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Journal of Cultural Heritage 13S (2012) S196–S202
Available online at
www.sciencedirect.com
econtamination and “deconsolidation” of historical wood preservativesnd wood consolidants in cultural heritage
chim Unger ∗
niversity of Applied Sciences Potsdam, Department of Architecture & Urban Design, DE-14469 Potsdam, Pappelallee 8-9, Germany
r t i c l e i n f o
rticle history:eceived 13 January 2012ccepted 26 January 2012vailable online 10 March 2012
eywords:ood artifact
reservativeonsolidantiocide
a b s t r a c t
In the past, wood artifacts were treated with a variety of wood preservatives formulated on the basisof inorganic and organic biocides. Most of these biocides have a high human toxic potential and pol-lute the environment. Some of them even cause damage to the objects they were meant to preserve.This poses a considerable challenge to the handling, exhibition, storage and restoration of such woodenworks of art. In addition, biocide-containing structural wood members in historic buildings pollute theindoor-air, and represent a permanent health risk. Wood artifacts previously damaged by organisms andsubsequently preserved and consolidated with mixtures of vegetable oils and natural resins now showcharacteristics of renewed deterioration. An important condition for the re-treatment of such objectsis the exact detection of the substances originally utilized for their conservation. Non-destructive and
etectionecontamination
in situ-measurements have priority among the listed analytical methods. The various decontaminationprocedures currently used are classified in regard to their mode of operation. Preferred methods includemechanical cleaning, thermo desorption, washing with water and detoxicants, and leaching as well asextraction with liquid or supercritical carbon dioxide. The masking with various sealers to prevent bio-cide evaporation into the indoor-air is limited to application to structural wood members. Leaching ofdegraded natural consolidants in wood artifacts is currently undertaken in a testing plant.
© 2012 Elsevier Masson SAS. All rights reserved.
. Research aim
The aim of this paper is to outline and discuss the research-basedurrent knowledge and know-how in the field of decontaminationnd “deconsolidation” of “dry” wooden artifacts. Finding reliableethods, for removal of agents used to preserve wooden artifacts
amaged by wood-destroying organisms, is urgently required toalt the damage directly inflicted on the wood artifact by the agents,nd to eliminate the danger they pose to human health and to thenvironment.
. Introduction
In the past, wood artifacts infested and damaged by wood-estroying organisms were commonly preserved with liquid andaseous agents. A chronological survey of these agents was pub-ished in 2001 [1].
Unfortunately, numerous wooden objects now turn out to haveuffered serious damage, caused by the past use of preservativesnd consolidants. Removal of these substances is therefore urgently
∗ Tel.: +49 33 34 22 32 5.E-mail address: [email protected]
296-2074/$ – see front matter © 2012 Elsevier Masson SAS. All rights reserved.oi:10.1016/j.culher.2012.01.015
required, not only to halt the damage directly inflicted on the woodartifact, but also because of the danger they pose as biocides tohuman health and the environment. Individual objects and struc-tural wood members with such biocides pollute the indoor-air ofdepots and exhibition rooms in museums and in historic buildings.In particular, conservators and other members of the museum staffare at risk [2]. Furthermore, there is a risk of contamination ofobjects stored near previously treated wood artifacts. The resultis that contaminated museum objects cannot be presented to thegeneral public or lent to other museums without special safetymeasures.
New restoration procedures to maintain the damaged woodenobjects are only effective and sustainable if hazardous wood preser-vatives and consolidants are removed.
Therefore, methods and technologies able to decontaminateobjects, especially such without decorative layers, without causingchanges to their shape and properties need to be developed.
The aim of any decontamination procedure is to achieve amaximum biocide reduction with an otherwise minimal impact onthe state of the object. Detailed analytical investigations are neces-
sary to prove the efficiency of the measures. Non-invasive methodsshould be preferred.The following text outlines the current knowledge and know-how in the field of decontamination and “deconsolidation” of wood
l Heritage 13S (2012) S196–S202 S197
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rtifacts with the exception of those methods used for re-treatmentf stabilized wet or waterlogged wood from archaeological exca-ations.
