Fig. 87.—Hand Shaving.
Fig. 88.—Shaving Machine.
It is a practically invariable rule that the leather-fibre must be wet when it is stuffed. The surface-tension between the water and the fats is less than that of either with regard to air; and therefore, as the water dries out of the small interstices of the leather, the fat follows it in, and gradually takes its place. Generally speaking, the amount of water should be such that some exsudes in minute drops when the leather is pinched, that is, that not only the minutest spaces between the fibrils are filled, but even the larger ones between the fibre-bundles to a considerable extent.
Fig. 89.—Band-Knife Splitting Machine.
In “hand-stuffing,” the leather is now coated on the flesh side, or occasionally on both sides with “dubbing,” which is a pasty mixture of fats usually mainly composed of cod-oil and tallow, which is applied rather thickly with a brush and smoothed down with the fleshy part of the forearm. When such constituents are melted together, the harder fats dissolve in the oils, and as the mixture cools, much of the hard fats again crystallise out. To make a good dubbing, the cooling fats must be stirred continuously till this has taken place, as otherwise the mixture separates into little globular masses of crystals with liquid oil between them, instead of forming a uniform body of salve-like consistency. The proportions of the hard and soft constituents of the dubbing should be adjusted to the season, and to the temperature at which the drying of the stuffed leather is to take place, so that on the one hand, the dubbing will not melt and run off, and on the other, that it should not solidify more than is necessary, as only the liquid solution which remains entangled among the crystals can be absorbed by the leather. The solid crystalline fats remain on the surface, and are scraped off by the sleeker in finishing, as “table-grease,” which is generally re-melted and used over again. It does not answer, in hand-stuffing, to carry this re-use too far, as the table-grease contains only the harder parts of the fat, with a continually increasing proportion of stearic acid, so that if a dubbing be made continuously of table-grease and oil, in the end little but the latter will be absorbed by the leather; while where fresh tallow is used, a portion of its softer constituents remains dissolved in the oil. The principal function of the harder fats is the mechanical one of retaining the oil on the surface of the leather; and to a certain extent they may be replaced by other solids, such as steatite (“French chalk”), or perhaps other pulpy materials. The use of a portion of soft fat, such as bone-fat, or the better sorts of glue-grease, is quite practicable, especially if mixed with the harder table-grease.
The drying of hand-stuffed leather should be slow, to allow time for the absorption of the grease; and the temperature should be so regulated as to keep the dubbing in a soft but not liquid condition. In winter, if the temperature of the outer air be raised sufficiently for this, the drying will be too keen (cp. [p. 426]) and the water will be dried out before the grease is properly absorbed. It is therefore best, in cold weather, to maintain the ventilation mainly by circulating the air in the room, with little admission from the outside, and in extreme cases even artificial damping of the air may be advantageous. Sometimes the tendency to mildew during slow and warm drying is very troublesome. This may be prevented by the addition of antiseptics to the stuffing grease. Carbolic acid and creasote are effective, but generally objectionable from their smell; rosin oil has considerable antiseptic power, and mineral oils also in a less degree. Probably α-naphthol would prove an efficient remedy, as it has little odour, and its antiseptic properties are very strong, but it has not been tried by the writer. (Cp. [Chapter V.])
Fig. 90.—Haley’s Injector Stuffing Drum.