Radlat. Phys. Chem. Vol. 32, No. 2, pp. 275-280, 1988 Int. J. Radlat. Appl. Instrum. Part C Printed in Great Britain

0146-5724/88 $3.00 + 0.00 Pergamon Press plc

APPLICATION OF RADIATION CHEMISTRY FOR COI~SERVATION OF ARCHAEOLOGICAL WATERLOGGED

WOOD AND OSTEOLOGICAL OBJECTS

TINO G.i.UMANN, THOMAS S. KOWALSKI and ANDR]~ MENGER Institute of Physical Chemistry, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland

(Received 6 July 1987)

Abstraet~The application of radiation chemistry to the conservation of archaeological objects is described. Methyl methacrylate is introduced into the voids of the objects to be conserved either by an exchange process or--if allowed by the mechanical strength of the object--by vacuum impregnation. It is neither necessary nor desirable to fill the object completely with the monomer that is subsequently polymerized with ~°Co ~-rays.

1. INTRODUCTION

Archaeological objects which have long been in con- tact with water or humidity cannot be dried without irreparable damage. The aim of this research was to improve the methods of conservation of archaeo- logical artifacts, such as woods, bones, horns, antlers, animal teeth, ivory and other organic objects, excavated from wet surroundings.

Several methods have been used to conserve waterlogged °'2) and dried n 0 3) bo es , and other organic artifacts. 0,4) However the efficiency and range of application of most of the classical methods of preservation are not sufficient, c~'5)

Radiation-indUced polymerization is one of the fields in radiation chemistry that has an important practical value.

Although development of radiation technology and its introduction to archaeology and conservation is relatively recent, the radiation curing can be traced back to ancient Egypt. Four thousand years ago the Egyptians exposed mummies to the sun in order to solidify paints prepared with (unsaturated) bitumes of Judea. ~6) This curing of paints is a polymerization process. Application of radiation and polymer chemistry for conservation of dry archaeological artifacts (ceramics, wood, stone, bone) ¢7) as well as waterlogged ones (wood, bone)¢S) are examples of the contribution of chemistry towards the preservation of our cultural heritagefl )

Our present work originates from experiments with waterlogged wood in this laboratory. °°) The nature of waterlogged archaeological materials is rather special and depends on many factors. ~4"H'~2) The role of water and a wet environment in the preservation of archaeological artifacts is crucial and can be either positive or negative. (4*'m3'n4)

Bones, horns and antlers form a class of their own among archaeological objects of biological origin, since they are composite materials. They contain

mineral salts (about 70%), deposited among a net- work of organic fibres (about 30%). ~2~.7° Fossil bones may have lost up to one third of their organic sub- stance, thus weakening their mechanical strength: 2~)

The aim of a conservation process should be to give back to the object its original appearance and robustness. In this work, we would like to report our experiences in this field using radiation-induced poly- merization. Most of the conventional methods of preservation of cultural goods are time consuming: 4") The vacuum impregnation used in this work followed by polymerization of a monomer with ~-rays is a relatively fast and promising technique which can be used for conservation of waterlogged objects. The simultaneous sterilizing action of ~-radiation is an additional advantage of the method.

2. EXPERIMENTAL

2.1. Mater ials and chemicals

The 3000 years old waterlogged wood was from Lake Zfideh, °3'15) the waterlogged osteological sam- ples from Lake Neuchfitel v3:~) (4000-6000 years old) and from Lake Bienne, tl3) (excavation site Sutz V, 4600- 4700 years old), all of them in Switzerland. The dimensions of the waterlogged osteological samples were: length 3--40 cra, width 0.5-8 crn and thickness 0 .5-3cm. The weight varied between 1 and 500g. The amount of water in the samples of waterlogged wood was about 90%; in the waterlogged osteo- logical samples it varied between 7 and 60%. Percents are given in terms of the initial weight.

The ethanol used was denaturated with 5% meth- anol and contained less than 0.2% water. Monomers used in our investigation were styrene ( > 99% purity, stabilized with 40-50 ppm, 1,2-dihydroxy-4-t-butyl- benzene) and methyl methacrylate ( > 99%, stabilized with 25 ppm hydrochinone). The samples were identi- fied, documented °Tas) and their age was determined by radiocarbon dating. °9)

275

276 TINO G.~UMANN et al.

2.2. Methods The percentage of water in waterlogged wood

was estimated after one week of oven drying at 120°C. The water content in osteological samples was calculated after one month of oven drying at 120°C. However this method could not be applied to con- served museum objects or combined bone-wood artifacts (e.g. bone axes). In these cases pumping to a residual vacuum below 1 torr was performed.

