14Purple colours are also made by tinging chalk with madder-root and hysginum. Divers colours are also made from flowers. Thus, when dyers are desirous of imitating the Attic ochre, they put dry violets into a vessel, and boil them. When so prepared, they pour the contents of the vessel on to a cloth, and, squeezing it with their hands, receive in a mortar the water thus coloured by the violet, and then, mixing Eretrian earth with it, and grinding it, the colour of Attic ochre is produced.
2In the same way an excellent purple is obtained by preparing vaccinium, and mixing it with milk. So also, those who cannot afford the use of chrysocolla, mix blue with the herb weld, and thus obtain a brilliant green. These are called factitious colours. On account of the dearness of indigo, Selinusian chalk, or that used for making rings, is mixed with glass, which the Greeks call ὑαλος; and thus they imitate indigo.
3In this book I have explained, as they have occurred to me, the methods of making colours for painting, so that they may be durable and appropriate. Thus, in seven books, are methodically laid down all the rules that relate to the perfection and convenience of buildings. In the following book I shall treat of water, how it is to be found and conveyed to any place; as also how to ascertain its salubrity, and fitness for the purposes to which it is to be applied.