Page:Vitruvius the Ten Books on Architecture.djvu/257

219 an oven. As soon as the copper and the sand grow hot and unite under the intensity of the fire, they mutually receive each other's sweat, relinquishing their peculiar qualities, and having lost their properties through the intensity of the fire, they are reduced to a blue colour.

2. Burnt ochre, which is very serviceable in stucco work, is made as follows. A clod of good yellow ochre is heated to a glow on a fire. It is then quenched in vinegar, and the result is a purple colour.

1. is now in place to describe the preparation of white lead and of verdigris, which with us is called "aeruca." In Rhodes they put shavings in jars, pour vinegar over them, and lay pieces of lead on the shavings; then they cover the jars with lids to pre­vent evaporation. After a definite time they open them, and find that the pieces of lead have become white lead. In the same way they put in plates of copper and make verdigris, which is called "aeruca."

2. White lead on being heated in an oven changes its colour on the fire, and becomes sandarach. This was discovered as the re­sult of an accidental fire. It is much more serviceable than the natural sandarach dug up in mines.

1. now begin to speak of purple, which exceeds all the colours that have so far been mentioned both in costliness and in the superiority of its delightful effect. It is obtained from a marine shellfish, from which is made the purple dye, which is as wonderful to the careful observer as anything else in nature;