2.2.1. Treatment of waterlogged samples. The samples were washed with tap water and then dehydrated in absolute ethanol during 1-3 weeks at room temperature before impregnation. The alcohol was changed several times. The length of time is determined by the smallest dimension of the piece. The concentration of water and later of monomer in the alcoholic solutions were determined by gas chromatography. The concentration profile in the wood and bone samples was not determined. The next step was the (partial) replacement of the alcohol by a monomer. The same procedure and time scale were applied as in the water/alcohol exchange.

2.2.2. Vacuum impregnation of wet samples. The idea of vacuum impregnation is to pump water, air and other volatile ingredients out of the voids of the osteological samples and to force the solvent under pressure into the sample. This method may also be applied to waterlogged wood, but because of its missing structure stability (which puts a limit to the vacuum impregnation procedure), we used in some experiments the milder procedure of lyophilic drying.

The samples were then placed in a vacuum glass vessel of an appropriate volume. The vessel is con- nected to a vacuum installation, first to a water pump and then to a rotary pump. When a residual vacuum of about 0.1 torr was attained, the pumping system was shut off and the previously degassed monomer was sublimed onto the sample. Finally air was admit ~- ted to atmospheric pressure. The time of pumping varies from 5 h to 3 days depending on the size and number of the pieces: the monomer solution used for impregnation had to be separated from the stabilizer prior to its addition to the sample. Degassing was done by several freezing-thawing cycles at liquid nitrogen temperature before distilling the monomer into the sample. The transfer of the monomer took between 3 and 8 h. The flask with the monomer was warmed in a water bath (30-37°C) and the sample vessel cooled in liquid nitrogen ( -196°C).

After melting at room temperature the liquid monomer penetrated the empty pores of the sample under its own vapour pressure (for methyl meth- acrylate, MMA, 50 tort). The object stays 3-6 days under these conditions. Then air is admitted under atmospheric pressure for another 2-4 days.

2.2.3. Radiation polymerization. Methyl meth- acrylate (MMA) was selected as monomer on the basis of our previous experiments. It showed that this monomer, compared to styrene, requires a smaller radiation dose for polymerization (2.5-3 Mrad = 25-

30 kGy) and thus the sample is less likely to change colour under irradiation. For example, an antler treated with styrene darked after a y-irradiation of 20 Mrad (200 kGy), the dose necessary for the com- plete polymerization of styrene. A further advantage of a M M A is its high reactivity even at low tempera- ture. This allowed us to use an irradiation tempera- ture of 0°C to prevent evaporation of the monomer.

The exchange in the system alcohol-MMA will never be complete. We determined in vitro that the polymerization under irradiation ceases at MMA concentration below 5%. A major disadvantage of M M A is the inhibition of its polymerization by oxygen, common to all radical polymerizations. ~2°) Evaporation of monomer before and during the polymerization and trace of impurities are among the reasons for an incomplete polymerization within the pores. Polymerization is an exothermal process. A temperature of about 80°C was obtained during radiation polymerization of vinyl acetate in archaeo- logical objects. ~) However it is possible to adjust the dose rate to limit the rise of the temperature to an acceptable level. ~7e,sb) In order to reduce the effect of monomer evaporation, we made a series of experi- ments with different irradiation temperatures between -200°C and 40°C. It turned out that 0°C was the most reasonable compromise between evaporation losses, rate and convenience of polymerization. The samples were wrapped in Al-foil, packed in PVC bags, which were sealed and put in Styropor con- tainer filled with ice before irradiation. It might be mentioned that with larger samples evaporation is less important.

The polymerization was carried out in a Sulzer radiation chamber using 6°Co radiation. The dose rate was 0.3 Mrad/h. We kept the samples in the sealed PVC bags for an additional time after irradi- ation ( I -3 days). This storing of the samples in the M M A vapour allows some post-polymerization to take place; this was checked in glass vials under the same conditions. After removing from the bags, the samples are dried to constant weight under room conditions.

