New trials in the consolidation of waterlogged archaeological wood with different acetone-carried products

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New trials in the consolidation of waterlogged archaeological wood with differentacetone-carried products

Gianna Giachi a,*, Chiara Capretti b, Ines D. Donato c, Nicola Macchioni b, Benedetto Pizzo b

a Soprintendenza per i Beni Archeologici della Toscana, via della Pergola 65, 50121 Firenze, ItalybCNR e IVALSA Via Madonna del Piano, 10 - 50019 Sesto Fiorentino, ItalycDipartimento di Chimica Fisica “F. Accascina”, Università degli Studi di Palermo, viale delle Scienze, Edificio 17 - 90128 Palermo, Italy

a r t i c l e i n f o

Article history:Received 2 August 2010Received in revised form6 June 2011Accepted 17 June 2011

Keywords:ColophonyRosinEsterified colophonyVinyl copolymerWood diagnosisWood decayPEG 3400Impregnation

a b s t r a c t

Some acetone-carried consolidants for waterlogged archaeological wood were tested in order to evaluatetreatments able to save time and energy. In details, colophony (rosin), two esterified colophonies (Rosin100� and Rosin 459�), a mixture of colophony with PEG 3400 and a vinyl acetate - vinyl versatecopolymer (Vinavil 8020S�) were tested. The treatments were carried out at temperatures of 20 and35 �C onwaterlogged maritime pine, elm, oak and beech. The materials came from the archaeological siteof the ancient ships of Pisa (Tuscany, Italy) and were dated back to VII cent. BC e II cent. AD. To evaluatethe processes, equilibrium moisture content and dimensional stability of treated wood samples atdifferent relative humidity, and retention of impregnating products were measured; moreover macro-scopic and microscopic examination were also run to respectively assess the shape and appearance oftreated wood and the way of deposition of consolidants. The results highlighted that natural andmodified colophony treatments gave the most satisfactory results both in the maintenance of shape anddimensions of samples and in the stabilization with respect to RH variations. Moreover, the equilibriummoisture contents of samples treated with R100 and R459 were much reduced in comparison to theother consolidants and to untreated archaeological wood. This fact was related to the high retentionvalues of those products that occluded most of the porosity including the microporosity of cell walls.Therefore, in terms of higroscopicity treated wood was more similar to impregnating substances ratherthan to decayed wood. This fact was considered helpful in contrasting the moisture-related negativeeffects in cases of eventual faults in the climate control during e.g. exhibition and in protecting treatedwood from the risks of new fungal attacks.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The discovery of wooden artefacts in archaeological contexts isnot as frequent as onemight expect when considering the large useof this material, mainly in the past. Only particular conditionsduring burial, as in the case of waterlogged environments, havesometimes allowed their preservation up today. However, also inthese cases very often wood underwent severe decay, even ifobjects preserve their original size and shape rather well.

The decay of wood is mainly due to biological attack (Bjordalet al., 1999; Kim and Singh, 2000; Jordan, 2001), but also chemicalfactors can have a role in this process (Unger at al., 2001). The decaycauses wood mass loss and increases both permeability and watercontent (Fengel, 1991; Hedges, 1990). As a consequence, weakened

woodcells shrink andcollapseduring theevaporationofwaterand ifdriedwithout any caution, decayed objects lose their original shape.Conservation of these findings necessarily implies the stabilizationof their shape and size.Moreover the approach to conservationmustalso ensure the safeguarding of physical integrity of wood and, asmuch as possible, of its mechanical properties, when the woodenfindings consist of large sized structures with a complex architec-ture, made of several wood species differently decayed.

Various methods have been tested and used so far for theconservation of degraded waterlogged wood. Recently, Hoffmann(2009) compared some of the treatments currently most usedand he set that whereas the success of a stabilizing interventiondepends on several factors, each method presents advantages anddisadvantages. Generally a compromise between contingentconservation exigencies, maintenance requirements, costs, safetyand easiness of procedures is chosen.

At present, the most used techniques to conserve waterloggedarchaeological wood are based on the impregnation of artefacts by

* Corresponding author. Tel.: þ39 0 55700953; fax: þ39 0 557131694.E-mail address: [email protected] (G. Giachi).

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natural or synthetic products, which penetrate into the objectsthrough the diffusion and permeation processes (Grattan andClarke, 1987). Treatments using PEGs (Polyethylene Glycols ofvarious molecular weights) are by far the most diffused, althoughmethods using sucrose and other sugars (Cott and Strigazzi, 1999)and more recently sugar alcohols (Zhang et al., 2009) andKauramin�, a melamine resin (Wittköpper, 1998), are also used.

In addition to the consolidant, also the final drying plays anessential role in the success of treatment of waterlogged archaeo-logical wooden objects. Two techniques are commonly used:controlled exsiccation and freeze-drying. The applicability of thesemethods mainly depends on both the chemical nature of thechosen substances and the size of the objects to be treated.However, although freeze-drying allows fast processes, it alsorequires expensive machinery and a procedure which is not easy tocorrectly carry out and control (Jensen and Jensen, 2006), mainly ifartefacts are of large dimensions: in these cases big size devices areoften exclusively dedicated, thus implying excessive costs.

On the other hand, controlled drying is very easy to manage andcarry on, and it generally does not require expensive equipments,except again for very large objects, such as entire ships. However,the controlled drying is often a time consuming process whenwater-carried consolidants are used.

