Charcoal-Black

Synonyms:

Charcoal-Black, Blue-Black, Vine-Black, Frankfort-Black, Noir De Vigne, Rebschwartz

When non-resinous woods and woody tissues are strongly heated in crucibles or other almost completely-closed vessels, the residue contains the greater part of the carbon of the original material, and preserves its form. As the charcoals thus made contain some soluble mineral matter, particularly potash salts, they should be coarsely ground and thoroughly washed with hot water previous to their being finally converted into paints. They are also hygroscopic, and therefore need to be dried before being ground in oil.

Amongst the best materials for producing these charcoal or vegetable blacks may be named the hard shells or stones of plums, almonds, and similar fruits; coco-nut shell broken into small pieces; cork cuttings, and the twigs of beech and shoots of vine. Coco-nut yields the densest, most velvet-like black; vine-shoots a less solid black, having a bluish hue. Washed wine-lees yield a rich black - Frankfort-black.

When charcoal has been prepared at a sufficiently high temperature, and then has been thoroughly washed, there is no question as to its permanence as a pigment in all media. But if it retains tarry matters, or has been imperfectly carbonized, then it may become greyer or less brown (when used in thin washes) after long exposure to light. This change is due to the oxidation of the tarry matters, or of the brown substances which are intermediate in composition between the original vegetable materials and the carbon, which is the final product of their torrefaction out of contact with the air. And one peculiar property, possessed in varying degrees by charcoal of every kind, must not be lost sight of. Charcoal withdraws the majority of organic colouring matters from suspension, and even from solution in water. A pale tint of rose-madder in water, if a pinch of coco-nut or vine-black be added, becomes rapidly decolourized, the pigment being completely absorbed, although not really destroyed. Animal charcoal (bone-black, ivory-black) exerts a still more energetic action of this character; but lamp-black is less powerful.

It is to be regretted that the beautiful bluish blacks, derived from such materials as the shoots of the vine and beech, are frequently fraudulently imitated by mixtures of lamp-black and indigo. The purple vapours of indigotin, given off when such a mixture is heated in a test-tube, betray the adulteration at once.