The Conservation of Amber

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<ul><li><p>Maney Publishing</p><p>The Conservation of AmberAuthor(s): David Thickett, Pippa Cruickshank and Clare WardSource: Studies in Conservation, Vol. 40, No. 4 (Nov., 1995), pp. 217-226Published by: Maney Publishing on behalf of the International Institute for Conservation ofHistoric and Artistic WorksStable URL: http://www.jstor.org/stable/1506496 .Accessed: 27/12/2013 07:22</p><p>Your use of the JSTOR archive indicates your acceptance of the Terms &amp; Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp</p><p> .</p><p>JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact support@jstor.org.</p><p> .</p><p>Maney Publishing and International Institute for Conservation of Historic and Artistic Works are collaboratingwith JSTOR to digitize, preserve and extend access to Studies in Conservation.</p><p>http://www.jstor.org </p><p>This content downloaded from 66.77.17.54 on Fri, 27 Dec 2013 07:22:05 AMAll use subject to JSTOR Terms and Conditions</p></li><li><p>THE CONSERVATION OF AMBER </p><p>David Thickett, Pippa Cruickshank and Clare Ward </p><p>Summary-The British Museum (London) has extensive collections of amber artifacts. Analysis is carried out on amber for both authentication and provenancing. Infrared spectroscopy has been widely used in the past but other methods are becoming available. Several factors, including environmental conditions, contribute to the deterioration of amber, making it vulnerable to damage through handling. Where consolidation is required, the materials used must not interfere with possible future analysis. This paper considers the selection of resin and solvent mixtures for the consolidation of amber, as well as improved methods for storage and display. The materials recommended have well-known properties and can be removed from small samples prior to analysis. </p><p>Introduction </p><p>The British Museum in London has an extensive range of archaeological and historic amber artifacts. Amber was frequently used in jewellery, and there are many examples of beads, necklaces, pendants, brooches and fibulae in the collections, as well as carvings and containers. Very little has been pub- lished on the conservation of amber, and what is available gives rather negative and confusing advice [1, 2]. Some amber artifacts have suffered deteriora- tion from chemical alteration, which results in increased fragility, and subsequently physical dam- age has occurred. Although some of the amber in the collections was provenanced by Curt Beck in the 1960s [3, 4], additional methods of provenanc- ing are now available and further analysis may be carried out in the future. The importance of the amber collections in the museum and concern about the possible effect of any consolidants, adhe- sives and solvents on future identification and provenancing studies of amber [1] led to this research being carried out. As part of this work, conditions of storage were also considered. </p><p>Amber chemistry and analysis Ambers are fossilized resins dating from the early Cretaceous (135-65 million years) to the Miocene (26-7 million years) periods. They occur in a num- ber of locations, the major European source of amber being the shores of the Baltic Sea. Although the chemistry of all types of amber has not been completely elucidated, that of Baltic amber has been fully investigated. Its main constituent is a polyester formed from communol and communic and succinic acids. Slight differences in chemical composition between ambers have allowed methods of provenancing to be defined. </p><p>Received 11 January 1995 </p><p>Analysis is carried out on ambers for both authentication and provenancing, which is impor- tant for the study of ancient trade routes. Many techniques have been used [5], of which infrared spectroscopy has been the most widely applied. This requires as little as 50gg of sample. The identi- fication of a sample as amber is based on a ter- penoid-type spectrum with additional absorptions due to exocyclic methylene groups occurring at 3095, 1625 and 880cm-'. It should be noted that both copal and sandarac also show these absorp- tions, but these materials can be distinguished easily from amber by their solubility in certain solvents. The region between 1100 and 1250cm-1 is used to discriminate between European ambers of Baltic and non-Baltic origin [6]. Baltic amber is unique in that it has a single absorption maximum near 1150cm-' and a broad shoulder between 1175 and 1250cm- . </p><p>Infrared spectroscopy has limitations as a tech- nique for analysis of amber, since it cannot distin- guish between all types. This technique is also sensitive to contamination, for example by conser- vation materials, making identification, and espe- cially provenancing, difficult. The sample size is extremely small and would be taken from the sur- face. Since the surface of deteriorated amber can be porous, a sample may take up a relatively high pro- portion of consolidant. This, coupled with