. Decontamination of historical wood preservatives
.1. Nature of the problem
.1.1. Inorganic biocidesFrom the beginning of industrial wood preservation in Europe
n the year 1832 up until the substitution of organochlorine bio-ides by synthetic pyrethroids between 1980/85, numerous woodreservatives with high toxic biocides were applied to wood arti-acts. These biocides can be classified into inorganic and organicompounds.
Inorganic compounds are mostly water-soluble, and are com-only of saline nature. Examples are fluorides, hydrogen fluorides,
uorosilicates of alkali metals and alkaline-earth metals, Mg-, Zn-nd Cu-salts (especially sulfates and chlorides), alkali arsenates andoron compounds (boric acid, borax). Mercury(II) chloride (subli-ate), mostly used when industrial wood preservation started, is
nother important inorganic biocide. All of them are prone to leach-ng, which resulted in chromium compounds being introduced asxatives in preservatives for wood impregnation at the beginningf the 20th century.
Most inorganic biocides have a negative impact on the humanastrointestinal tract and skin. Absorption occurs orally, topicallynd through inhalation of contaminated dust particles.
Fluorides and fluorosilicates can cause skin irritation, nausea,bdominal-pain and conjunctivitis. Arsenic compounds can resultn headaches, nausea, and vomiting, and they are carcinogenic.hromium compounds are known to trigger chronic skin irrita-ion and inflammation (“chromium eczema”). Mercury(II) chlorideauses visual and motor impairment as well as nerve inflammationneuritis).
Despite these health hazards, the toxic potential of inorganiciocides is smaller than that of organic biocides. This is because ofhe low vapor pressure of most of these compounds, as well as theirartial chromium fixation, which result in a reduced leaching. Some
norganic biocides such as fluorides, fluorosilicates, and mercury(II)hloride directly attack wood and other materials. For example,ome wood preservatives (e.g., fluorides or hydrogen fluorides) canestroy the cell structure of wood, usually starting on the surfacef the object. This is also known as “maceration” and occurs underpecific environmental conditions and in combination with fireetardant salts (such as ammonium hydrogen phosphates) appliedo roof trusses.
.1.2. Organic biocidesOrganic biocides are soluble in several organic solvents. How-
ver, their solubility in water is minimal. Some organic biocidesan be converted to salts, which are soluble in water (e.g., sodiumentachlorophenolate, Na-PCP).
The following chemical substance classes include organic bio-ides with a high toxic potential:
chlorinated hydrocarbons:◦ mono- and dichloronaphthalenes, 1,2- and 1,4-Dichloroben-
zene, Dichlorodiphenyltrichloroethane (DDT), �-Hexachloroc-yclohexane (�-BHC), Chlorothalonil;
cyclodiene compounds:◦ Aldrin (HHDN), Dieldrin (HEOD), Chlordane, Heptachlor, Endo-
sulfan;phenol derivatives:
Fig. 1. DDT crystals on a wood surface.
◦ dinitrophenols, Tetrachlorophenol, Pentachlorophenol (PCP)and Na-PCP;
• organometallic compounds:◦ organotin compounds: Tributyltinoxide (TBTO), Tributyltin-
naphthenate (TBTN), Tributyltinbenzoate (TBTB),◦ organoaluminum compounds: Xyligen-Al (Xylasan-Al),◦ organomercury compounds: Phenylmercuryoleate;
• organophosphates:◦ Parathion, Diazinon.
Many of these biocides affect the nervous system, liver, kidney,blood and skin. Some compounds such as PCP have carcinogenic,mutagenic and teratogenic effects. The utilization of technical PCPcontaminated with highly toxic polychlorinated dibenzodioxines(PCDD) (one of the compounds is also known as the “Seveso” poi-son) and polychlorinated dibenzofuranes (PCDF) often increasesthe toxicity of old wood preservatives.
DDT is another important biocide. It was used in wood preser-vation for protecting wood against wood-destroying insects insome European countries from about 1955 to 1990. This insecti-cide is also suspected to cause nerve damage and cancer, and itirritates the eyes, skin and mucous membranes. Furthermore, DDTaffects the human immune and endocrine systems. Wood artifactsimpregnated with DDT-containing oil-borne wood preservatives(e.g., Hylotox 59 produced in former East Germany) commonlyshow exudates of DDT in form of crystals on their surfaces (Fig. 1).
