Most leather articles while in use require the periodical application of a preservative varnish to give them a finished appearance, and protect them from decay and surface wear. Such varnishes go by various names, e.g. "dubbing," "gloss," etc, but are most commonly known as " blacking,' being usually intended to give a black polish. Blacking is a pasty compound used 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 lime carbonate and phosphate, 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 lime acid phosphate, sulphate and chloride. The lime sulphate 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, iron sulphate, 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 neutralise any excess of acid which may remain. The consistence of different blackings varies widely.
(1) 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 converting into sulphate a large proportion of the lime contained in the animal charcoal, thickens the mixture into the 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.
(2) Animal charcoal, 5 oz.; treacle, 4 oz.; sweet oil, 3/4 oz.; triturate until the oil is thoroughly incorporated, then stir in gradually 1/4 pint each vinegar and beer lees.
(3) Animal charcoal, 1 lb.; sperm oil, 2 oz.; beer and vinegar, each 1 pint, or sour beer, 1 qt.
(4) Bryant and James's indiarubber blacking. Indiarubber in very fine shreds, 18 oz.; hot rapeseed oil, 9 lb. (1 gal.); animal charcoal in fine powder, 60 lb.; treacle, 45 lb.; gum arabic, 1 lb., previously dissolved in vinegar, No. 24 strength, 20 gal. The mixture is triturated in a colour-mill until perfectly smooth, then placed in a wooden vessel, and sulphuric acid is added in small successive quantities amounting altogether to 12 lb. This is stirred for 1/2 hour daily for 14 days, then 3 lb. of finely-ground gum arabic are added, and the stirring is repeated for an additional 14 days, when the blacking will be ready for use.
(5) 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 almost immediately after application, shines without the necessity of being brushed, and is much less liable to soil the clothes.
(6) Acme blacking. To 1 gal. rectified spirit is added 21 dr. blue aniline, and 31 dr. Bismarck brown aniline, the solution of the two last being effected by agitation for 8-12 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 space of 10-15 minutes. Of this liquid, 1/4 gal. is added to 1 gal. rectified spirit, and in this are dissolved 11 oz. camphor, 16 oz. Venice turpentine, 36 oz. shellac. To 1 qt. benzine, add 3 1/5 fl. oz. castor oil, and 1 3/5 fl. oz. boiled linseed oil. The two solutions are then united by agitation, but should not be allowed to stand over 2 days in any vessel of Iron or zinc, as in the presence of the gums the colours will be decomposed by contact with zinc in 8 days, and with iron in 18-24 days.
(7) A quantity of ordinary starch is dissolved in hot water, and while still hot, oil or wax is added; the mixture is stirred and allowed to cool. When cold, a small quantity of iodine is added to give a bluish-black colour. To 1 gal. of this are added 8 oz. of a solution of iron perchloride or other per salt, a small quantity of gallic or tannic acid (or both), and sometimes about 2 dr. of oil of cloves with 8 oz. glycerine. The whole is thoroughly stirred.
(8) Nicolet, of Lyons, prepares boot blacking by dissolving 150 parts wax and 15 of tallow in a mixture of 200 of linseed oil, 20 of litharge, and 100 of molasses, at a temperature of 230° to 250° F. (110° to 120° C.). After this, 103 parts lampblack are added, and when cold it is diluted with 280 of spirits of turpentine, and finally is mixed with a solution of 5 of gum lac and 2 of aniline violet in 35 of alcohol.
(9) Hein, in Kaufering, makes another kind of shoe blacking by melting 90 parts beeswax or ceresine, 30 of spermaceti, and 350 of spirits of turpentine, with 20 of asphalt varnish, and adds 10 of borax, 20 of lampblack, 10 of Prussian blue, and 5 of nitro-benzol.
(10) Brunner uses 10 parts bone-black, 10 of glucose syrup, 5 of sulphuric acid, 20 of train oil, 4 of water, and 2 of soda carbonate. The bone-black and glucose are stirred with the acid in a porcelain vessel until the whole mass is homogeneous and has a shining black surface when at rest. The soda is dissolved in a little water, and boiled with the oil under constant stirring until it forms a thick liquid; then the other mixture is stirred into it. By varying the proportions of these two mixtures, the blacking is made thinner and softer, or harder and firmer. The substances sold as French polish are mostly composed of these ingredients. In this and all other kinds of shoe blacking made with bone-black and sulphuric acid, the precaution must be observed of stirring rapidly and evenly after the acid is added, otherwise lumps will be formed that are difficult to crush, and the blacking will have a granular condition that does not belong to it. Good shoe blacking must always remain soft, and show a smooth uniform surface when applied to the leather.