the sen- sitivity of the conditions used for provenancing, means that even the small amounts of conservation materials used for consolidation can have an effect on the infrared spectrum of amber. </p><p>More advanced techniques such as pyrolysis-gas chromatography-mass spectrometry are reported to be able to distinguish between all amber types [7]. Methods based on gas chromatography can easily distinguish between amber and modern synthetic resins used as conservation materials; hence these would not affect analysis using such techniques. </p><p>Studies in Conservation 40 (1995) 217-226 217 </p><p>This content downloaded from 66.77.17.54 on Fri, 27 Dec 2013 07:22:05 AMAll use subject to JSTOR Terms and Conditions</p></li><li><p>D. Thickett, P. Cruickshank and C. Ward </p><p>However, because of its wide availability, infrared spectroscopy is still often used for amber analysis and has been used in this study. </p><p>Degradation of amber </p><p>Amber degrades with addition of oxygen across the double bonds of the exocyclic methylene groups. This degradation can lead to one of two physical changes: a surface crazing (Figure 1), resulting in areas becoming weakened and susceptible to loss; and surface powdering (Figure 2), also resulting in weakening and loss of material. The British Museum's collection is used for study and research purposes and therefore sometimes needs to be han- dled. Degraded ambers may require consolidation to prevent mechanical damage through handling, and detached fragments need to be reattached. In addition, amber retrieved from archaeological exca- vations is often badly weathered and deteriorated. However, as it is important that any conservation treatment does not interfere with future analysis of the amber, non-interventive methods should always be considered first. Good storage and display meth- ods may eliminate the need for consolidation. </p><p>Figure 1 Detail of amber beads showing surface crazing. </p><p>1 Crm </p><p>Figure 2 Detail of surface powdering of an amber bead. </p><p>Review of past and current conservation treatments </p><p>A wide range of methods and materials has been used to conserve amber. 'Oil of amber' is often rec- ommended as a suitable consolidant [2, 8]. Its com- position is not well known. It may be a distillate of amber [9], or amber in a solvent such as linseed oil or aspic oil. Copal-derived 'oil of amber' is also available. Other varieties of natural terpenoid resin, including dammar, have also been utilized to con- solidate amber [1, 10, 11]. </p><p>Waxes, such as paraffin wax and beeswax, have been used as consolidants [1], and there are exam- ples in the British Museum collections of the use of waxes both to consolidate and to fill missing sec- tions of amber [11]. </p><p>Synthetic resins used as consolidants include polyvinyl acetates and acrylic resins. Synacryl 9122X (an acrylic resin) in dimethylbenzene (xylene) has been favoured in Denmark [12]. Paraloid B72 (an ethyl methacrylate methyl acrylate copolymer) in dimethylbenzene has recently been the preferred consolidant and adhesive for amber at the British Museum. Vacuum pressure has been applied to aid the deeper penetration of a consolidant into the amber [10, 12]. </p><p>The partial solubility of amber in certain sol- vents, such as propanone (acetone) or potassium hydroxide solution, has been utilized to make joins by softening the edges of the amber. Heat in con- junction with linseed oil has also been used to join amber sections [13]. </p><p>Waterlogged amber from archaeological excava- tions presents its own specific problems. Methods that have been used to dry out the amber prior to treatment include gradual dehumidification [14], desiccation under vacuum [12], and dewatering in propanone [15]. Consolidants applied to wet amber </p><p>218 Studies in Conservation 40 (1995) 217-226 </p><p>This content downloaded from 66.77.17.54 on Fri, 27 Dec 2013 07:22:05 AMAll use subject to JSTOR Terms and Conditions</p></li><li><p>The conservation of amber </p><p>include solutions of polyethylene glycols and polyvinyl acetate emulsions. Waterlogged amber has also been freeze-dried [16]. </p><p>Selection of conservation materials </p><p>Several resin and solvent mixtures were selected for evaluation as consolidants and adhesives for amber. It was decided to investigate materials with known consolidating properties, taking the approach that any added consolidants would be removed from small pieces of the conserved amber prior to analy- sis. </p><p>The properties considered important for a consol- idant or adhesive for amber are good chemical and colour stability, easy reversibility, good adhesive properties, minimal interference with the infrared spectrum of amber, and minimal alteration of the visual appearance of amber. Although it is desirable that a consolidant be reversible, any attempt to remove a consolidant from a friable surface will lead to some loss of material; hence, removal of consolidant from a complete object would not usu- ally be attempted. For analysis