As a result of both the frequently poor fixation of organicbiocides in wood and their considerable vapor pressures, smallamounts of biocide continuously diffuse from wooden surfaces intodust and into the indoor-air. Spaces, which contain impregnatedwooden objects, pose a health hazard to individuals. Increasingtemperature and humidity as well as low air exchange rates inten-sify the compounds’ harmful effects.
3.2. Chemical analysis
3.2.1. Sampling and simple testsAny restoration effort should begin with a thorough and careful
inspection of wood artifacts with regard to the presence of woodpreservatives. Objects often show discoloration of their surfaceswhen impregnated previously with oily wood preservatives. Fur-thermore, they may have surface layers consisting of exudates,
show maceration, and may be malodorous. Historic restorationdocuments containing information on wood preservatives previ-ously applied can help to identify the component biocides withoutlarge-scale analytical investigations.
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The nature of any substance identified should be characterizedy simple tests such as the determination of the solubility and com-ustibility. If the substance is insoluble in water and combustible,he Beilstein test as a first step test for the detection of organohalo-en compounds is carried out. Organohalogen compounds on theurface of objects yield a green to blue-green flame when com-usted using a copper wire or copper tube. However, this test isot very specific. The melting point should be determined for thoserystalline exudates, which show a positive Beilstein test. Suchrystallized exudates could represent DDT, lindane (�-1,2,3,4,5,6-exachlorocyclohexane), and PCP, and they have a melting pointf 109 ◦C, 113 ◦C, and 190–191 ◦C, respectively. All these investiga-ions should take place in a cupboard with an extraction fan.
Biocides of wood preservatives can be detected in wood, in dustr in the indoor-air.
Dust particles from the wood artifacts should be analyzed at firsts they may contain both inorganic and organic biocides.
Obtaining and preparing appropriate samples is a key stepowards successful analysis. It is of the utmost importance thatamples obtained represent a realistic picture of the object to bessessed. This can be achieved by obtaining samples from variousites of the object, in sufficient numbers, and of adequate sam-le amount. However, in the case of valuable wood artifacts, theemoval of material for the purpose of analysis needs to be mini-ized and non-invasive methods may be preferable.
.2.2. Qualitative analysis of biocidesThe characteristic ions of inorganic biocides contained in wood
an be detected by means of specific color reactions. The analysis isarried out by spraying a reagent solution over an increment borerore or over the back of a small section taken from the wood surface.owever, this method is invasive and observing a color change after
reatment is difficult when dealing with wood surfaces that are oldr have an intense color themselves. This approach can also be usedo identify several organic biocides such as PCP, Na-PCP and TBTO.
The determination of heavy metals of inorganic biocides onood surfaces by x-ray fluorescence spectroscopy (XRF) usingortable devices is non-destructive and may be performed in situ.
high chlorine signal indicates chlorides (e.g. NaCl, KCl or MgCl2)nd/or organochlorine compounds such as PCP, lindane, DDT, andthers [3]. Organochlorine biocides can then be distinguishedhrough the Beilstein test. The composition of organic biocides inood or biocide exudates on wood surfaces can be detected by
pplication of thin layer chromatography (TLC, HPTLC), high perfor-ance liquid chromatography (HPLC), gas chromatography (GC),
nd Fourier transform infrared (FTIR) spectroscopy.
.2.3. Quantitative analysis of biocidesAtomic absorption and atomic emission spectroscopy (AAS,
ES), both commonly used in conjunction with the induced cou-led plasma (ICP) stimulation, are suitable for the determination ofhe amount of heavy metals (As, Cr, Cu, Hg, Pb, Sn) originating fromnorganic biocides in wood. Anions of saline wood preservativesan be identified and quantified by ion chromatography (IC).
