Reactions of the Black Pigments.
Almost all black pigments consist of carbon, upon which reagents have no action. They should be at once heated on charcoal. If they burn away completely in the oxidising flame they consist of lamp black or carbon obtained by some process of incomplete combustion; if a white, infusible residue is left, the pigment is bone (ivory) black; if the residue is black the substance under examination must be ”neutral tint,” chrome black, or chrome-copper black. The two former give a pale green borax bead, whilst chrome-copper black gives a deep green bead, and when heated, moistened with hydrochloric acid, and again heated, it colours the flame green.
CHAPTER LXXI.
EXAMINATION OF LAKES.
In examining lakes it is necessary to ascertain the nature of the organic colouring matter, the base with which this is united, and also the nature of the substances mechanically mixed with the lake to lighten its shade.
It is most convenient to examine the pigment first for mechanical admixtures. If an effervescence follows the addition of hydrochloric acid, denoting the presence of a carbonate, this is generally calcium carbonate. White lead is rarely used, since it is very heavy and is more expensive. Gypsum and magnesia are occasionally added to lakes; they are recognised by examining the residue left on ignition.
In testing for the metallic oxide which is united with the colouring matter a small quantity of the lake is heated in a little porcelain crucible until all organic substances are completely destroyed. The residue is then examined for aluminium and tin oxides, by means of which lakes are most frequently prepared.