1. PURPLE colours are also manufactures by dyeing chalk with madder root and with hysginum. Other colours are made from flowers. Thus, when fresco painters wish to imitate Attic yellow ochre, they put dried violets into a vessel of water, and heat them over a fire; then, when the mixture is ready, they pour it onto a linen cloth, and squeeze it out with the hands, catching the water which is now coloured by the violets, in a mortar. Into this they pour chalk and bray it, obtaining the colour of Attic yellow ochre.
2. They make a fine purple colour by treating bilberry in the same way and mixing it with milk. Those who cannot use malachite green on account of its dearness, dye blue with the plant called dyer's weed, and thus obtain a most vivid green. This is called dyer's malachite green. Again, for want of indigo, they dye Selinusian or anularian chalk with woad, which the Greeks call i)sa/tis, and make an imitation of indigo.
3. In this book I have written down, so far as I could recall them, the methods and means of attaining durability in polished finishings, how pictures that are appropriate should be made, and also the natural qualities of all the colours. And so, having prescribed in seven books the suitable principles which should govern the construction of all kinds of buildings, I shall treat in the next of water, showing how it may be found in places where it is wanting, by what method it may be conducted, and by what means its wholesomeness and fitness may be tested.