Capillary GC, combined with special detectors such as the flameonization detector (FID), the electron capture detector (ECD) andhe mass-selective detector (MSD), is the preferred method for theetermination of the amount of organic biocides in treated wood,eposited dust or indoor-air. For the quantification of biocides,btaining small samples of treated wood is imperative. The protonransfer reaction (PTR)-mass spectroscopy (MS) offers a new possi-ility for in situ quantification of volatile organic compounds (VOC)
inclusive biocides) in indoor-air of treated roof trusses and roomsontaining contaminated objects. Quantifying the exact amount ofiocides is a prerequisite for the assessment of the overall healthisk.tage 13S (2012) S196–S202
3.3. Current procedures
3.3.1. IntroductionPrior to treatment of contaminated objects, it is necessary to
distinguish between movable artifacts from museums, and fixedstructures such as wooden elements of roof trusses, altars orwooden floors and paneling. The number of possible preventionand decontamination measures is smaller for fixed structures com-pared to movable objects.
There are three principal approaches to handling woodenobjects containing biocides:
• removal and disposal of contaminated wood;• containing biocides in wood and prevention of release into the
environment;• removal of wood preservatives (biocides and solvent residues)
from wood.
Removal and disposal of contaminated wood are not an optionfor unique museum objects and well-maintained timber structuresof historic buildings.
3.3.2. Prevention of emissionSeveral methods are available to reduce the emission of bio-
cides from treated wood into indoor-air. Preventive measures forcontaminated museum objects include:
• packing and sealing in plastic or aluminum foil, impermeable tobiocide vapors;
• sealing with activated carbon fabric;• storage in protected cabinets or special storage areas with an air
circulation system.
Contaminated wood surfaces of structural wood members inbuildings are brushed or coated with special liquid sealers or var-nishes (masking). However, the efficiency of most of these productsis limited with regard to the degree of biocide reduction over longerperiods of time, and currently not enough is known about the effectof ageing of their ingredients. Valuable museum objects shouldnot be treated with sealers or varnishes aiming to reduce biocideemission.
Covering timber constructions with carbon tissue or boards aswell as aluminium foil offers protection from biocide emission.However, the measures are of temporary nature only.
A particular challenge represents the conversion of contami-nated attics into rooms for use as living spaces. In order to excludeany health risks, newly created rooms must be completely sealedoff from contaminated areas by use of biocide-inhibiting construc-tion materials. In addition, they need to be fitted with a separateventilation system, increasing the cost and scale of such a project.
3.3.3. Decontamination3.3.3.1. Mechanical procedures. These procedures can be dividedinto methods aiming to remove contaminated dust, and abrasiveprocedures for treated wood.
During the former, the wood surface is dry, liberated from dustwith special vacuum cleaners fitted with high-efficiency particulateair (HEPA) filters.
Museum objects that are contaminated with biocides attract-ing dust and dirt particles to their surface can also be cleaned withthe aid of laser beams [4]. Laser ablation is based on the applica-tion of short pulses (about 10 nanoseconds) of high energy, which
is preferentially absorbed by dark-colored dirt and dust particles.When using a wavelength of 1064 nm, the device removes the con-taminated layer from the surface of the object without damage tothe wood.
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It is known that about 90% of the total amount of biocide in mosttructural wood members is situated in the first 5 mm just belowhe wood surface. Therefore, it is possible to remove the contami-ated area by application of abrasive procedures. Such proceduresre already used when cleaning dirty facades of historic buildingsnd removing graffiti. These methods utilize a low-pressure vortexf air, water mist, glass powder or cork (Jos®-procedure). Biocide-ontaining wood layers are removed under suction.
The “dry ice” (solid carbon dioxide) technology can be appliedor abrasive decontamination of fixed structural wood members inistoric buildings. It is already being used for the removal of cor-osion products or aged varnish layers. Here, coatings or exudatesf biocides on wood are initially rendered brittle by using differentarticles (pellets) of dry ice (–78 ◦C). The brittle residues are then
oosened during the sublimation of dry ice as a result of a signifi-ant increasing of its volume. Subsequently, the complete removalakes place by help of the kinetic energy of the impacting particles.ontaminated wood areas can also be removed using this tech-ique, depending on the dimension of the dry ice particles, chosenressure, shape of the nozzles and treatment time. The procedure isarticularly suitable for large surfaces, maceration layers and wood
oints with cracks and gaps. The waste includes the loosened sur-ace material only, because the dry ice particles sublimate directlynto gaseous carbon dioxide. Unfortunately, this technology is asso-iated with a significant noise and dirt pollution. Furthermore, theperator may sustain skin lesions caused by dry ice and the dangerf suffocation through inhalation of gaseous carbon dioxide shoulde taken into account.