3. RESULTS AND DISCUSSION

3.1. The diffusion problem

We had to exchange the water with a solvent where the monomer is soluble. The speed of exchange of water with the solvent, followed by another exchange between the solvent and the monomer is limited by the speed of the diffusion that is determined by the viscosity and the pore size: ~) The concentration change can be expressed to a first approximation by:

c, = cr (I -- exp(--t]¢))

where c, is the concentration after the time t and Cr is the final concentration. The diffusion coefficient is

Radiation chemistry in archaeological conservation 277

contained in the time constant r that gives the time for a change of c, by two thirds. As a rule one has to wait five to ten times the value of the time constant for a complete exchange. The exchange of water by alcohol is a rather fast process. According to Graham's law the diffusion coefficient of a compound is approximately inversely proportional to the molec- ular weight. Thus the second exchange (alcohol- MMA) is much slower than the replacement of water by alcohol. An increase of temperature by 30°C decreases the time constant by a factor of two corresponding to an apparent activation energy of about 25 kJ/mol. This figure is representative for our standard samples of waterlogged wood. The time constant will increase approximately with the square of the smallest distance d of the sample (in our case often about 2 cm). The time constant reaches 6 h. The pore size of non-wooden sample is often smaller, thus slowing down the speed of the diffusion. In order to decrease the time constant we increased the exchange temperature. An upper limit of 50°C is given by the stability of the system and by practical reasons. That is why we preferred to conduct the experiments along the following lines (whenever feasible):

----diminishing the polymer content of the sample - -vacuum impregnation in order to avoid exchange

processes.

3.2. Evaluation o f the results

The purpose of this investigation was to look at some factors influencing the treatment of the water- logged objects. Comparing samples of different archaeological origin is always difficult, even when the objects are similar. Though bones have laid in the same environmental conditions for thousands of years, they are not similarly degraded; the degree of deterioration depends upon the condition of the object and the length of the time before they were immersed in water. (::) The results of the treatment depend on the structure of the material to be con- served. The relative fraction of the spongy and the compact parts of an object is an important factor, because the spongy part is more easily filled with the monomer.

Another problem is the choice of reference samples. Whenever feasible, we preferred the comparison with an identical sample, especially for ageing tests. (~) Otherwise, we divided the objects into two similar parts, one serving as a reference. This method may alter the properties of the sample and is not applicable for valuable objects.

The extent of impregnation of the waterlogged sample was calculated from the known water content, assuming a 100% impregnation if all water was replaced by an equal volume of monomer (Table 1). The results of polymerization were assessed on the basis of the resultant polymer content and on the final appearance of the object after treatment. In

¥ + ~ . ~ .~ ~ + o

<

÷

- 5 =

7-

e q t ¢ ~

,~ "r, • ~1" r - - o o

e ~ e ~ o o ¢ q

m . .-r ,

,.~ ,z-- L~ ~

278 TINO Gk, UMANN et al.

order to evaluate the results of polymerization, we use the following definitions:

- - re la t ion of the polymer content in the sample to its maximum possible content. This relation is defined as the percent of volumetric replacement of water in waterlogged samples by polymer:

Rv = (VpMMA/Vnzo) X 100 VO1% (1)

where VVMMA: volume of the polymer (PMMA) in the sample after y-irradiation, VH~o: volume of the water in the waterlogged sample;

- - t he yield of polymerization:

: (GpMMA/G'PMMA) 100%

- - (GpMMA/GMMA) >( 100% (2)

where GpMMA: weight of polymer (PMMA) in the sample after ?-irradiation, G~MMA: weight of polymer corresponding to complete polymerization of monomer, GMMa: weight of monomer in the impregnated sample before ~-irradiation;

- -po lymer content of the treated sample based on its dried weight:

C d = (GpMMA/Gd) × 100 Wt% (3)

where Gd: weight of the dried sample.

After polymerization a polymer layer is formed which covers the inner surface of the wood or bone (14) and retards the uptake of water, gives them mechan- ical strength and makes them more solid. The results of the treatment of waterlogged osteoiogical samples (extreme value for each batch) are given in Table I.