In this perspective, it can be easily understood how alternativetechniques in treatment of archaeological wood are continuouslysearched. Research efforts have been directed towards variousdirections. Among them high volatility solvents are used, becausethey are able to more favourably carry consolidants and quicklyevaporate after treatment. Several attempts were made in the pastby using acetone as dehydration agent and various products asconsolidants: Tetraethyl orthosilicate (TEOS) (Irwin and Wessen,1976), Methacrylate, Methyl methacrylate, Ethyl methacrylate andButyl methacrylate to be polymerised in situ (Brendel, 1966;Donato and Agozzino, 2004; Tran et al., 1990), Unsaturated poly-ester resins (Ketelsen, 1959).

Within this picture, the most successful procedure, making useof acetone, is the impregnation of waterlogged objects with colo-phony (rosin). McKerrell et al. (1972) firstly proposed the acetone/rosin method. Colophony treatments present several advantages interms of stabilization of both dimensions and shape of wood and ofsurface details retain (Unger et al., 2001). Moreover, colophony islittle hygroscopic and hence treated objects do not require partic-ular maintenance equipment (such as climate-control devices)during their exposure in museums. On the other hand, colophony isa rigid resin and therefore once treated and dried the shape of theobjects can be adapted very limitedly and with the greatest caution.

Moreover, procedures making use of acetone evidence somedisadvantages. In facts, while the safety problems can be overcomeby a careful control of the process during treatment, this sameaspect appreciably limits the size of wooden findings that can betreated.

Aim of this work is to investigate on some issues related to thetechnique of impregnation of waterlogged wood samples withdifferent acetone-carried consolidants, most of which have beennever considered so far, including colophony as a reference. Moreprecisely, several aspects were considered:

� setting a treatment temperature closer to room temperature, inorder to decrease the acetone evaporation and the energyconsumption;

� increasing the plasticity of colophony using amixturewith PEG3400;

� evaluating some chemically modified types of colophonycommercially available, which possess excellent stability tothermal and oxidative degradation;

� evaluating a chemically different type of product, namelya vinyl copolymer. It was never tested so far for applications onwaterlogged wood.

Various treatments were carried out on different wood speciesand levels of decay, and they were evaluated by comparing somephysical parameters, such as the retention of the products, thewater absorption capability, the dimensional stability in differentthermo-hygrometric conditions and also the deposition of theconsolidant at microscopic level.

2. Materials

2.1. Wood samples

Wood samples were collected from findings pertaining theancient vegetation of the archaeological Site of the ancient ships ofPisa (Tuscany, Italy). They are dated from VII cent. BC to II cent. ADaccording to the archaeological stratigraphy. Cubes of 5 cm edgeswere cut from discs obtained from selected stems of: one conifer,one diffuse-porous and two porous ring hardwoods (Fig. 1).

2.2. Products used for wood treatment

All the wood samples, the impregnating solutions and thecomposition of their mixtures and their acronyms are reported inTable 1. Two series of different samples of the same species weretreated with each solution: a series was utilised for the physicalmeasurements and the other for the examination at microscopiclevel of the deposition of the impregnating substances into wood.

Samples were impregnated by immersion in acetone solutionswith:

1. Colophony2. Rosin 100�: a chemically modified colophony (stabilised rosin

ester of pentaerythritol),3. Rosin 459�: a chemically modified colophony (stabilised rosin

ester of polyalcohols),

Fig. 1. Example of obtaining of 5 cm cubes from the discs of the selected stems.

G. Giachi et al. / Journal of Archaeological Science 38 (2011) 2957e29672958


4. Mixture of colophony with PEG 3400,5. Consolidant Vinavil 8020S�: a commercial copolymer (vinyl

acetate and vinyl versate).

The various types of colophony (points 1, 2, 3) were supplied byBresciani s.r.l., PEG 3400 was purchased from Fluka, Consolidant8020S was provided by Mapei Company whereas acetone fromJ.T. Baker.

3. Methods

3.1. Wood characterisation

Wood characterisation was performed following the pathestablished by the Italian standard UNI 11205:2007, that consistsin:

� micro-morphological analysis on wood sections along thethree diagnostic directions (UNI 11118:2004);

� decay assessment by means of physical measurements ofMaximum Water Content (MWC), Basic Density (Db), ResidualBasic Density (RDb) and total dimensional variations along thethree characteristics directions (Jensen and Gregory, 2006;Macchioni, 2003; Schniewind, 1990). Reference values forbasic densities of sound wood were taken from literature(Giordano, 1981);

� decay assessment by means of chemical measurements ofresidual amounts of holocellulose (according to Norman andJenkins, as reported in Browning, 1967) and lignin (TAPPI T222,2002), and their ratio H/L, which is considered as representa-tive of the extent of the chemical decay (Pizzo et al., 2010).

Anatomical examination pointed out that the wood specieswere: elm (Ulmus cf. minor), samples marked with B and D (twoseparate stems), maritime pine (Pinus pinaster Aiton) marked withC and F (two separate stems), oak (Quercus sp. caducifolia) markedwith Q, beech (Fagus sylvatica L.) marked with G.

The decay of waterlogged archaeological wood involves the lossof substance (mostly polysaccharides) and its replacement bywater. The increase of MWC, the decrease of Db and the decrease ofH/L indicate an increasing level of decay for the archaeologicalwood. In our case, the diagnostic evaluation resulting from bothchemical and physical characterisations gave comparable resultsfor each wooden sample, as shown in Table 2.