.3.3.2. Thermal processes. Whilst mechanical procedures areimited to the wood surface or surface layers, thermal processesan mobilize volatile biocides from inside wood as well as fromood surfaces by using heated and humidified air. The presentlysed methods include the Thermo Lignum® procedure for movablebjects, and the humidity-regulated Thermo Lignum® warm airrocedure for interior spaces in buildings. The former is carried out
n a climate controlled thermo chamber, which can be stationaryr mobile. Both procedures are normally used to control wood-estroying insects at a temperature of 55 ◦C. By modification of therocess parameters it is possible to liberate and reduce pollutants
n movable wooden works of art and in rooms with contaminatedtructural wood members.
Another procedure is the vacuum desorption of volatile biocidesrom movable wood artifacts by means of a vacuum (≤ 150 mbar)nd increased temperature (≥ 35 ◦C) in a closed chamber (auto-lave). The treatment is performed at a largely constant relativeumidity of 45% to 65% in order to avoid changes in wood dimen-ions and damage to decorative surface layers. The treatment timef this procedure can be extended depending on the dimension ofhe object and how it has been maintained, as well as the prop-rties of the contaminating biocide and its amount. However, aonsiderable reduction of the amount of low-volatile biocides suchs DDT is hardly possible using this technique.
Contaminated built-in structural members without decorativeayers such as wood columns, door panels or sleeper timbersan also be treated by means of portable microwave and high-requency devices. Portable microwave devices work at a frequencyf 2.45 GHz and with a wavelength of approximately 12.2 cm. Theeating time depends on the wood density, wood moisture content,nd the amount of the identified biocide. The temperature shouldot exceed 100 ◦C. Hidden metal parts and timbers with high resinontent represent a fire hazard and can cause the formation of exu-
ates. To avoid electro smog and health risks by microwaves, it isecessary to determine the hazard of respective microwave fieldsuring the treatment and to plan protection measures. Currently,he method is not tried and tested sufficiently.Fig. 2. Cleaning of roof timbers by using the vacuum washing procedure.
The simultaneous vacuuming of the polluted indoor-air and asafe adsorption of the included biocide dust is a very importantpart of all these procedures.
3.3.3.3. Solvent-based methods. These methods are based on oneof four principles: damp cleaning, washing, leaching or extraction.The depth at which decontamination can be achieved increases inthis order.
Damp cleaning consists of wiping contaminated surfaces withsmall cotton buds and pads, pulp swabs and compresses, and fab-rics based on micro fibers using water with addition of a surfactantor alternatively with organic solvents. The utilization of water ororganic solvents is not unproblematic. Firstly, most organochlorinebiocides have a poor aqueous solubility. Water acts as a trans-port medium for biocide dust and dirt removed mechanically only.Secondly, sensitive wood artifacts can swell following the use ofwater. On the other hand, organic solvents may damage coatings,varnishes, and paint layers. Besides, the predominant part of thesolved biocides may be transported more deeply into wood asa consequence of its high capillary suction potential. Residues ofthe solvent mixture used for the formulation of the former woodpreservative may also be activated. After some time, they can trig-ger a biocide migration to the wood surface, and new exudates canbe formed.
