Charcoal blacks include blacks made from soft woods, the best being that charred from the willow, but some of the wood pulp mills have also charred their refuse and offered the resulting product as charcoal or carbon black. The latter has not found much of a market, as its texture seems to be too fibrous and refractory in grinding in oil or water. Charcoal black made from willow twigs, however, has been made use of in some special structural iron paints, and has been especially recommended in that connection by late researchers in paint problems, with what real practical value is yet to be fully determined. It is claimed, however, that it has rust inhibitive properties, while gas carbon black is said to lack that feature.
However this may be, gas carbon black is a safe pigment when used with the proper inert material. It is a queer fact that this pigment has a specific gravity of 1.85 and requires anywhere from 80 to 84 per cent of oil for grinding, while willow charcoal black has a specific gravity of 1.40 and yet requires only 35 to 40 per cent of oil for grinding. This can only be accounted for by its very coarse texture. Vine black is very similar to willow charcoal black in every way - specific gravity and absorption of oil, texture, etc. Coal blacks have been put on the market under the name of carbon black, but have been found to promote rust and will not pass chemical investigation, for which reason these pigments are undesirable.
Frankfort black is a pigment that derives its name from the fact that it was originally made in that town, Frankfort-on-Main, in Germany, being calcined in closed vessels until thoroughly charred, consisting of a mixture of vine twigs, hop vine, the pressed residue of grapes, peach and prune stones, ivory and bone chips and shavings - in fact, every imaginable residue of that character. The calcined pigment is ground up in water, then well washed and floated and finally formed in drops and dried. The average chemical composition is about 60 to 65 per cent carbon, 3 to 5 per cent moisture, the balance mineral matter, containing more or less phosphate of lime, according to the amount of bones or animal matter used in calcination. This black shows much variation and is scarcely ever used by color grinders at the present day in the United States.