3.2. Technique of impregnation

Waterlogged archaeological wood was consolidated by animpregnation process carried out by immersing the samples inacetone solutions of each one of the selected products.

Firstly, the wood was desalinised by washing it with deionisedwater at 25 �C until a conductivity �10 mS cm�1 was reached. Then,water was replaced by acetone, and refractive index (nD)measurements were used to follow the process. Measurementswere carried out at constant temperature (20 �C) by means of anAutomatic Refractometer GPR 11e37 (Index Instruments). Theacetone-water exchange process was supposed to be completewhen the refractive index of the solutions in which the sample wasimmersed approached the value of pure acetone, within the limitsof the measurement error.

Finally, acetone-filled samples were immersed in the consoli-dating solutions. The diffusion of these solutions into wood wasfollowed by measuring the variation of the consolidant concen-tration during time through viscosity measurements. Flow-timewas measured at constant temperature by means of a capillarymicro-viscosimeter Ubbelhode-type, equipped with optical sensorand automatic unit AVS 440 of Schott-Geräte. A second impreg-nation step was carried out after the first one, and in some cases

Table 1The utilised samples, impregnation solutions and treatment temperatures. For eachtreatment, samples in row ‘T’ are those used for the physical characterisation, andsamples in row ‘O’ are those used for microscopic observations.

Samples Impregnationsolutions

Acronym Treatmenttemperature

T D18 F17 F24 G17 63% colophony COL 20 �C; 35 �CO D17 F18 F23 G18T B24 Q15 C14C C18B 30% colophony

and 30% PEG 3400CP3400 35 �C

O B16 Q14 C14D C18AT D9 F7 F8 G9 60% Rosin 100 R100 20 �C; 35 �CO D6 F9 F6 G6T D10 F10 F14 G11 60% Rosin 459 R459 20 �C; 35 �CO D11 F11 F13 G10T D16 F15 F61 G15 11% consolidant

8020S8020 20 �C

O D15 F16 F21 G16

Table 2Results of chemical and physical characterisation of selected wood samples. The values marked with * were measured with a helium-pycnometer by using the methodologydescribed in (Donato and Armata, 2009). The other values were obtained through the water-displacement method (Tsoumis, 1991). In the table, WAW is the WaterloggedArchaeological Wood. H/L of non-degraded wood was measured according to the same procedures as for WAW.

Sample Wood Species Db of non-degradedwood (g/cm3)

Db of WAW (g/cm3) RDb (%) MWC (%) H/L of WAW H/L of non-degradedwood

B29 Ulmus cf. minor 0.57 0.15 26.3 602 0.11 2.1D3 0.18 30.7 514 0.14D4 0.19* 33.3 475 e

D25 0.19* 33.3 483 e

D26 0.18 30.9 505 0.22Q18 Quercus sp. cad. 0.67 0.17 26.0 515 0.18 2.1C12A Pinus pinaster 0.53 0.13 24.0 727 0.07 2.2C12B 0.27 52.0 435 0.49F3 0.15 27.6 630 0.12F4 0.16* 30.2 567 e

F27 0.16* 30.2 550 e

F28 0.15 29.1 594 0.16F30 0.20 38.3 440 0.45G3 Fagus sylvatica 0.60 0.18 28.9 507 0.10 3.0G4 0.13* 21.7 700 e

G27 0.14* 23.3 652 e

G28 0.16 26.3 557 0.08

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(see Table 1) temperature was increased in order to speed up theprocess. Each process required at least 30 days to reach the equi-librium. In Table 3 the equations of the logarithm of flow-time, Ln(t)versus concentration (in percentage) are reported for eachconsolidating mixture.

After treatment, each sample was placed into a 500 cm3 box atroom temperature to slowly remove the acetone; the box wasperiodically open to let the acetone vapours go out.

3.3. Evaluation of the treatments

Treatments were evaluated as described in Giachi et al. (2010).More specifically, the following parameters were calculated:

- equilibrium moisture contents,- dimensional stability at different relative humidity,- retention of impregnating products.

Moreover, some macroscopic evaluations on the shape andappearance of samples, and micro-morphological characterizationof samples after impregnation were added.

3.3.1. Physical characterisationTwo series of measurements were carried out on treated

samples: a) determination of moisture absorption in climaticchamber, and b) determination of retention of consolidants.

To evaluate the effect of treatments with respect to moistureabsorption, treated samples were put in a climatic chamber at20 �C, varying the RH values according to the sequence 35, 45, 55,65, 75, 85% and reaching, in each step, the equilibrium moisture

content (EMC) (Giordano, 1981). In each step samples reached theequilibrium approximately after 30 days.

Measurement of the samples along the three anatomicaldirections after each humidity step was carried out in order tocalculate the dimensional variations of treated wood, consideringthe dimensions before starting the conditioning sequence as theinitial ‘reference’ ones (21 �C, 50% RH). Moreover, at the end of thewhole climatic sequence, samples were completely dried in a ovenat 103 �C up to constant weight, reached after approximately 4days, in order to calculate the moisture content of each step.

The retention of consolidants, Y, was calculated by means of theincrease of weight of treated wood with respect to weight of thesame untreated sample, according to the following equation:

Y ¼ 100$mAT �mANT

mANT

where mAT is the oven-dried mass of treated samples and mANT isthe estimated oven-dried mass of the same samples when non-treated. This latter was calculated by attributing to each samplethe same MWC of the nearby sample utilised in diagnostic evalu-ations. In particular, the following expression was used in theestimation of mANT :

mANT ¼ 100$mWNT

100þMWC

where mWNT is the wet mass of untreated samples (i.e. the mass ofeach waterlogged wood sample before the treatment).

