BLACICING. (FR., Cirage ; GER., Wichse, Stiefelwichse.) Blacking is a pasty compound used for producing a polish on black ltatlaer surfaces, especially on the "uppers " and the edges of the soles and heels of boots and shoes. There are numerous methods of manufacturing this substance ; but in nearly all, the base is a black colouring matter, usually animal charcoal, mixed with substances which acquire a gloss by friction, such as sugar and oil. The carbon employed should be in the form of a very deep, finely powdered black. Since it always contains carbonate and phosphate of lime, it is treated with a mineral acid in order to decompose these salts ; a mixture of sulphuric and hydrochloric acids is frequently used, the salts produced being acid phosphate of lime, sulphate and chloride of lime. The sulphate of lime gives consistence to the pasty mass, and the two other salts being deliquescent help to keep the leather flexible. No more acid should be used than is sufficient to decompose these salts, or the leather will be destroyed. It is probably to prevent this that some makers add a small quantity of alkali to the blacking. Sometimes powdered gall-nuts, sulphate of iron, indigo, and Prussian blue are incorporated with the blacking in order to impart to it a good colour. Fatty or oily matters are also sometimes added in order to preserve the flexibility of the leather, and to neutralize any excess of acid which may remain. The consistence of different blackings varies widely ; they may be classed either as liquid or as solid blacking.
The well-known liquid blacking of Day and Martin is composed in the following manner. Very finely ground animal charcoal, or bone-black, is mixed with sperm oil till the two are thoroughly commingled. Raw sugar or treacle, mixed with a small portion of vinegar, is then added to the mass. Next a small measure of dilute sulphuric acid is introduced, which; by con verting into sulphate a large proportion of the lime contained in the animal charcoal, thickens the rnixture into tbe required pasty consistence. When all effervescence has subsided, but while the compound is still warm, vinegar is poured in until the mass is sufficiently thinned ; then it is ready to be bottled for the market.
The following are other ways of making liquid blacking :—
. 1. Animal charcoal, 5 oz. ; treacle, 4 oz.; sweet oil, oz. ; triturate until the oil is thoroughly incorporated, then stir in gradually pint each vinegar and beer lees.
2. Animal charcoal, 1 lb.; sperm oil, 2 oz. ; beer and vinegar, each 1 pint, or sour beer, 1 quart.
3. Bryant and James's indiarubber blacking. Indiarubber in very fine shreds, 18 oz.; hot rapeseed oil, 91b. (1 gallon); animal charcoal in fine powder, 601b.; treacle 451b.; gum arabic, 1 lb., previously dissolved in vinegar, No. 24 strength, 20 gallons. The mixture is triturated in a colour-mill until perfectly smooth, then placed in a wooden vessel, and sulphuric acid added in small successive quantities amounting altogether to 121b. This is stirred for half an hour daily for fourteen days, then 31b. of finely ground gum arabic are added, and the stirring repeated for an additional fourteen days, when the blacking will be ready for use.
4. It has been proposed to treat the leaves and other portions of the mastic gum tree, Pistacia lentiscus, by decoction or distillation, principally to obtain from them a blacking which dries ahnost immediately after application, shines without the necessity of being brushed, and is much less liable to soil the clothes.
5. Acme' blacking. To rectified spirit, 1 gallon, is added blue aniline, 20.80 drachms, and Bismarck brown aniline, 31.20 drachms, the solution of the two last being effected by agitation within eight to twelve hours. After the solution is completed, the mass is allowed to settle, and the liquid portion is drawn off by spigots above the sediment, and filtered if necessary. The alcohol is placed in the apparatus first, then the colours, and the mixture agitated every hour for a span% of ten to fifteen minutes. Of this liquid gallon is added to rectified spirit, 1 gallon, and in this are dissolved gum camphor, 11 oz.; Venice turpentine, 16 oz. ; gum shellac, 36 oz. To benzine, gallon, add castor oil, Sk fluid oz., and boiled linseed oil, 11 fluid oz. The two solutions are then united by agitation, but should not be allowed to stand over two days in any vessel of iron or zinc, as in the presence of the gums the coloura will be decomposed by contact with zinc in eight daya, and with iron in eighteen to twenty-four days.