The vacuum washing process is a surface cleaning procedurewith water containing some surfactant. It can be applied to museumobjects and structural wood members in buildings without anycoating [5]. Dusty and dirty surfaces are cleaned by means of a spe-cial washing device and different hand hoods wrapped with rubbersleeves (Fig. 2). The hoods are fitted with spray nozzles, a scrubberstrip and suction openings. The use of a rubber sleeve on the sur-face avoids the loss of water and is necessary for the suction undervacuum. Contaminated dirty water is collected in canisters for theircareful disposal as toxic waste. Residual water on the surface of the
object quickly evaporates.As an example, the decontamination rate for DDT and lindanefrom wood surfaces using this method is about 50%. This can beimproved by adding a special detoxicant based on orange terpenes
S200 A. Unger / Journal of Cultural Heritage 13S (2012) S196–S202
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Fig. 3. Prototype of a plant for dry cleaning industrial goods with liquid CO2.
o the process. It is applied to the object surface following treatmentith water. After a reaction time of approximately 5 to 10 minutes,
he surface is re-sprayed with water, resulting in formation of anmulsion. This biocide-containing emulsion is subsequently suc-ioned off. Using this approach, about 70% of DDT and lindane cane removed from wood surfaces. However, this detoxicant does notork for removal of PCP.
Liquid carbon dioxide (CO2) has excellent cleaning abilities,n particular, for nonpolar compounds. It is already used for dryleaning of textiles. Most organochlorine biocides have a nonpo-ar character. Thus, liquid CO2 can be used for leaching of biocidesrom ethnographic objects, textiles, and wood artifacts [6]. Theecontamination rate depends on the dimensions of the object.bjects with a small cross-section are particularly suitable. Atresent, a prototype of a plant using liquid CO2 designed foregreasing industrial goods is being tested for its application inhe context of biocide decontamination of cultural heritage objectsFig. 3). The circular cleaning process is carried out in an auto-lave at 15 ◦C–20 ◦C and at 50–60 bar (industrial goods as well asultural heritage objects) and can be supported by additional clean-ng mechanisms. The reduction of DDT for treated test specimensnd timber boards (12,0 × 6.0 × 0.9 cm3) of an historical xylothequecollection of wood species) after a leaching time of 2 hours wasound to reach about 80% on the wood surface, and up to 35% overhe entire cross-section [7]. The physical properties, shape and colorf these test specimens and original timber boards following liquidO2 treatment remained unchanged in most cases. However, woodpecies with high resin content showed some resin exudates on thend grain and around knot zones.
Carbon dioxide exists as a supercritical fluid above theritical point (31 ◦C and 74 bar). Supercritical carbon dioxidescCO2) has very good solvent properties for nonpolar organicompounds. It has gas-like low viscosities, a high diffusivitynd a very low surface tension. Semi-porous materials such asood are rapidly and completely penetrated. ScCO2 is currentlysed in the food industry, e.g., for extracting caffeine from cof-ee beans or hop extracts from hop cones. On account of itsdvantageous physical and chemical properties scCO2 can besed for the removal of organochlorine biocides from movableood artifacts over their entire cross-section [8]. The extrac-
ion with scCO2 is a closed circular process and the CO2 can
e reused. Toxic biocides extracted by scCO2 are collected in aeparator and fixed to activated carbon. By means of scCO2,he DDT and lindane content in polychrome wooden sculpturesas reduced up to 90% at 40 ◦C and 150–250 bar, a CO2-flow ofFig. 4. Decontamination of wooden works of art with supercritical CO2 in a testingplant.
15–45 kgh−1 and an extraction time of 0.5–4 hours [9]. Sculpturesshowed no changes in dimension, color and gloss followingtreatment (Fig. 4). Weak paint layers were not altered. PCP andinorganic biocides such as arsenic or mercury(II) chloride couldnot be extracted to a greater extent. These biocides require theadditional application of co-solvents (modifiers).
Unfortunately, industrial plants designed for the decontamina-tion of wood artifacts and other cultural heritage using liquid and/orsupercritical CO2 do not exist yet.
3.3.3.4. Other methods. It is possible to detoxify old woodcontaining PCP or tar oil by breaking down these compounds withthe help of enzymes derived from fungi, or through the actionof bacteria. For example, enzymes of certain white-rot fungi canmarkedly reduce the DDT and lindane content on the surface of con-taminated wood. Such microbiological approaches are importantfor the treatment of industrial wood waste contaminated with dan-gerous wood preservatives. However, contaminated wood needs tobe broken down into small wood chips prior to treatment to achievemaximum efficiency of these decontamination processes. The useof microorganisms for the decontamination of wood artifacts, inparticular their surfaces, remains largely undeveloped.