In addition to this ‘practical’ value, a ‘theoretical’ value, Y*, of theretention of impregnation products was also calculated. This latterwas estimated by hypothesizing the complete substitution of theimbibition water with the solution of consolidant. By this way, thevolume of impregnating mixture penetrated into the wood can beassumed as equal to the original water volume, estimable bymeansof MWC. Then, the mass of retained consolidant can be calculatedby means of the density value and of the concentration of eachimpregnating mixture (dCOL ¼ 0.970 g/cm3; dR100 ¼ 0.957 g/cm3;dR459 ¼ 0.950 g/cm3; d8020 ¼ 0.817 g/cm3; dCP3400 ¼ 0.959 g/cm3).

3.3.2. Macroscopic evaluationsAll treated samples were examined and photographed after the

end of the whole humidity sequence in order to evaluate theiraspect and shape after drying.

Table 3Equations of logarithm of flow times, Ln(t), versus concentration in percentage, C, at25 �C. R2 are the correlation coefficients.

Consolidatingsolutions

Ubbelohde TypeViscosimeter

NominalConstant(cSt/s)

Equation R2

COL 53810/I 0.01 Ln(t) ¼ 0.067 C þ 2.674 0.974CP3400 53813/Ic 0.03 Ln(t) ¼ 0.073 C þ 1.725 0.999R100 53810/I 0.01 Ln(t) ¼ 0.069 C þ 2.922 0.984R459 53810/I 0.01 Ln(t) ¼ 0.072 C þ 2.604 0.9658020 53810/I 0.01 Ln(t) ¼ 0.152 C þ 4153 0.993

Table 4Values of MWC and Db used in the calculation of mWNT and VTREAT, of DVW�TREAT, of the retention of products calculated by the oven-dry method (Y), and of the retention ofproducts calculated by hypothesizing the complete substitution of water by the consolidants (Y*).

Treatment Sample MWC (%) Db (g/cm3) mWNT (g) VTREAT (cm3) DVW�TREAT (%) Y (%) Y* (%)

COL D18 480 0.18 120.96 110 5 259 295F17 567 0.14 142.56 129 15 336 348F24 440 0.20 153.07 138 3 208 270G17 680 0.15 140.18 108 10 394 417

R100 D9 480 0.18 134.43 103 20 226 281F7 567 0.14 131.51 112 20 273 331F8 440 0.20 139.33 120 7 312 257G9 680 0.15 146.15 97 22 353 397

R459 D10 480 0.18 134.38 101 22 155 274F10 567 0.14 138.81 113 24 195 323F14 440 0.20 141.43 123 6 132 251G11 680 0.15 148.31 86 32 272 388

CP3400 B24 602 0.15 126.98 88 27 302 346Q15 515 0.17 137.28 88 33 231 296C14C 435 0.20 135.19 126 0 187 250C18B 435 0.20 126.76 118 1 191 250

8020 D16 480 0.18 153.56 104 30 27 43F15 567 0.14 136.76 101 31 20 51F61 440 0.20 149.44 124 10 25 40G15 680 0.15 144.40 50 60 27 61



Moreover, after oven drying they were crosscut and scanned ona high definition scanner (Epson 1640 XL) together witha measuring ruler. In such a way the cross section surface of eachsample could be quantitatively evaluated by using an image pro-cessing software. The volume of treated samples, VTREAT, wastherefore calculated by multiplying the cross section by the lengthof each specimen, before their cutting. This volume was comparedwith that estimated on the same sample before treatment, inwaterlogged conditions. This initial volume, VW, was evaluated onthe basis of MWC and of basic densities values. Finally, initialwaterlogged volume and volume after treatment were comparedand their variation expressed in percentage as:

DVW�TREAT ¼ 100 $VW � VTREAT

VW

Values of MWC, Db, mWNT, VTREAT, DVW�TREAT, Y and Y* are reportedin Table 4.

3.3.3. Microscopic observationsSome samples of untreated waterlogged maritime pine (C5C,

F1), elm (D5) and beech (G1) were chosen to evaluate the micro-morphological feature of the decay of wood.

Treated samples of the same species were examined to assessthe deposition of the consolidating substances into the wood cells(series ‘O’ in Table 1). With this aim, small fragments of about2 � 2 � 2 mm3, sectioned along the diagnostic direction of wood,were cut with a razor blade approximately below 1mm and 25mmfrom the transversal surface of treated cuboids.

‘O’ samples were treated as the ‘T’ ones but not oven dried at103 �C in order to avoid any possible variation in the deposition ofthe consolidants within wood cells due to heating. The samples,gold coated, were examined by means of a scanning electronmicroscope. Untreated waterlogged wood was dehydrated inacetone and exsiccated by means of critical-point dryer. Subse-quently, it underwent to the same procedure as for treated samples.

A FEI Company Quanta 200 scanning electron microscope wasused, with acceleration voltage of 20 and 25 kV and filamentemission current of 40 mA.

4. Results and discussion

4.1. Macroscopic observation

The macroscopic observations allowed evaluating the effect ofeach treatment on samples at different levels of decay. Only themost exemplificative cases of elm (Fig. 2), beech (Fig. 3) and pinesamples (Fig. 4) are shown. Pictures allow perceiving immediatelythe final dimensions and shape of the samples and also checkingany eventual openings and collapse as consequences of both thetreatment phase and the conditioning one.

