become brittle. The preservation of animal skinsfollowing the removal of fatty material involved drying by salting withsodium chloride (NaCl) or potassium chloride (KCl) and ammoniumchloride (NH4Cl) or sulfate addition with lime to adjust the pH. Thiswas followed by treatment with potash alum mixed with flour and eggyolk to produce a supple substrate. Poorer parchments were treated withpreparations such as sodium sulfite suspended in natural oils in order tomimic the more translucent vellums.

Paper is believed to date back to China in 200 AD [Hon, 1989]. Sheetsof paper were first made by drying fibres, such as flax and rice stalks, mac-erating in water and draining on moulds. Wood became the main sourceof paper fibres during the 19th century after the development of a grinderfor the mechanical pulping of wood. During the manufacture of paper,sizing agents are usually added to the pulp or the paper surface afterformation to make the cellulose component more hydrophobic [Carter,1997]. This is required to prevent printing inks from running. Cellulosecontains hydroxyl groups which make it a naturally hydrophilic mate-rial. Until the 1950s, a rosin precipitated by alum (Al2(SO4)3.18H2O)was used as a sizing agent. However, the acid hydrolysis of the com-plex hydrated aluminium ions produced by this salt of cellulose results


in the depolymerisation of the cellulose chain and the paper becomesbrittle as a consequence. Deacidification is used in an attempt to protectpaper from degradation. The goal is to establish an alkaline buffer thatcompensates for the acidic species responsible for degradation. Organicbasic solutions are usually used to minimise reactions which can affectthe appearance of paper and those that may involve inks.

Writing inks of various compositions have been used for millen-nia [Brunelle and Reed, 1984]. Carbon inks consisting of fine grains ofcarbon black suspended in liquid were used as early as 2500 BC. Suchinks are known as India ink or Chinese ink. A red version used cinnabarinstead of carbon and was used on the Dead Sea Scrolls. Carbon inksare still used and the carbon is usually suspended in a gum or gluesolution which also acts as a binder. Another ink that has been usedsince the middle ages is iron gall ink. The main ingredients of this inkare gallic acid, tannic acid (extracted from gall nuts), iron[II] sulfate(FeSO4.7H2O) and gum arabic. When iron gall ink is applied to paperor parchment, the ink darkens due to the oxidation of Fe2+ to Fe3+.Iron gall ink was regarded as the most important ink up until the 20thcentury. However, the corrosive nature of some compositions led to irongall inks falling out of favour. Modern inks contain an array of syntheticresins and pigments.

Medieval manuscripts have survived well with time, largely becausesuch materials have always been regarded as valuable and so werehandled with care. The materials used for the production of thesemanuscripts were parchment, pigment, binder and ink [Vandenbeele andMoens, 2004]. Parchment was the important writing material duringmedieval times, although during the early middle ages, papyrus wasused as well. Some pigments used during this period were vermillion,red lead, haematite, azurite, lapis lazuli, smalt, lead–tin yellow (type I),massicot, limonite, verdigris, malachite, copper phosphate, basic coppersulfate, copper resinate, Veronese green earth, chrysocolla, carbon blackand iron black [Vandenbeele and Moens, 2004]. Protein-based materialssuch as casein, egg white and egg yolk were used as binding media. Starchand gum arabic were also employed as binders. Animal glues were notused at the time for binders, but were used for applying gold leaf. Irongall ink and charcoal were the most commonly used medieval inks.

1.12 GLASS

Glass is an amorphous solid that has been used widely in the productionof heritage objects [Henderson, 2000; Lambert, 1997]. Man-made glass

GLASS 33

is believed to have originated in northern Mesopotamia (Iraq) prior to2500 BC. The mass production of glass objects began in the secondmillennium BC in Mesopotamia and in the Mediterranean region. Glasstechnology was further developed during the Roman period and greenglass was mass produced during this era. Glass was also produced inAsia from early times. Glass production spread to centres throughoutEurope during medieval times.

Glass is made by melting a mixture of three main components [Cronyn,1990; Davison, 2003; Henderson, 2000; Lambert, 1997; Pollard andHeron, 1996]. The basic glass forming material is silica (SiO2), whichcomprises 60–70 % of the mixture. An alkaline flux, such as sodium orpotassium derived from minerals or plant ash (Na2O, K2O), is addedto lower the melting temperature of the batch. Fluxes interrupt some ofthe Si–O bonds and so disrupt a continuous network produced by silica.The unattached oxygen atoms become negatively charged and looselyhold the monovalent cations in the spaces of the network (Figure 1.13).As the bonding is weak, the cations can migrate out of the network inwater. A stabiliser, such as lime (CaO) or magnesia (MgO), is needed tomake the glass water resistant. The stabiliser is doubly charged so is heldmore tightly than the monovalent ions, thus the fluxes are held withinthe network. The most commonly produced glass is soda-lime, whichcontains silicon dioxide (SiO2), sodium oxide (Na2O) and calcium oxide(CaO). From about 1675, lead oxide was used in glass, acting both as aflux and a stabiliser. The batch is melted in a furnace at a temperature ofapproximately 1500 ◦C until it reaches a liquid state. The qualities andcharacteristics of glass may be changed by varying the proportions of themain components and the addition of other components. These varia-tions affect the manner in which the hot glass behaves when it is shaped.

