Commentary Page 183



	THEOPHRASTUS ON STONES

	to change yellow ochre into red ochre by roasting it, though no definite evidence exists for any earlier discoverer. Undoubtedly the effect of heat on yellow ochre must have been noticed in earlier times, but it is not at all improbable that the artist Kydias was actually die first who used the process deliberately to make a red pigment. 54. New earthen vessels are covered with clay and placed in ovens; for when the vessels become red-hot, they heat the ochre, and as they become hotter in the fire, they ma\e the color darker and more li\e glowing charcoal. It is not clear from the text whether open or closed pots were used. However, it is unlikely that closed pots were used for roasting yellow ochre since some vent must be provided for the steam that is produced when the ochre is heated. Probably the clay was applied to the exterior of the pots to protect them to some extent from the intense heat of the fire; this procedure would minimize breakage and the consequent loss or contamination of the product. Since new pots were used, it is probable that the vessels often cracked or broke on heating and so could not safely be used again. The conversion of yellow ochre into an artificial red ochre by roasting is essentially a process of dehydration: the water in the hydrated ferric oxide of the yellow ochre is expelled to form anhydrous ferric oxide of a characteristic red color. Though later ancient writers on technical subjects also allude to this process, they add very little to what Theophrastus says about the procedure employed by the ancient technicians. Pliny⁴⁰⁵ merely repeats the account given by Theophrastus, and Dioscorides⁰⁸ only mentions the process. Vitruvius⁴⁰⁷ does, however, describe very briefly a somewhat different process. He states that burnt ochre was made by heating a clod of good yellow ochre to a glow on a fire and then quenching it in vinegar. This is actually a more primitive process than the one described by Theophrastus much earlier. Since many technical works composed through the centuries contain accounts of very similar processes for preparing artificial red ochre, it seems that the procedure here described by Theo- 0'XXXV, ₃₅. 4oe_V, 112 (Wellmann ed., V, 96). *o⁷VH, 11, 2.

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