Generally speaking, except for beech samples, treatments R459,R100 and COL maintained the shape and dimensions of thesamples, while the efficiency of CP3400 and 8020 stronglydepended on the level of decay of treated samples. For oak andbeech, all treatments failed in the task of the dimensional stabili-zation of samples, but beech samples treated with R100, R459 andCOL resulted less distorted than the 8020 treated samples. It isinteresting to note that most of the damages on the samplesdeveloped just after the drying phase, before starting the climaticvariations.

The considerations based on macroscopic observations werealso confirmed by the volumetric measurements. Table 4 reportsthe values of volumetric variation for treated samples, DVW�TREAT.For all the considered treatments this variation increased by

increasing MWC and it was related to both wood species andmolecular structure of consolidants. In fact, in all samplesDVW�TREAT increased in the order COL < R100 < R459 < 8020.

Fig. 2. Aspect of selected elm samples after treatments. D18 was treated with COL, D9with R100, D10 with R459, D16 with 8020. See Table 1 for details.

Fig. 3. Aspect of beech samples after treatments. G17 was treated with COL, G9 withR100, G11 with R459, G15 with 8020. See Table 1 for details.



4.2. Physical characterisation

Results of Equilibrium Moisture Content (EMC) of treatedsamples are reported in Table 5, whereas their dimensional varia-tions are reported in Tables A1 e A3.

Moisture content of samples varied sharply according to theimpregnating substances. Samples treated with R100 and R459gave very similar, and very low, values of EMC at the different RHlevels. During the complete cycle, from 35% to 85% RH, the average

EMC (calculated from the values on Table 5) passed from 6.89% to7.39% for all the R100 treated samples, whereas in the case of R459the variation was from 5.61% to 7.23%. COL treatment gave to woodmore hygroscopicity, with EMC varying on average from 8.16% to10.39%. Essentially, the presence of PEG 3400 influenced thebehavior of mixtures PEG-Colophony, causing the rising of EMCfrom 9.25% to 14.25%. Finally samples treated with 8020 had anaverage EMC variation from 10.94% to 16.45%. In the same RHvariation span, the untreated samples passed from 10.31% to18.70%, while normal sound wood (from literature, Giordano, 1981)varied from 7 to 18%. Analogously, moisture absorption of pureconsolidants were measured at 20 �C and 85% RH and the obtainedvalues were: EMCCOL ¼ 3.87%, EMCR100 ¼ 0.22%, EMCR459 ¼ 0.29%,EMCPEG3400 ¼ 6.46%, EMC8020 ¼ 1.25%.

The treatments with natural or modified colophony preventedvery decayed wood from absorbing environmental humidity. FromTable 5 it is easy to calculate that the variation of EMC of normalsound wood in the considered hygroscopic range is 11%, whereas itwas averagely 8.4% for untreated decayed wood. However, in thesame RH span the EMC variations for treated samples were 0.5% inthe case of R100, 1.6% for R459 and 2.2% for COL, and hence muchmore limited. It means that eventual faults in the climate control,during e.g. exhibition, would have less negative effects oncolophonies-treated samples. Moreover a very low EMC of samplescould protect treated wood from the risks of new fungal attacks(Eaton and Hale, 1993). In this context, in the case of samplestreated with 8020 and when PEG 3400 was added to colophony(samples CP3400) the treatment was less effective (the EMC vari-ations were 5.5% and 5.0% respectively, double compared to COL)but still improving compared to untreated wood.

The dimensional variations during the conditioning phases inthe climatic chamber were practically stable as well in all samples.This circumstance was related both to the low values of absorbedmoisture and to the fact that most part of deformations occurredduring the drying phase. Moreover, the small variations observedseemed more influenced by the accuracy of measurements than bya real movements of samples. It is worth to note that the accuracy ofthe measurements was not affected by the utilized tool, but ratherby the low deformations of samples and consequently by thedifficulties in taking measurements always in exactly the sameposition on distorted samples.

Table 5Equilibrium moisture content (EMC) of the treated samples in the entire RH cycle.

Sample Consolidant EMC at RH (%)

Initial 35 45 55 65 75 85

F7 R100 7.52 7.25 6.69 6.93 7.19 7.58 7.57D9 7.36 6.33 6.56 6.76 6.99 7.34 7.23F8 7.64 6.64 6.78 6.93 7.12 7.40 7.18G9 8.38 7.33 7.41 7.49 7.64 7.91 7.57D10 R459 6.34 5.04 5.43 5.80 6.19 6.76 6.94F10 6.29 5.16 5.49 5.82 6.18 6.67 6.71F14 7.24 5.77 6.20 6.62 7.11 7.80 7.99G11 7.75 6.45 6.67 6.92 7.24 7.71 7.63F17 COL 9.37 8.55 9.08 9.60 10.16 10.82 11.27G17 8.42 7.47 7.87 8.30 8.76 9.35 9.62D18 9.35 7.50 7.57 7.78 8.12 8.57 9.02F24 10.31 9.12 9.59 10.03 10.51 11.15 11.64B24 CP3400 10.31 8.35 8.98 9.66 10.58 12.11 12.86C18B 11.46 9.42 10.08 10.83 11.75 13.45 14.12Q15 12.75 9.99 10.61 11.39 12.53 14.47 15.46C14C 11.45 9.23 10.06 10.93 12.03 13.96 14.57D16 8020 14.66 11.38 12.35 13.09 14.11 15.82 17.00F15 13.29 10.39 11.26 11.91 12.82 14.40 15.51F61 14.74 11.36 12.38 13.19 14.30 16.11 17.45G15 13.50 10.62 11.52 12.22 13.13 14.71 15.85Control (untreated samples) 14.20 10.31 11.86 12.76 14.25 16.68 18.70Sound wood e 7 9 10 12 15 18

Fig. 5. Graph of Y (symbols) and Y* (lines) vs. MWC for the treated samples. Thedotted line refers to values of Y* for samples treated with colophony, the discontinuousline to Y* for samples treated with 8020 whereas the solid line refers to Y* for all othersamples.