SiO2 glass

Si4+

Na+O2−

Na2O modified glass

Figure 1.13 Unmodified and Na2O modified silica glass networks


The colour of glass is determined by the presence of metallic oxides.The silica used to produce glass generally contains natural impurities ofiron that result in a green colour in the glass. Pure silica must be used inorder to produce colourless glass. Alternatively, the natural green can beneutralised by the addition of a small quantity of manganese. The greentint of reduced iron is diminished by the pink manganese ions becausethe iron is oxidised to a yellow colour that then appears colourless inthe presence of the pink manganese. If a surplus of manganese is added,a pink coloured glass results. Other colourants include copper or cobaltoxides for blue, copper or iron oxides for green, gold oxide for rubyred and uranium oxide for a radiant green. Table 1.6 summarises someof the metal ions responsible for colouring glass [Pollard and Heron,1996]. The colour produced by an ion depends on its oxidation stateand on the position it occupies in the glass structure. For instance, it issignificant as to whether the metal forms a tetrahedral or an octahedralcoordination. Opacifiers, such as tin oxide and calcium antimonite, canbe added to create opal-like or opalescent glass. Glass objects can bedecorated by firing on gilding or unfired paint applied in a lacquer,varnish or oil. Window glass is painted by enamelling, firing on amixture of powdered glass and iron oxide, or stained yellow by firingwith silver sulfide.

Table 1.6 Metal ions in glass

Raw material Colouring ion Colour intetrahedral

coordination

Colour inoctahedral

coordination

Cr2O3 Cr3+ – GreenK2Cr2O7 Cr6+ Yellow –CuO Cu+ – Colourless to red,

Brown fluorescenceCuSO4.5H2O Cu2+ Yellow–Brown BlueCo2O3, CoCO3 Co2+ Blue PinkNiO, Ni2O3, NiCO3 Ni2+ Purple YellowMnO2 Mn2+ Colourless–Pale

yellow, Greenfluorescence

Pale orange, Redfluorescence

KMnO4 Mn3+ Purple –Fe2O3 Fe3+ Brown Pale yellow–PinkU3O3, U6+ Yellow–Orange Pale yellow,Na2U2O7.3H2O Green fluorescenceV2O5 V3+ – GreenV2O5 V4+ – BlueV2O5 V5+ Colourless–Yellow –

CERAMICS 35

The deterioration of glass is dependent upon its composition, age,firing history and storage environment [Davison, 2003]. When glass isin contact with water, the alkali metal ions are leached out and replacedby hydrogen ions. The surface of the glass will change its refractive indexand appear dull. Colouring metal ions may also be leached from glassby water and this leads to discolouration. Alternatively, the ions maychange colour by oxidation. For example, Mn2+ can be deposited byblack manganese dioxide (MnO2). In addition, colouring ions from theenvironment may be taken up by the glass and result in a colour change.Encrustation may also be observed due to the deposition of insolublesalts. Excess lime can leach out of the glass and deposit as a white cruston the surface.

1.13 CERAMICS

Ceramic objects have been produced for tens of thousands of years bymany cultures and so form an important aspect of cultural heritage.Ceramics are produced from fired clay [Cronyn, 1990; Henderson,2000; Lambert, 1997; Pollard and Heron, 1996]. Clay is a widelyavailable material with plastic properties that enable it to be shapedwhen wet, producing a hard object when heated. The main componentsof clays are minerals such as kaolinite, montmorillonite and illite.Different clay types contain layers of silica, alumina (Al2O3) and waterin different proportions. The structures of kaolinite and montmorilloniteare illustrated in Figure 1.14. Kaolinite is composed of a silicate sheetionically bonded to a sheet of AlO(OH)2 and is often represented by theformula Al2Si2O5(OH)4. Montmorillonite (Al2(SiO5)2(OH)2) containstwo silicate sheets around an AlO(OH)2 layer. Clays also contain fillers(or temper) that can aid in the physical properties of the ceramicproduced. A variety of materials including sand, limestone, mica, ashand organic matter have been used.