it would only be necessary to remove the consolidant from a small sample. The materials investigated are shown in Table 1. </p><p>Paraloid B72 was chosen for evaluation since it has recently been used with considerable success to conserve amber at the British Museum. Paraloid B67 was also selected, because it is soluble in less polar solvents than Paraloid B72, the solubility of </p><p>Table 1 Materials investigated </p><p>Material Chemical type Film cast from </p><p>Paraloid B72 Ethyl Dimethylbenzene methacrylate </p><p>Paraloid B67 Butyl White spirit methacrylate </p><p>Synacryl 9122X Acrylic resin Dimethylbenzene, as supplied </p><p>Acrysol WS 50 Acrylic colloidal dispersion Water, as supplied </p><p>Primal WS 24 Acrylic colloidal dispersion Water, as supplied </p><p>Mowilith DMC2 Polyvinyl acetate emulsion Water, as supplied </p><p>amber being greater in polar solvents. Polar sol- vents such as alcohols and ethers dissolved up to 25% of the amber in solubility tests [5, 6]. This obviously ruled out solvents such as industrial methylated spirit (IMS). White spirit, butanone (methyl ethyl ketone) and dimethylbenzene have been evaluated for their effects on amber and proved to have a low solvent effect [5]. They were therefore selected for use in this evaluation. Paraloid B67 has not been widely used in conserva- tion as it has been shown to be less flexible than Paraloid B72. However, amber is a brittle material and so flexibility is not a required property. Also it has been reported that butyl methacrylates such as B67 crosslink on light-aging to become less soluble [17]. However, this behaviour has not been observed in tests carried out at the British Museum. Synacryl 9122X was also tested, as good results have been reported [12]. </p><p>Consolidants that might be suitable for the con- servation of wet or waterlogged amber on archaeo- logical excavations were also included in the evaluation. Mowilith DMC2 has been used in the treatment of waterlogged amber. Primal WS 24 and Acrysol WS 50, were selected because they have been recommended as consolidants for organic materials, such as waterlogged bone and glass [18-20]. </p><p>The introduction of a material chemically similar to amber was thought to be inadvisable-for exam- ple, adding Baltic amber to non-Baltic amber would confuse analysis-and therefore 'oil of amber' was not included. Beck has suggested that only materi- als with no absorptions in the 1100-1250cm-1 region of the infrared, used for provenancing, should be employed to treat ambers [1]. Paraffin wax is such a material but was discounted in this study because of unsightly past repairs made using wax in the British Museum. </p><p>Evaluation of suitability for use with amber Thin films of each of the chosen materials (listed in Table 1) were cast and aged by exposure to heat (70TC for 28 days) and light (Microscal MTL 4000 bulb for 28 days). The reflectance spectrum of the films was collected using a Perkin Elmer 551S UV/visible spectrophotometer and CIELAB co- ordinates were calculated to allow the colour differ- ence (AE) to be determined. Infrared spectra of the films were obtained by sampling onto silicon car- bide discs and using DRIFTS on a Nicolet 510 FTIR spectrometer. A thin film of each material, aged in contact with amber, was also evaluated to determine any synergistic effects. The solubility of the 'as cast' and heat-aged films in white spirit, butanone and dimethylbenzene was determined. </p><p>Studies in Conservation 40 (1995) 217-226 219 </p><p>This content downloaded from 66.77.17.54 on Fri, 27 Dec 2013 07:22:05 AMAll use subject to JSTOR Terms and Conditions</p></li><li><p>D. Thickett, P. Cruickshank and C. Ward </p><p>The solubility of the light-aged B67 was also deter- mined. The results and rankings for interference of the infrared spectrum of a material with that of Baltic amber are shown in Table 2. </p><p>As expected, the Paraloids exhibited good stabil- ity and reversibility. Mowilith DMC2 and Synacryl 9122X yellowed on aging, and changes in the infrared spectra indicated that chemical changes had occurred. Further investigation of these materi- als was therefore abandoned. Acrysol WS 50 and Primal WS 24 showed reasonable stability (some changes in infrared spectrum) and reversibility and may be suitable for consolidation of wet or water- logged amber, where their water base would be an advantage. They were not tested further, as no such amber was available. </p><p>The removal of Paraloid B72 and B67 from amber was assessed as follows. Pieces of Baltic </p><p>amber were soaked for eight hours in a 5% w/v solution of Paraloid B72 in dimethylbenzene or 5% w/v B67 in white spirit. They were then allowed to dry under ambient conditions for seven days. Removal of the B72 from the consolidated amber was attempted by soaking the sample in dimethyl- benzene for 16 hours and allowing it to dry. The piece consolidated with B67 was similarly treated by soaking with white spirit. Infrared spectra of the amber pieces before consoli...</p></li></ul>