It is not so easy as it appears at first sight to judge of the quality of a black pigment by its appearance. To the unaccustomed eye a pigment may appear to be of an unexceptionable black, which to the expert appears decidedly brown; only long practice can give the eye the requisite keenness. In addition to the test already given—of smearing on white paper—there is another especially to be recommended to the inexperienced for discriminating between pigments. A small quantity of the black under examination is intimately mixed with a white pigment; white lead or zinc white is very suitable. If the mixture has a pure grey shade, the black may be regarded as of good quality, but if it contains brown substances the mixture has an indefinite, dirty shade instead of a pure grey. This is a sure sign that the black requires further purification.
Although caustic soda is now very cheap, its use for the purification of soot black is tolerably costly, because it entails much labour. This process is therefore only used for the finest qualities which are to be employed for copper-plate inks and black coach colours. For inferior qualities the method of calcination is used, which produces, when properly carried out, a black of such purity that it can be used for the preparation of even the finest black paints.
Calcination of the Soot.—The substances which give the soot its brown colour are products of dry distillation, and hence are all volatile at a certain temperature; they can be separated from the soot by heating it in the absence of air. The temperature necessary completely to volatilise these compounds is fairly high. The soot must be heated to a good red heat to obtain a safe result.
When the soot is heated too quickly or to too high a temperature it undergoes an alteration which affects the quality of the product. By too long or too vigorous ignition the soot changes its flocky consistency to a sandy nature; it will then require much longer grinding with oil to produce a uniform mixture than is the case with the light flocculent soot, which very readily mixes with oil.
The soot is ignited in sheet-iron boxes with a coating to protect the metal from burning. This coating is best made from clay and hair. A very thin paste of clay and water is painted uniformly over the boxes with a brush; when the first coat is dry a second and third are given. When once the metal is completely covered with clay, several coats are given of clay mixed with chopped tow until the layer is several millimetres thick. The coating carefully made in this manner is very durable, and the boxes can be used for a long time, whilst without the coating they would very soon be burnt. Particular care must be given to the manufacture of the boxes themselves; the bottoms must fit very accurately, and should be coated with clay in order to ensure an air-tight joint. The lids also must fit accurately, and when they are closed must equally be made tight by clay.
The soot is at first loosely packed into the box and each portion then pressed with a rammer so that it tightly fills the boxes. In the lid is a very small opening, through which the volatile products can escape. The heating begins quite gently at the back, and proceeds gradually to the front; the boxes are finally brought to a good red heat, at which they are maintained for about half an hour; at this temperature the substances mixed with the carbon are almost entirely volatilised, and the soot acquires its true black appearance. The soot itself attains a red heat in the boxes and in consequence of its loose nature it very readily burns. The precautions given above must be observed in order to protect completely the soot against the action of the air. The boxes should not have the smallest opening besides that in the cover necessary for the escape of the volatile matters; through an opening invisible to the naked eye so much oxygen may enter during the cooling of the boxes that a considerable quantity of carbon will burn to carbonic acid.
To avoid losses through the carbon burning precautions must equally be taken in the cooling of the boxes. When the ignition is finished they are drawn out of the furnace by tongs and placed upright on a stone floor; as the cool air enters, and in contact with the red-hot carbon would burn a portion of it. This can be prevented by a simple artifice: a red-hot coal is placed on the small opening in the lid; this converts the oxygen of the air which enters the boxes into carbonic acid. When all the boxes are taken out of the furnace the doors of the room in which they are placed are opened, so that they are cooled in the draught. Finely divided carbon takes fire at a temperature far below a red heat, so that the boxes should not be opened until their contents are quite cold. If they were opened whilst hot the soot might take fire.
Pine Black.—Under this name a poor quality of black comes into the market, which is much used for ordinary black paints, etc. The pine black formerly brought into the market was what its name purported; it was made from the roots of the pine in the primitive fashion already described. The black was sold without further purification: it was a soft light powder, very variable in shade on account of the absence of control over the process: it varied from pure black to a dark brown. At present, under the name of pine black many substances are sold, generally without purification, made from the most different materials, frequently from rosin, rosin residues, and other cheap materials.
CHAPTER XLII.
THE MANUFACTURE OF SOOT BLACK
WITHOUT CHAMBERS.
A certain temperature is necessary for the combustion of every substance; if a cold body is placed in the flame of a candle or an oil lamp it becomes covered with soot, because it cools the flame to such an extent that the carbon is no longer heated to the temperature necessary for its combustion, and is, therefore, separated in a finely divided condition. Use is made of this phenomenon in a method of making soot black which has many advantages, of which the most important is that expensive buildings are not required, and the manufacture can be conducted in restricted space.