4. “Deconsolidation” of historical wood consolidants
4.1. Nature of the problem
In the 18th and19th century, it was common practice thatwood artifacts damaged by wood-destroying insects and fungiwere stabilised with hot solutions of animal glue. Rathgen [10]described the utilization of dammar resin or collodion for the con-solidation of decorated wooden works of art. Treatments with pureand modified waxes were the preferred methods around 1900.Between 1905 and 1912 in Saxony (Germany), wooden objects
were immersed in hot baths of linseed oil and linseed oil varnish.The use of cold linseed oil, with an addition of French turpentineand phenol as a biocide, was recommended since 1912/13. Thepreparation “Puckelin”, probably a mixture of linseed oil varnish,
A. Unger / Journal of Cultural Heritage 13S (2012) S196–S202 S201
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Fig. 5. Exudates on a wooden sculpture caused by an aged consolidant.
mber varnish and camphor oil [11], was applied for consolidationurposes in Saxony to a greater extent since about 1921. Chineseung oil, paraffin, dammar resin and rosin were mentioned asonsolidants in 1924. The Berlin conservator Sommerfeld treateduropean and East Asian furniture with a special resin solution“Sommerfeld resin solution”) using dichloromethane as solventn an impregnation facility between 1935 and 1939 [12].
Numerous objects previously consolidated with drying oils suchs linseed oil, Chinese tung oil, or their mixtures with natural resinsave now developed a sticky and soft consistency and show exu-ates on the surface (Fig. 5). Such impregnated wooden works ofrt often smell rancid. In several cases, the softening, in combina-ion with the increased mass of the wood because of consolidation
easures, is responsible for individual parts of treated objects toecome detached. This is due to the incomplete polymerization ofhese oils inside the objects. Polymerization is initiated by oxy-en in room air, which slowly penetrates the wooden object overime. This results in an increase in the volume of the object, andracks appear on the surface of consolidated objects. At the sameime, bacteria already present in wood metabolize drying oils andssociated short-chain fatty acids such as butyric acid. This processauses the typical rancid odor.
Resin pearls/drops can now be found on decorated surfaces ofurniture previously impregnated with “Sommerfeld resin solu-ion”. These were formed from solvent residues contained inhis solution, and changes in climatic conditions accelerated theirppearance [12]. There is an urgent need to develop proceduresor the non-destructive removal of outdated consolidants (termeddeconsolidation” in analogy to “decontamination” in this review).
.2. Chemical analysis
Fourier transform infrared spectrometry (FTIR) and gashromatography–mass spectrometry (GC-MS) are suitable meth-
ds for evaluating the composition of aged organic consolidants.owever, the exact composition of historical wood consolidantsuch as “Puckelin” and the “Sommerfeld resin solution” mentionedbove remains to be determined. The results from the chemical
Fig. 6. Testing plant for leaching of consolidated sculptures with vaporous solvents.
analysis of samples carried out by GC-MS and FTIR confirmed theuse of tung oil as a drying oil in the “Puckelin” mixture, whereasthe use of linseed oil cannot be excluded. Colophony, and, pos-sibly, amber varnish, were also used as resinous constituents in“Puckelin”. Furthermore, degradation products of drying oils wereidentified [13].
The resin of the Maritime pine (Pinus pinaster syn. Pinus mar-itima), solved in dichloromethane, was identified as consolidant inthe “Sommerfeld resin solution” [12].
4.3. Current procedures
Aged consolidants can be leached from treated wood artifactswith the help of several organic solvents, provided that the irre-versible three-dimensional cross-linkage of consolidants withinthe object has not occurred yet.
Liquid tetrahydrofurane and 1.3-dioxolane are suited for theremoval of mixtures consisting of vegetable oils and resins. Thisis carried out in a bath with or without circulation [14].
For instance, it was possible to leach “Puckelin” from a poly-chrome sculpture by use of 1.3-dioxolane. Repeat stabilization wasachieved by use of Paraloid B 72 with 1.3-dioxolane acting as sol-vent.