Fig. 4. Aspect of selected pine samples after treatments. C14C was treated withCP3400, F8 with R100, F14 with R459, F61 with 8020. See Table 1 for details.



4.3. Retention of impregnation products

In Table 4 both the real retention (Y) and the theoretical one (Y*)are shown, thus allowing a direct comparison between the twovalues. It resulted evident how both Y and Y* increased byincreasing MWC. Moreover, for all the consolidants the real yield ofimpregnation, Y, was always lower than the theoretical one, Y*, foreach considered sample and type of treatment, except for F8. Whilethe relationship between Y* and MWC was correctly linear for allthe considered treatments (Fig. 5), a highly linear relationshipbetween Y and MWC was also observed, if the anomalous sampleF8 was excluded, for all consolidants except for 8020 which wasessentially invariant with MWC. This fact evidenced that while forCOL, CP3400, R100 and R459 the impregnation process can beeffectively modelised by the solvent/consolidant mixture exchangewithin the wood samples, in the case of 8020 this simplisticmechanism was no more applicable and other factors (for instanceproperties and polymer rheology, structure and conformation ofmacromolecules etc.) played a considerable role.

The appreciable difference between Y* and Y observed for alltreated samples was due to the fact that during the real process ofimpregnation the theoretically full exchange between solvent andconsolidating mixture was never obtained, despite the processcould be considered complete on the basis of concentration

evaluations. Moreover, a specific set of measurements allowedexcluding the possibility that the observed lower values of Y wasimputable to a weight loss of consolidants during the oven dryingphase, when they were kept at 103 �C for 4 days. In facts,measurements evidenced that in the same conditions as for ovendrying, R459 did not lose any mass at all, whereas colophony (theleast thermally stable product among those considered) loosedonly less than 2% of its original weight.

As shown in Fig. 5, for all MWC values the observed differencebetween Y* and Y decreased in the order: R459 > R100 andCP3400> COL. In fact, Y*-Y was included in the range 116e130% forR459, in the range 40e60% for R100 and CP3400 and it wasgenerally lower than 40% for COL. This observed ranking, togetherwith the invariance of Y*� Ywith MWC, let deduce that dimensionand shape of the molecules played a key role in the diffusion ofthese consolidants and, additionally, that for a same product alsochemical properties influenced the interaction between it andwood substrate. For example, colophony (that evidenced thehighest retention value) has an acid number of 150 mg KOH/g,higher than those of R100 (20 mg KOH/g) and R459 (10 mg KOH/g).

Retention values could also be related with EMCmeasurements.In fact, by comparing data reported in Tables 4 and 5 it could beobserved that EMCs were associated to retentions in a waydepending on the hygroscopicity of consolidants. The lowest values

Fig. 6. SEM images of untreated pine samples. Sample C5C (left), longitudinal section: weak troughs parallel with cellulose microfibrils due to erosion bacteria activity are shown.Sample F12 (right), transversal section: the attack due to soft rot fungi and bacteria caused the production of large cavities in the S2 layer of the cell walls, consequently, frequentdetachment of the secondary wall from ML, distortions and collapses are evident.

Fig. 7. SEM images of transversal sections of pine treated with COL (sample F18): cell walls and lumina are almost completely filled in the latewood (left) whereas in the earlywood,lumina appear empty (right).



of EMC were observed for R100 and R459: these products hadsimilarly lowvaluesof EMCwhenpure (0.22%and0.29% respectivelyat 20 �C and 85% RH) but they were retained in wood differently(Y¼ 291%, average value for R100 treated samples, Y¼ 189% averagevalue for R459, see Table 4). As a consequence, the EMCsvariationsoftreated wood samples in the considered RH range were 0.5% in thecase of R100 and 1.6% for R459. On the other hand, the highest valuesof EMCs in treated samples were associated to the scarcely hygro-scopic 8020 (EMC¼ 16.45% on average) because of the lowest valuesof retention (Y ¼ 25% on average), and hence the highest relativeamount of wood substance associated to this product.

4.4. Microscopic observation

In almost every examined case the micro-morphological anal-ysis highlighted a severe decay of the wood due to biological attack(bacteria and fungi). The two samples of pine differed each other(Fig. 6): while the C5C sample showed a light decay, the F12 oneappeared highly degraded, similar to elm and beech. In the lessdecayed pine, only weak troughs parallel to cellulose microfibrilswere evident in the inner part of the cell wall: they were due to theactivity of erosion bacteria. In pine sample from stem F, the attackby soft rot fungi in the S2 layer of the secondary walls caused theproduction of several cavities and some of these coalesced in themost decayed areas. At the same time the activity of erosionbacteria, which consumed the secondary wall starting from thelumen, was evidenced. As a consequence, wood had a markedspongy consistency. Moreover, mainly in the latewood and some-times even in earlywood, the S2 layer of the secondary wallappeared detached from middle lamella (ML) and distorted. The

decay was severe also in elm and beech: the walls of fibres werecompletely detached from ML, sometimes collapsed and distorted,the S2 layer became spongy and swollen so that often the luminawere no longer distinguishable.

