[271] The German blue was also liable to turn to a bright emerald green if exposed to damp air. This change has taken place in a great part of the painted ceilings of the Villa Madama, which Raphael designed for Cardinal de' Medici (afterwards Clement VII.) on the slopes of Monte Mario, a little distance outside the walls of Rome. [272] Because it was used by goldsmiths in _soldering gold_. [273] _Minium_ was largely used in the manuscripts of classical times; this is mentioned by Pliny (_Hist. Nat._ XXXIII. 122) who says _minium in voluminum quoque scriptura usurpatur_. [274] All natural earthy pigments owe their colours to the various metals, which in combinations with different substances give a great variety of tints. Thus, _iron_ gives red, brown, yellow and black; _copper_ gives many shades of brilliant blues and greens; and _manganese_ gives a quiet purple, especially in combination with an alcaline silicate. [275] Plutarch (_De defec. Or._ § 41) mentions flour made from beans as being used with _murex_ purple and _kermes_ crimson to give them sufficient body for the painter's purpose. [276] Kermes is the Arabic name for this insect. [277] It should be remembered that a large number of the mediaeval receipts and processes were not based on any reasonable principle, and endless complications were often introduced quite needlessly; this is well shown in a very interesting paper by Prof. John Ferguson of Glasgow on _Some Early Treatises on Technological Chemistry_, read before the Philos. Soc. Glasgow, Jan. 6, 1886. [278] The use of litharge as a drier was one of the most important improvements made in the technique of oil painting by the Van Eycks of Bruges in the first half of the fifteenth century. Before then, oil paintings on walls had often been laboriously dried by holding charcoal braziers close to the surface of the picture. Among the accounts of the expenses of painting the Royal Palace of Westminster in the thirteenth century (see above, page 110) charcoal for this purpose is an important item in the cost. Paintings on panel, being moveable, were usually dried by being placed in the sun; but, in every way, a good drier like litharge answers better than heat, either of the fire or of sunshine. [279] See Theophilus, I. 39. [280] See Vitruvius, VII. 10; and Pliny, _Hist. Nat._ XXXV. 41; and Dioscorides, V. 183. [281] Sometimes accidentally produced in domestic life by some overdrawn tea remaining on a steel knife. [282] The modern "lead-pencil" is wrongly named, being made of _graphite_, which is pure carbon. This does not appear to have been used in mediaeval times.