(OH)−

O2−, (OH)−

O2−

Kaolinite Montmorillonite

Al3+

Si4+

O2−, (OH)−

O2−, (OH)−

O2−

O2−

Al3+

Si4+

Si4+

Figure 1.14 Kaolinite and montmorillonite clay structure


The formation of ceramics when clay is heated is a complex processdue to the mixture of components present. When wet clay is heated,water molecules that are not bound are first lost by evaporation at100 ◦C. At temperatures near 600 ◦C, the clay loses water bound to theclay structures. The aluminosilicate component rearranges its structureto accommodate the loss of the water molecules and the material losesits plasticity. Above about 850 ◦C, vitrification occurs where glass-likeregions are formed in the aluminosilicate structure.

The porosity of a fired ceramic is dependent on how the clay mineralsfuse together, on whether the pores have been filled with glass, and onthe original size of the components. Terracotta describes the oldest formof man-made ceramic and is fired at about 900 ◦C. The addition of ironand heating in the range 900–1100 ◦C produces red earthenware, whichis stronger and less porous. The addition of lime, containing calcium,and heating at 1100–1200 ◦C produces a cream colour in earthenware.Faience or majolica is earthenware covered with a glaze. Stoneware isproduced by heating at 1200–1300 ◦C, which intensifies vitrificationand produces a strong material which is non-porous. Heating above1300 ◦C produces porcelain, which is highly vitrified and translucent.

A number of surface finishes can be applied to ceramics. Colour maybe added to the surface using graphite (black), haematite (black–red)or mica powder (golden). A gloss may be added by the application of athin coat of diluted fluid clay, known as a slip. Glazes, which are glasses,are frequently added for decoration or to improve the impermeabilityof the ceramic. Glaze is made of quartz and is formed by heating, oftenapplied to an object and created in a second firing. Paint can be fixed tounglazed ceramics by applying pigments such as iron ores that containsilica or clay minerals that bind the colour to the ceramic during firing.

1.14 STONE

A number of different types of stone have found application in manyhistoric objects including statues, building stone, jewellery and manyarchaeological artefacts [Henderson, 2000; Lambert, 1997]. Stones canbe made up of a broad range of minerals. Marble has been used formany thousands of years as a building stone and for carved statues.Marble is largely made up of calcium carbonate. Quartzite, mainlysilicon dioxide, has also been used for building stone since antiquity.Some rocks, sedimentary rocks, are formed from marine sedimentsover time. Sandstone is formed from quartz sand and also containsclays such as kaolinite. Over time and exposure to pressure and heat,

METALS 37

sandstone is transformed into quartzite. Like quartzite, flint is a stonemainly composed of silica. As flint was widely used for tool making andonly mined in particular locations, this material is of great interest toarchaeologists. Limestone is formed from shells and coral and can betransformed into marble over time. Granite is highly heterogenous andmore difficult to characterise. The presence of trace elements in stonescan be used to identify the source of many stones.

These types of stone can be susceptible to deterioration due to envi-ronmental factors. Compounds in air pollution are responsible fordeleterious reactions on the surface of certain stones. For instance, thesulfur from burnt coal forms sulfur oxides which deposit on marble andtransform into calcium sulfate (CaSO4). The calcium sulfate is morewater-soluble than calcium carbonate and so detail on carved objectsis lost. Attempts to counteract such degradation are made by apply-ing materials known as consolidants. Consolidants are often made ofpolymers such as silicones.

A stone that has been widely studied is obsidian because of its wide usefor the manufacture of tools and weapons [Henderson, 2000; Lambert,1997]. Obsidian is a naturally occurring glassy material that is found involcanic regions of the world. Obsidian is predominantly made of silicondioxide, but also contains aluminium oxide (Al2O3), sodium oxide(Na2O), potassium oxide (K2O) and calcium oxide (CaO). However,the composition of obsidian will vary depending on the source and thepresence of trace elements can also be used to determine the origin.

There are many gemstones used in the creation of precious objects,such as jewellery [O’Donoghue and Joyner, 2003]. A gemstone is amineral or petrified material that is cut or faceted. An array of mineralsproduce such precious stones and Table 1.7 lists the structures andproperties of some common gemstones. Most gemstones are mined, butthere are some stones derived from other sources. Pearl is an iridescentgem produced by molluscs including oysters. Pearls are produced oflayers of calcium carbonate in the form of the minerals aragonite and/orcalcite. Nacre, also known as mother-of-pearl is a naturally occurringorganic-inorganic composite. Mother-of-pearl contains a combinationof calcium carbonate and the protein conchiolin and is found in theinner surface of mollusc shells.

