A light coating of oil is usually applied to the wet leather before drying in order to act as a protective film against the drying of the grain surface and to ensure that the evaporation of the moisture is chiefly from the flesh side. The regularity of the colour of the leather when dry is in the main dependent upon the rate of evaporation of the moisture. Thick sole leather requires to be dried more slowly than thin goods (calf or sheep leather) ; and vegetable tanned leather much more slowly than mineral tanned leather, which is dried in a few hours at a comparatively high temperature and with little ventilation.
During the past few years considerable improvements have been effected with a view to accelerating the process. The method now employed for all of the lighter classes of leather and also for some of the heavy classes is to dry the goods by machine. There are various machines, which differ mechanically, but are similar in general principle and are modelled more or less on the continuous type of baker's oven.
Currying.—The old time currier was a skilled craftsman who was capable of undertaking the whole of the processes from the crust tanned condition to the finished commercial product, when the leather was required to be of a greasy type. The term currying is applied to-day to the impregnating of leather with grease. The goods after tanning and drying are split or shaved, and then scoured either by hand or machine until the grain sur face is thoroughly cleansed from "bloom" and excess tannins. The leather is then usually re-tanned with sumach to obtain a better colour, after which it is ready for grease impregnation.
Grease impregnation may be done in three different ways (r) hand stuffing, (2) drum stuffing and (3) dipping. Hand stuffing consists in applying to the previously moistened leather a thick layer of dubbin prepared by mixing together about equal parts of beef tallow and cod liver oil. The application is made, usually to the flesh side of the leather, with a brush and the leather is afterwards slowly dried. As drying proceeds by evaporation the more oily portions of the dubbin are absorbed by capillarity, and become thoroughly distributed and absorbed by the fibres. This method is now only practised upon a few high grade leathers and has been replaced by drum stuffing and dipping. Drum stuffing consists in drumming the leather, which has been previously got into an equable condition of moisture by damping, in a drum which has been heated either by live steam or by hot air cir culating through the vessel.
Dipping is chiefly practised upon heavy leathers, for example, harness backs, strapping, etc., and consists in immersing the
leather, which has been previously well dried, in a tank filled with molten grease (usually paraffin wax). The dried leather offers little resistance to the penetration of the molten wax and complete permeation is effected in a few minutes. This impregna tion is not so satisfactory as drum or hand stuffing because of the non-oxidisable and little lubricating properties of the wax.
Dyeing.—The dyeing process is accomplished by the follow ing methods : (I) brush dyeing or "staining", (2) dipping, (3) dyeing in paddle or drum, (4) spraying. The first mentioned. is used in the colouring of the heavy types of leathers, such as hides for upholstery, bag and portmanteau work, etc., which on account of their size and weight are inconvenient to handle by dipping or other immersion processes. This method consists of brushing a weak solution of the dye on the surface of the pre pared leather, usually two or three applications being given to obtain regularity of colour. The dyes employed are the usual coal tar colouring materials of the acid or basic class.
Dyeing by immersion in rectangular wooden trays or by dip ping pairs of skins at a time in a shallow wooden or porcelain vessel either by machine or by hand is practised when small quantities of leather are to be processed.
The spray method in which the dyestuff is applied to the leather in an atomised state through a spray pistol is used upon leather for the application of dyestuffs and also of insoluble paint-like compositions. The application of mineral pigments, such as the ochres and umbers in suspension in a water medium in which has been dissolved some binding agent such as casein or shellac, has effected a considerable improvement in the regu larity of shade of colour. The modern practise is to dye with coal tar dyestuffs with a top coating of a suitable pigment mixture.