Each of the batches of osteological objects was treated separately according to our method. The values of the factors defined by equations (1) to (3) varied between batches. It can be assumed that the reasons for these differences result from structural variations of the bones and the degree of their deterioration. It is also possible that the conditions during the impregnation process were not completely reproducible and thus influence the results. Other aspects of these results are discussed elsewhere314)

In Fig. 1 we show samples of waterlogged wood (a) containing 90% wt of water and (b) after drying in an oven. Linear shrinkages from waterlogged to oven dry conditions are the following: longitudinal 10-20%, radial 25% and tangential 70%. This illustrates the destruction of waterlogged wooden archaeological objects caused by uncontrolled drying.

In Fig. 2 we show a sample which was dehydrated in ethanol, subsequently impregnated with MMA and irradiated to 3 Mrad (30 kGy). In this case the polymer content in the wood was about 40% of the maximum amount possible. Shape and structure of the wood is well preserved. It allows a determination of the species as well as detailed inspection of the kind of treatment that had been applied to the wood (scanning electron microscopy).

Figure 3 shows a comparison of the appearance of two parts of the same osteological sample with and without treatment. It can be seen that after drying in the atmosphere and even after artificial ageing tests (14/ the treated sample preserves its shape better.

Only a few samples of antlers collapsed during the vacuum impregnation process and none during the exchange process. However a later examination of their structure suggested a iligh degree of deterior- ation previous to impregnation revealing internal cracks and the presence of stresses which could not be seen before. They were most probably responsible for the destruction of the samples.

Although the first aim of developing conservation methods is the stabilization of the dimensions of the samples, the ethic and aesthetic aspects should also be taken into account. Therefore well conserved objects should retain their natural matt look without any additional glossy "plastic" appearance of the sur- face. (1'8b'23'24) Our treated samples seem to the most extent to fulfil such requirements. The colour of bones (grey-brown) is similar to their natural appearance, the surface is not sticky and the traces of polymer on the surface are reduced to a minimum. We checked also the dimensional changes of the treated objects, their cracks and their general appearance after several months of storing under normal conditions. No

(a) (b) Fig. 1. Samples of waterlogged wood (a) containing 90% of water, (b) after drying one week in an oven.

Radiation chemistry in archaeological conservation 279

(a) (b)

Fig. 2. Samples of waterlogged wood (a) after exchange with alcohol, (b) after polymerization (compare with Fig. lb).

(a) (b)

Fig. 3. Sample of antler after dividing into two pieces (a) fight is the untreated sample, left after treatment; (b) appearance of both pieces after artificial ageing.

dimensional changes of their initial cracks could be observed. A final evaluation of the applied method depends on the results of tests during which the treated samples underwent the artificial ageing test. The examples and results of these tests are presented elsewhere. (14)

3.3. Conclusions

1. The vacuum impregnation method followed by polymerization of a monomer with ~,-irradiation is a relatively fast method which can be used for conservation of waterlogged and wet objects.

2. Radiation-induced polymerization of archaeo- logical organic objects with methyl methacrylate seems to give satisfactory and promising results. In order to judge the final results of our treatment, it is necessary to consider substantial factors of radiation polymerization of objects, the appearance after treat- ment and after artificial ageing.

3. The substantial factors characterizing results of treatment depend on the type and the structure of the osteological objects.

4. Methyl methacrylate was preferred for the treat- ment because it requires lower doses of radiation for polymerization and can be carried out even at low temperatures.

5. It is not necessary to fill the objects completely with polymer. Samples with polymer content close to its lower limit show a better appearance while still preserving sufficient mechanical strength.

6. Special packing and cooling during irradiation results in a higher polymer content within the object.

7. The prolonged time of keeping the sample in its packing after irradiation enable a post-polymeriz- ation process which results in an increase of the total yield of polymerization.

Acknowledgements--The authors would like to thank Dr B. Miihlethaler and Mr W. Kramer of the Swiss National Museum in Ziidch, Mr G. Kaenel, Director of the Cantonal Museum of Archaeology and History in Lausanne and Dr H. Grfitter of the Cantonal Office of Archaeology in Berne, all in Switzerland, for supplying archaeological waterlogged wood and osteological objects for our research.

We thank the Swiss National Science Foundation for a grant.

280 TINo G.~UMANN et al.

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