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THEOPHRASTUS ON STONES
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several times a day with a spatula until all the copper had reacted. Dioscorides437 also describes similar processes, including one tliat resembles the method described by Theophrasrus. In this process plates of copper were buried in sour grape skins for a number of days.
A method of preparing verdigris by means of acetic acid vapor acting on a sheet of pure copper is described in detail in die Stockholm Papyrus*3* and other accounts appear in medieval technical works.
It is interesting to note that die particular method mentioned here by Theophrastus is, in principle at least, still in use today and probably has been used more or less continuously throughout tlie intervening centuries. The method of preparing verdigris by die action of sour grape skins on copper plates is mentioned by early modern writers on chemistry and chemical technology. Boerhaave,439 for example, refers to it, and Hill440 gives an account of die commercial process that was usual in his time. At present, die manufacture of verdigris by this method is centered in die wine districts of France, particularly at Grenoble and Montpellier.441 Usually the marc, which is the waste matter consisting of die skins and stems of grapes, is first allowed to ferment, eidier in large vessels or in special rooms. After the fermentation has proceeded to the proper stage, die pasty mass is spread on diin sheets of copper. Next, piles of alternate sheets of copper and layers of fermented marc are built up. These are allowed to stand from two to five weeks, depending upon the temperature, and are then dismantled. If die process has been successful, die copper sheets are covered widi fine green crystals of copper acetate. The sheets are then exposed to die air and moistened from time to time widi water or damaged wine. As a result of this treatment they become coated with a tJiick layer of basic copper acetate which is detached, kneaded widi a little water, and pressed into cakes or
437 V, 91 (Wellmann ed., V, 79).
438 Lagercrantz, Papyrus Graecus Holmiensis, pp. 20, 194.
439 H. Boerhaave, Elements of Chemistry, trans. T. Dallowe (London, 1735), Vol. II, p. 152.
440 Theophrastus's History of Stones, p. 134.
441T. E. Thorpe, Dictionary of Applied Chemistry (London, 1921), Vol. I, p. 24; F. Ullmann, Unzyhjopadie der technischen Chemie (Berlin, 1929), Vol. IV, p. 676.
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