Verdigris[1] is also applied to many purposes, and is prepared in numerous ways. Sometimes it is detached already formed, from the mineral from which copper is smelted: and sometimes it is made by piercing holes in white copper, and suspending it over strong vinegar in casks, which are closed with covers; it being much superior if scales of copper are used for the purpose. Some persons plunge vessels themselves, made of white copper, into earthen pots filled with vinegar, and scrape them at the end of ten days. Others, again, cover the vessels with husks of grapes,[2] and scrape them in the same way, at the end of ten days. Others sprinkle vinegar upon copper filings, and stir them frequently with a spatula in the course of the day, until they are completely dissolved. Others prefer triturating these filings with vinegar in a brazen mortar: but the most expeditious method of all is to add to the vinegar shavings of coronet copper.[3] Rhodian verdigris, more particularly, is adulterated with pounded marble; some persons use pumice-stone or gum.

The adulteration, however, which is the most difficult to detect, is made with copperas;[4] the other sophistications being detected by the crackling of the substance when bitten with the teeth. The best mode of testing it is by using an iron fire-shovel; for when thus subjected to the fire, if pure, the verdigris retains its colour, but if mixed with copperas, it becomes red. The fraud may also be detected by using a leaf of papyrus, which has been steeped in an infusion of nut-galls; for it becomes black immediately upon the genuine verdigris being applied. It may also be detected by the eye; the green colour being unpleasant to the sight. But whether it is pure or adulterated, the best method is first to wash and dry it, and then to burn it in a new earthen vessel, turning it over until it is reduced to an ash;[5] after which it is pounded and put by for use. Some persons calcine it in raw earthen vessels, until the earthenware becomes thoroughly baked: others again add to it male frankincense.[6] Verdigris is washed, too, in the same manner as cadmia.

It affords a most useful ingredient for eye-salves, and from its mordent action is highly beneficial for watery humours of the eyes. It is necessary, however, to wash the part with warm water, applied with a fine sponge, until its mordency is no longer felt.