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.
Because it was used by goldsmiths in soldering gold.
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.
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.
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.