The examination of samples treated with COL, R100, R459 andCP3400 highlighted that the impregnation process of woodinvolved the filling of both the inside of cell walls and/or of celllumina depending on the different anatomical elements of eachspecies. In pine almost all the latewood cell walls and luminaappeared completely filled, whereas in earlywood the impregna-tion involved almost exclusively the filling of the inside of the cellwalls (Figs. 7 and 8). In both elm and beech only the fibres werefilled, while the vessels lumina remained empty. Moreover, in elmthe spaces originating in the fibres from the detachment of S2 layerfrom ML, were often not completely filled (Fig. 9). This differentbehaviour seemed to be related to the different size of the pores. Infact, consolidating solutions were not retained in larger cavities (inaverage above 30e40 mm in all treatments) after the extraction ofsamples from their dipping bath.

As expected, the addition of PEG 3400 to colophony gavea “waxy” appearance to the deposition and amore soft resistance tocut (Fig. 10).

In all the examined samples the impregnating products pene-trated deep inside the cuboids, up to their centre, with no signifi-cant variations in theway of their deposition. Both less decayed andmore decayed pine samples showed a similar filling after treat-ments. This fact validated the observation reported above that themore was the porosity (and consequently the decay), the higherwas the quantity of impregnating products retained into wood.Moreover, the observed occlusion of most of the porosity, included

Fig. 8. SEM images of the transversal section of pine treated with R100 (sample F9): the way of deposition is similar to that of COL.

Fig. 9. SEM images of transversal sections of elm treated with COL (D17, left and centrum) and R100 (D6, right). Fibers are filled by the impregnating products while vessels luminaare still empty. Sometimes the space resulting from the detachment of S2 layer from ML remained empty.



the microporosity of the cell walls, allowed to give explanation forthe reason why treated wood had an affinity to environmentalhumidity more similar to that of the impregnating substancesrather than to that one of decayed wood.

Searching the consolidant 8020 inside treated samples wasmuch more difficult than for other impregnating substances. Thematerial did not occlude any cell lumina and the microporosity ofthe decayed cell walls. Only sporadically the deposition of a film onthe lumina walls was detected (Fig. 11). What observed would notexplain any wood stabilization, but, on the other hand, the fact thatmost of wood surface remained uncovered justified that treatedsamples behave in a way very similar to untreated wood respect toenvironmental humidity variations.

5. Conclusions

Several methods have been tested and are currently in use forthe conservation of waterlogged archaeological wood. Amongthem, those using volatile solvents, like acetone, are able to morequickly carry the consolidants into wood and require a shorter timeto dry the treated material.

The aim of this work was to investigate on some issuesregarding the conservation methods by using different acetone-carried consolidants. In particular were tested: colophony (COL);two esterified colophonies Rosin 100� and Rosin 459� (R100,R459); a mixture of colophony with PEG 3400 (CP3400); a vinylacetate-vinyl versate copolymer, Vinavil 8020S� (8020). Theexperiments were carried at 20 and 35 �C on archaeologicalwaterlogged maritime pine, elm, oak and beech with differentdecay level. Macroscopic observations put in evidence that treat-ments with COL, R100 and R459 maintained well shape andappearance of samples, while the efficiency of the mixture CP3400and of 8020 strongly depended on the level of decay of samples tobe treated. Volumetric measurements confirmed this trend: thevolume variations after treatments were dependent on MWC andhence on the decay level of wood. However, the volumetric varia-tions increased in the order COL < R100 < R459 < 8020 and theywere also dependent on MWC. It is worth to note that colophonywas more retained by wood with respect to its derivates.

However, it was observed that the retention of impregnants (Y)was always lower than the expected theoretical value (Y*) due tothe incomplete exchange between solvent and consolidatingmixture. It resulted that dimension and shape of the molecules ofconsolidants played a key role in their diffusion, and that theirchemical properties influenced the interaction between substancesand wood substrate. More specifically, the molecular structure ofconsolidants affected both volumetric variations and retentionafter wood treatments.

The conditioning of treated samples to different RH levels didnot lead to significant results in dimensional changes. This wasbecause equilibrium moisture contents of treated samples weregenerally reduced in comparison to those of untreated archaeo-logical wood and the reduction value was to be related to both theretention and the properties of each impregnating substance. Infact, samples treated with R100 and R459 had very similar and verylow EMC values at the different RH levels, whereas a higherhygroscopicity was shown by wood treated with COL and CP3400.Treatment 8020 evidenced EMC values more similar to untreatedwood because samples treated with it had the lowest value ofretention. The reduction of moisture absorption was consideredalso helpful in contrasting the moisture-related negative effects incases of eventual faults in the climate control (e.g. during exhibi-tion) and in protecting treated wood from the risk of new fungalattacks.

The different behavior among the various treated wood sampleswas clearly understandable by their microscopic examination.Samples impregnatedwithCOL,R100, R459andCP3400highlightedthefilling of themicroporosity inside both cell walls and cell lumina.

Fig. 10. SEM images of the transversal section of pine treated with CP3400 (C14D). The deposition is very similar to that observed for COL, R100 and R459 except for a smooth and“wax-like” appearance.

Fig. 11. SEM image of the tangential section of elm treated with 8020 (D15): sporad-ically a film covers the lumina walls.