1.15 METALS

Metals have been used to produce many historic artefacts such as coinsand tools over many thousands of years [Cronyn, 1990; Henderson,


Table 1.7 Properties of common gemstones

Commonname

Chemical structure Properties

Agate SiO2 White–Grey, Blue, Orange–Red,Black

Emerald Be3Al2(SiO3)6 with Cr Green–Blue colourGarnet A3B2(SiO4)3 Red and other colours

A = Ca2+, Mg2+ or Fe2+B = Al3+, Fe3+ or Cr3+

Jade Nephrite Green and other coloursCa2(Mg,Fe)5Si8O22(OH)22Jadeite NaAlSi2O6

Lapis lazuli (Na,Ca)8(AlSiO4)6(S,SO4,Cl)1–2 Mineral lazurite with calcite,Sodalite and pyrite, Blue

Opal SiO2.nH2O Iridescent, Range of coloursRuby Al2O3 with Cr Red colourSapphire Al2O3 Blue, Pink, Yellow, Green or WhiteTopaz Al2SiO4(F,OH)2 Translucent, Yellow with other

colours due to impuritiesTurquoise CuAl6(PO4)4(OH)8.5H2O Blue–green colour, Opaque

2000; Lambert, 1997]. Originally metal was used as found, but later itwas recognised that metal containing ores could be mined and the metalisolated. It was also recognised that with heating, the physical propertiesof metals could be controlled and the metal worked into various shapes.

Metals form crystalline solids due to the regular packing of the atoms.When describing metallic structures, the repeating structures are definedusing unit cells. Each unit contains a specific number of atoms in aparticular pattern depending upon the element. Metals are made ofcrystals known as grains and the physical properties are affected bythe size and shape of such grains. While pure metals are used, it is alsocommon to find alloys, which are mixtures of metals and other elements.

The ability to deform metals has enabled them to be used in a widerange of applications. Metals can also be joined mechanically as wellas by soldering, which involves the formation of an alloy between thesurface of the metal and another metal known as the solder. Metalsurfaces are sometimes coated with a thin layer of a different metal inorder to change the appearance or to protect against corrosion.

Metals are susceptible to deterioration. The good electrical conduc-tivity of metals also means that they can undergo oxidation-reductionreactions. Corrosion can occur resulting in tarnishing, the formationof patinas or the significant disintegration of the surface. A layer ofoxide or sulfide can form on the surface and may form a protective

METALS 39

layer. More damaging is corrosion in a wet environment. A combinationof water and oxygen cause severe deterioration of particular metals.The nature of corrosion products depends on the potential reactants inthe metal and the environment and crystals are usually formed on thesurface.

Gold is an attractive malleable metal that is highly prized due to itscomparatively low abundance. The yellow lustrous qualities make itpopular for use in jewellery and coins. Gold is also used in the form ofgold leaf in picture frames and in illuminated manuscripts. The metal isalso found in alloys with silver and copper to improve the mechanicalproperties. Gold is very resistant to corrosion.

The appearance of silver and its relatively low abundance make thisa valuable metal used for jewellery, coins and cutlery. Sterling silver(92.5 % silver/7.5 % copper) is commonly used to produce jewelleryand cutlery. Silver plate involves the deposition of a thin layer of silveron a base metal (e.g. copper, brass). Silver objects tarnish due to theformation of a black silver sulfide (Ag2S) layer. Buried objects or itemsremoved from the sea may be coated with grey silver chloride (AgCl)and copper corrosion products.

Copper is a malleable and lustrous red metal that is commonlyused in alloys. Copper with zinc is brass and bronze is copper withtin. Some common corrosion products are copper oxides, carbonatesand sulfates, but they can form a protective layer. However, coppercorrosion products can breakdown in the presence of chlorides fromsea water or ground water. The formation of light blue–green growthsdue to the reaction with chlorides in a humid environment is referredto as bronze disease: the surface deposits crumble and a pitted surfaceremains.

Iron is an abundant metal that is found in many museum collections.This metal is used in a variety of forms: cast iron (2.5–5 % carbon),wrought iron (0–0.07 % carbon) and steel (0.07–0.9 % carbon). Ironmay also be plated with zinc. Iron and most of its alloys are susceptibleto rust formation by oxygen in the presence of moisture. Red–browniron oxides form that do not provide a protective layer and, in fact,accelerate corrosion.

Lead is a very malleable metal and was used to produce pewter bycombining with tin prior to the 19th century. The major corrosionproducts of lead are white–grey lead carbonates which provide a pro-tective layer. A grey–black patina of lead sulfide (PbS) may also beobserved. Acetic acid produced by wood can react with lead to formlead acetates.


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Conservation Materials COPYRIGHTED MATERIAL€¦ · Analytical techniques for studying culturally important objects have been developed using a wide range of established experimental