596 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



are now being used on an extensive scale as adhesives and consoli- 

 dants. The factor of paramount importance in the choice of a par- 

 ticular material is the amount of shrinkage which occurs when the 

 resin sets. If the degree of shrinkage is excessive, considerable con- 

 tractile forces may develop and there is a danger that serious dis- 

 tortion of a fragile object may occur. Tests have been carried out 

 on many synthetic resins, and it has been found that the epoxy resins, 

 which set by a condensation reaction that does not involve the loss 

 of any volatile material, show a minimum contraction [5]. 

 These materials are, therefore, particularly suitable in conservation, 

 because they not only set without appreciable contraction, but they 

 are also easy to use because certain of them set in situ from the liquid 

 to the solid state at room temperature using a special hardener and 

 catalyst. One example will suffice to show the great advantages 

 of the epoxy resins in the consolidation of fragile objects. In 1958 

 a number of silver objects of early Christian origin were discovered 

 on the site of the ninth-century church in St. Ninian's Island in the 

 Shetlands. Among these there was a silver hanging bowl, which 

 was of particular archeological significance because it is the only 

 surviving example of a silver hanging bowl found in these islands. 

 The bowl was covered with corrosion products formed by the 

 mineralization of the copper present in the silver alloy, and when 

 these had been removed, the silver which remained was egg-shell 

 thin and brittle so that the object was extremely fragile. It was, 

 therefore, necessary to devise some form of internal support to act 

 as a permanent reinforcement using a material which must have 

 the following properties: (1) it must be colorless and transparent, 

 (2) it must be strong but not brittle, (3) it must adhere well to the 

 silver, (4) it must set without appreciable shrinkage, and (5) it 

 must be easy to apply without using too much heat. The only 

 material which appeared to satisfy these criteria was a cold-setting 

 epoxy resin, which could be brushed on the inside of the bowl as a 

 viscous liquid and allowed to set as the bowl was slowly rotated on 

 a turntable. It was by this means possible to build up a uniform 

 thin layer of clear resin which gave sufficient mechanical strength 

 to the bowl so that it could be handled with safety [6] . 



The only disadvantage of these epoxy resins from the conservation 

 point of view is the fact that the hardeners used in the formulation 

 of the cold-setting types are amines, and therefore not entirely 

 suitable for the consolidation of bronze objects because of the risk 

 that the hardeners may react with the bronze forming unsightly 

 green or blue compounds. Hence, for the consolidation of bronze 

 objects, it is necessary to employ a different type of synthetic resin. 

 One of these is a special preparation of a polymethylmethacrylate 

 resin manufactured in Germany under the trade name "Technovit." 