Additionally, the filling of wood elements was abundantly observedfrom the surface to the centre of cuboids, thus justifying howmuchthehygroscopicity of the impregnating substances and their amountinto the wood influenced the EMC values of treated wood ratherthan with decayed wood. On the contrary, 8020 produced a film,which was detected only sporadically over the lumenwalls. For thisreason this treatment influenced only limitedly the moisture prop-erties of waterlogged wood.

Acknowledgements

The Authors whish to thank Leonardo Rescic for his work inacquisition and elaboration of pictures.

Appendix

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Table A1Longitudinal deformations of the different treated samples for each treatment(negative values ¼ swelling).

Sample Consolidant Longitudinal deformations at RH (%)

35 45 55 65 75 85

F7 R100 �0.3 �0.8 �0.1 �0.1 �0.4 �0.6D9 �1.7 �2.0 �1.4 �1.4 �0.9 �1.5F8 �0.5 �0.6 �0.2 0.0 �0.4 �0.4G9 �0.5 �0.8 �1.2 �1.3 �0.5 �0.8D10 R459 �0.3 �0.2 �0.3 0.1 �0.3 �0.2F10 �0.3 �0.6 �0.4 �0.3 �0.4 0.4F14 �0.9 �1.0 �0.7 0.7 0.6 �0.6G11 0.5 0.4 0.1 3.1 1.5 0.9F17 COL �0.4 �0.2 �0.1 �0.1 0.1 0.4G17 �0.5 �2.1 0.6 0.4 1.6 �0.2D18 �0.4 �0.9 �0.5 0.5 �0.8 �0.8F24 �0.4 �0.7 �0.4 0.1 �0.1 0.2B24 CP3400 1.3 0.9 �0.4 1.5 �0.2 �0.5C18B 0.1 �0.1 0.1 0.3 0.2 �0.4Q15 �0.6 �0.3 �0.7 0.1 �0.4 �0.1C14C 0.3 0.2 0.2 0.3 0.3 0.0D16 8020 1.8 0.9 0.7 0.9 0.5 1.1F15 �1.0 �0.7 �0,8 0.2 0.4 0.3F61 �0.3 �0.4 0.1 0.3 0.6 0.4G15 �0.8 0.2 0.0 �1.4 0,6 �1.2

Table A2Average tangential deformations of the different treated samples for each treatment(negative values ¼ swelling). Empty cells correspond to too distorted samples.

Sample Consolidant Tangential deformations at RH (%)

35 45 55 65 75 85

F7 R100 �2.3 �2.4 �1.0 �0.5 �0.7 �1.0D9 1.1 0.9 0.1 2.2 1.9 1.2F8 �1.2 �0.9 �0.6 0.8 �0.4 �0.7G9 �0.2 �1.8 �1.1 �1.1 �0.3 �1.0D10 R459 �3.4 �0.9 �0.7 0.0 �3.5 0.5F10 �1.0 �1.9 �1.4 �1.2 �0.6 �1.1F14 �0.9 �1.1 �0.9 �0.8 �0.5 �0.5G11 �2.5 0.2 1.2 �0.3 �0.9 0.0F17 COL �0.5 �0.5 0.4 �0.2 �0.3 �0.3G17 0.2 0.0 0.0 0.6 �0.2 0.3D18 �0.9 �0.3 �0.6 �1.5 �0.6 �0.8F24 �1.1 �1.7 �1.4 �1.3 �1.0 �1.3B24 CP3400C18B �0.2 0.0 0,9 0.4 0.6 0.6Q15C14C �0.1 �0.1 �0.4 0.1 �0.2 �0.6D16 8020 0.2 1.3 0.4 1.8 1.3 2.4F15 �2.5 0.6 0.8 1.9 �2.4 �2.1F61 �0.9 �0.9 0.3 �1.1 0.6 0.0G15

Table A3Average radial deformations of the different treated samples for each treatment(negative values ¼ swelling). Empty cells correspond to too distorted samples.

Sample Consolidant Radial deformations at RH (%)

35 45 55 65 75 85

F7 R100 �0.6 �2.8 �0.3 �0.2 �0.4 �0.2D9 0.4 0.5 0.3 �1.0 0.4 0.4F8 1.0 0.5 1.0 2.5 1.9 0.6G9 �0.9 �1.1 �0.7 0.5 0.4 �0.5D10 R459 �2.3 �1.5 �2.2 �0.1 �1.4 �0.3F10 �0.8 �1.0 �0.6 �1.0 �0.7 �0.2F14 �0.1 0.6 �0.3 0.2 0.7 0.1G11 �1.4 0.1 �2.5 �1.3 �0.6 �1.4F17 COL 1.0 0.2 1.1 �0.3 0.1 �0.2G17 0.9 0.3 0.4 1.1 0.6 0.9D18 0.2 �3.1 0.0 �1.2 0.0 �0.5F24 0.1 0.2 0.1 �0.3 0.4 0.2B24 CP3400C18B 0.5 0.6 �0.2 0.7 0.6 �0.2Q15C14C �0.1 0.9 �0.4 0.5 0.8 �0.4D16 8020 0.2 0.5 0.6 1.6 1.6 �1.3F15 �0.2 �1.4 �0.7 �1.3 �0.8 0.0F61 �0.9 �1.0 �0.9 �1.2 �0.5 �0.5G15



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Documents

New trials in the consolidation of waterlogged archaeological wood with different acetone-carried products