| Lime white. |
Lime white.Bianco di San Giovanni was in all respects one of the best of the whites used by illuminators.
This was simply pure lime-white, made by burning the finest white marble; the lime was then washed in abundance of pure water, then very fine ground and finally dried in cakes of a convenient size; see Cennino Cennini, § 58; and Theophilus, I. 19. The medium used with it was the purest size or gum Arabic of the most colourless kind.
Another white pigment was made of powdered chalk and finely ground egg-shells; this was a less cold white than the bianco di San Giovanni.
| White lead. |
White lead.White lead (cerusa or biacca) was also used[[279]], especially by the later illuminators, but with very unfortunate results, since white lead is liable to turn to a metallic grey or even black if exposed to any impure sulphurous atmosphere.
Many beautiful manuscripts have suffered much owing to the blackening of their high lights which had been touched in with white lead; especially manuscripts exposed to the gas- and smoke-poisoned air of London or other large cities.
| Process of manufacture. |
Process of manufacture.The biacca of the mediaeval illuminator was made in exactly the same way that Vitruvius and Pliny describe; see Vitr. VII. xii.; and Pliny, Hist. Nat. XXXIV. 175.
A roll of lead was placed in a clay dolium or big vase, which had a little vinegar at the bottom. The top was then luted down, and the jar was left in a warm place for a week or so, till the fumes of the acetic acid had converted the surface of the lead into a crust of carbonate. This carbonate of lead was then flaked off and purified by repeated grinding and washing.