The Ten Books on Architecture, 7.9
Of the Preparation of Vermilion
9I now return to the preparation of vermilion. When the clods are dry, they are pounded and reduced to powder with iron beaters, and then, by means of repeated washings and dryings, the colour is produced. When this is effected, the vermilion, deprived of the quicksilver, loses its natural tenacity, and becomes soft and disconnected; and used in the last coat of the plastering of rooms, keeps its colour without fading.
2But in open places, such as peristylia or exedræ, and similar situations whereto the rays of the sun and moon penetrate, the brilliancy of the colour is destroyed by contact with them, and it becomes black. Thus, as it has happened to many others, Faberius, the scribe, wishing to have his house on the Aventine elegantly finished, coloured the walls of the peristylia with vermilion. In the course of thirty days they turned to a disagreeable uneven colour; on which account he was obliged to agree with the contractors to lay on other colours.
3Those who are particular in this respect, and are desirous that the vermilion should retain its colour, should, when the wall is coloured and dry, rub it with a hard brush charged with Punic wax melted and tempered with oil: then, with live coals in an iron pan, the wall should be thoroughly heated, so as to melt the wax and make it lie even, and then rubbed with a candle and clean cloth, as they do marble statues. This practice is called καῦσις by the Greeks.
4The coat of Punic wax prevents the effect of the moon’s as well as that of the sun’s rays thereon which injure and destroy the colours in work of this nature. The laboratories which were formerly carried on at the mines in Ephesus are now transferred to Rome, on account of mines of the same sort having been discovered in some parts of Spain, whence the clods are brought and worked by manufacturers at Rome. These laboratories are situated between the temples of Flora and Quirinus.
5Vermilion is occasionally adulterated with lime. The following is a method by which its goodness may be proved. Let the vermilion be placed on an iron plate over the fire, and remain till the plate is red hot: when the heat has changed the colour, and it appears black, let the plate be removed from the fire. If, when cooled, it returns to its original colour, it may be considered pure. but if it remain of a black colour, it is quite clear that it has been adulterated.
6I have written all that I remember respecting vermilion. Chrysocolla comes from Macedonia, and is found in the vicinity of copper mines. Minium and indigo, by their names, indicate the places from whence they are obtained.