Leaching with 1.3-dioxolane in the vapor phase is a more gentleprocedure (Fig. 6), but it is time consuming. Its efficiency fluc-tuates depending on the amount of consolidant in the object,the consolidant’s exact composition, and the achieved penetra-tion depth. A recently started pilot project assesses the effect ofa cyclic pressure process (vacuum-normal pressure) where leach-ing will take place over several months. It is hoped that this willhelp optimize leaching of aged consolidants.
The use of liquid and supercritical carbon dioxide for theextraction of mixtures of vegetable oils and resins used for theconsolidation of wooden objects is in its infancy. First resultsobtained from experiments with the pure phases were disappoint-ing. Co-solvents (modifiers) were found to improve the extractionrate. Optimal process parameters remain to be determined.
Another pilot study recently carried out uses �-rays to polymer-ize residual liquid linseed oil contained in wood artifacts. However,the dosage required for polymerization in wood is very high (morethan 75 kGy), and this may result in further damage to the object.
The search for efficient “deconsolidation” methods to removehistorical consolidants and to mitigate damage to valuable woodartifacts represents a significant challenge for future woodconservators.
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eferences
[1] A. Unger, A.P. Schniewind, W. Unger, Conservation of wood artifacts, Springer,Berlin, 2001.
[2] N. Odegaard, A. Sadongei, Old Poisons, New Problems: A Museum Resourcefor Managing Contaminated Cultural Materials, Alta Mira Press, Walnut Creek,2005.
[3] J. Bartoll, A. Unger, K. Püschner, H. Stege, Micro-XRF investigations of chlorine-containing wood preservatives in art objects, Stud. Conserv. 48 (3) (2003)195–202.
[4] E. Jelen, G. Widemann, K. Püschner, Decontamination of biocide loaded woodenart treasures, Poster abstract, 6th International Congress on Lasers in the Con-servation of Artworks, Vienna, 21–25 September 2005, Book of Abstracts, PosterP64.
[5] K. Winkler, A. Föckel, A. Unger, Das Vakuumwaschverfahren, Restauro 108 (5)(2002) 339–343.
[6] H. Tello, A. Unger, Liquid and supercritical carbon dioxide as a cleaningand decontamination agent for ethnographic materials and objects, in: A.E.Charola, R.J. Koestler (Eds.), Pesticide mitigation in museum collections: Sci-ence in Conservation. Smithsonian Contributions to Museum Conservation, No1, Smithsonian Institution Scholarly Press, Washington, 2010, pp. 35–50.
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tage 13S (2012) S196–S202
[7] S. Zoppke, A. Unger, J. Mankiewicz, M. Eisbein, Decontamination of a historicalxylotheque with liquid carbon dioxide, The International Research Group onWood Protection (IRG), Queenstown (New Zealand), 08–12 May 2011, IRG/WP11-10757.
[8] S.M. Kang, A. Unger, J.J. Morrell, The effect of supercritical carbon dioxideextraction on color retention and pesticide reduction of wooden artifacts, J.Am. Inst. Conserv. 43 (2004) 151–160.
[9] A. Unger, M. Eisbein, E. Jelen, T. Berger, F. Gockel, Gentle decontamination ofart treasures, gas aktuell 66 (2004) 21–25.
10] F. Rathgen, Die Konservirung von Alterthumsfunden, W. Spemann, Berlin, 1898.11] W. Bachmann, Das Imprägnieren wurmkranker Hölzer, Zeitschrift für
Denkmalpflege, Jg.1 (1926/27) 156.12] T. Weidner, U. Eichner, G. Heck, A. Unger, Die “Harzlösung Sommerfeld”,
Restauro 105 (6) (1999) 453–459.13] A. Schönemann, M. Eisbein, A. Unger, M. Dell’mour, W. Frenzel, E. Ken-
ndler, Historic consolidants for wooden works of art in Saxony. An
investigation by GC-MS and FTIR analysis, Stud. Conserv. 53 (2008)118–130.14] A. Unger, Historic consolidants for wooden works of art in Ger-many. Abstract COST IE 0601-Meeting, Prague, Czech Republic,30–31 March 2009.
