Seite - 160 - in The Vienna Genesis - Material analysis and conservation of a Late Antique illuminated manuscript on purple parchment

160 Alteration study of silver inks on parchment 1. Silver leaves were ground by hand in a porcelain bowl or with a pestel in a mortar. A grinding aid (e. g. sodium chloride, honey, gum arabic, glue) helped to finely disperse the silver. 2. Silver chips were ground with a pestel in a mortar with the addition of a grinding aid (as above). Coarse silver chips cannot be ground by hand in a porcelain bowl. 3. Silver leaves or silver chips could be used for the third method employing amalgama- tion. The silver was ground in a mortar together with mercury while being heated. The first possibility to achieve silver powder is by the mechanical grinding of silver leaves by hand or with a suitable tool like a mortar and a pestle. The use of a grinding aid is recommended in the literature: grainy additives like salt help to achieve a homogen- ous powder8. Sodium chloride was detected on different silver inks by analysis9. During the process of grinding with sodium chloride, silver can easily react with it forming silver chloride. Viscous materials like honey, gum or glue are mentioned in the Late Antique Greek Leiden and Stockholm papyri for helping to disperse the metal powder10. Addition- ally, a small amount of water can be added as a lubricant11. Mechanical grinding by hand (using the index finger) was executed in a large, smooth porcelain bowl without any inside edges, using highly viscous gum arabic as a grinding aid. The technique, still used in oriental book illumination, was demonstrated by Cahit Karadana12. After the silver leaves were mixed with gum arabic, they were ground with circular movements starting from the bottom of the bowl for up to two hours. During this process the mixture of gum arabic and silver was thoroughly dispersed to the rim of the bowl. The process was continued while single drops of water were added from time to time, until no coarse particles were visible (Fig. 2). Trost cites recipes calling for salt, red wine and vinegar to be used as grinding aids. The second way to obtain silver powder is by grinding silver chips in a mortar. A marble mortar and pestle were used. Similar to the first method, coarse grinding aids like salt were added. Water, red wine or vinegar were added dropwise as lubricants, depending on the recipe (Fig. 3). The grinding process took between 10 and 20 minutes. In the case of both methods, grinding aids as well as larger particles and possible impur- ities had to be removed after the refining process, by flotation, the thorough washing of the silver powder. Therefore, the silver powder was transferred into a porcelain bowl, if it had not already been ground in one, then finely dispersed in a great volume of water (Fig. 8 Oltrogge, 2011, p. 66. 9 Schreiner and Oltrogge, 2011, p. 107. 10 Oltrogge, 2011, p. 66. 11 Schreiner and Oltrogge, 2011, p. 107. 12 Cahit Karadana, book conservator at the Institute for Conservation, Austrian National Library. Open-Access-Publikation im Sinne der CC-Lizenz BY 4.0