On tempering azure. I have nothing very definite to say about azure, for some temper it with goat's-milk, others with woman's-milk, others with white of egg; and any of these is good enough. Now as soon as I see that it is necessary, I put the azure, and at the same time, a little water, into a basin, and rub it with my finger until it is all moistened. Then pour in water, and when it has all been mixed up thoroughly, and the blue has settled to the bottom, I pour off into another vessel the water thus soiled by the impurity, keeping back the precious colour, which remains in the bottom of the vessel. For it is of such a nature that, the purer it is, the more quickly it sinks to the bottom; the impurities, being lighter, rise to the top. But if it seems to be necessary, repeat the operation several times, pouring water in and decanting it repeatedly, until the blue is purified. And when it is well purified and ground with water, I place it in a receptacle. And when the water has been removed, and I wish to do anything with the colour, I put some very clear egg white into it, and this I remove again at the end of one hour. For if it should stay in the mixture longer, it would injure the colour, taking away something of its natural brilliance. And after I have removed the egg, I fill the vessel immediately with cold water, and then decant that, in the way I have already described. You apply this colour on a wall with water and wine, but upon wood, with oil.