Page - 91 - in The Vienna Genesis - Material analysis and conservation of a Late Antique illuminated manuscript on purple parchment

Sophie Rabitsch, Inge Boesken Kanold, Christa Hofmann 91 the shells (Fig. 16). Together with some adherent tissue, the glands were removed from the animal with the help of tweezers and then put into a glass jar with water. The liquid turns violet within an hour, but the dyestuff has to be reduced to its soluble (leuco) form first, so that it is able to penetrate the (textile) fibres. This process takes time and is achieved in the following days by raising the pH value to 8–9 by adding potash or putrid urine. The vat, kept in a double boiler with lid, was heated to approx. 50 °C, and was kept warm and away from sunlight for at least a week. After a few days, the fermentation process starts. During the first days, the vat is purple coloured, but then changes into blue, blue-green and even- tually lime green, meaning the dyestuff is on its way to reduction118. This begins normally after three days, but for successful dyeing, one has to wait until the end of one week, as it was done in the described experiments. The liquid should be clear at this time, be- 118 Boesken Kanold, 2017, pp. 69–70. Fig. 15: Hexaplex trunculus. Fig. 16: Change of colour of the dyestuff in the hypobranchial gland from transparent to purple. Open-Access-Publikation im Sinne der CC-Lizenz BY 4.0