In drum-stuffing the conditions differ materially from those of hand-stuffing. The goods, in a damp condition, are placed in a drum ([Fig. 90]), which has been heated by steam to as high a temperature as the leather will safely stand. Cold damp leather may be stuffed in a drum heated to 60° C. and the grease may be run in at the same temperature. The grease should generally be melted and mixed at a somewhat higher temperature. Sometimes steam is merely blown into the drum before introducing the leather, to heat it to the required temperature; sometimes a steam-coil is placed in the drum itself. A more modern method, which is now largely used in the United States, is to heat by hot air, which is circulated by a fan over an external steam heater and through the drum. The drum is set in rotation, and the stuffing grease in a melted condition is run in through the hollow axle, or if this is not provided, it is introduced through the door, and the rotation is maintained for twenty to thirty minutes. During the last few minutes the door is frequently replaced by an open grating or cold air is drawn through the drum by means of the fan, in order to cool the goods, which are set out with the sleeker on the table while yet somewhat warm, and dried under much the same conditions as have been described with regard to hand-stuffed goods.
In drum-stuffing, the hardness of the grease is limited by its melting-point, which must not be so high as to damage the leather, but it may be soft as is desired. As the grease is forced by mechanical means into the interior of the leather, there is no danger of its running off, but the drying must take place at such a temperature as to keep it at least in a partially soft condition, as the drumming only forces it into the coarser spaces of the leather, and does not complete its distribution on the fibre. By the use of exceedingly hard greases, such as “stearin” ([p. 359]) and oleo-stearin ([p. 356]), sometimes with additions of paraffin wax, it is possible to introduce immense quantities of grease, and yet to obtain a leather which will board up to a good colour. In America, it is not unusual to reckon 100 or even 115 lb. of greases to 100 lb. of leather weighed dry after scouring, or estimated from its wet weight; and the whole of this is absorbed, scarcely anything coming off in “setting.” The leather, as it comes from the drum, is dark brown, but when bent sharply in “boarding” to form the grain, after cooling and drying, the very hard and crystalline fats crumble into white powder, and the leather takes a light and pretty colour. Such leather would of course darken at once if it were held to the fire, but would again brighten on cooling and breaking up with the “board.” Some portion of liquid fats, such as dégras or fish oil, should be contained in the stuffing grease, as the solid fats alone will not penetrate to the heart of the fibres, but will leave the leather dry and harsh.
By drum-stuffing, it is possible to incorporate solid matter with the leather, and barytes (ground heavy-spar or barium sulphate) was formerly much used for this purpose, but has now been nearly abandoned. Glucose is still used as an adulterant of leather, but is not introduced in the drum, but by painting the goods with syrup before stuffing. It not only adds weight, and gives the leather a lighter colour than an equivalent quantity of grease, but at the same time lessens its toughness, and ought to be prohibited in England, as it already is in Germany. On the detection of adulteration of leather, see L.I.L.B., p. 212. Drum-stuffing is in this country mainly applied to shoe-leathers, but in America, with the hot-air drum, is coming into increasing use for harness, and even belting.
A method of stuffing is used in Germany for heavy belting and the like, which appears at first glance to contradict the axiom that leather must be stuffed wet. It is called Einbrennen (to burn in), and consists in first drying at a high temperature (50° C.), to ensure the absence of all moisture, and then either pouring hot melted tallow over the leather on a table, and holding it over a brazier, to allow the grease to sink in, or dipping it completely in a bath of melted tallow. The exception is only apparent, because, though the leather is at this stage completely saturated with tallow, it is only after wetting and drumming that it attains the flexibility due to true stuffing. Similar methods are applicable to alumed leathers, and even to chrome-leather; and so-called “waterproof” or “anhydrous” leather is made by immersing thoroughly dried leather in a bath of 2 parts of resin and 1 of paraffin, or some similar mixture. If the leather is not first thoroughly dried, it is scalded and destroyed by the hot grease.
The most troublesome defect to which stuffed leathers are liable, is known as “spueing,” and is of two kinds, of which the first and less serious (perhaps more properly distinguished as “striking out”) consists of a white efflorescence rather like incipient mould, which is easily wiped off, but generally reappears. This is due to the crystallisation of the harder fats, and especially of the free fatty acids, on the surface of the leather, and is almost sure to occur in greater or less degree when the hard fats such as tallow or stearine are combined with a non-drying oil such as neatsfoot, or when soft fats are present in the leather. It is sometimes combined with actual mildew, from which it is rather difficult to distinguish, even under the microscope, and may even be caused by fungoid plants, which not only mechanically expel the fats by their growth, but probably promote their rancidity and the separation of the crystalline fatty acids. It is at most only a defect of appearance, and does not in any way injure the leather. It is constantly present in calf-kid, from the neatsfoot oil used in finishing, and is in this case rather liked by the buyers, who for some reason regard it as a proof of quality. A very similar appearance may be caused by the use of solutions of barium chloride, alum or other mineral salts, for weighting or other purposes; but is persistent when the leather is held to the fire, while the crystallised fatty acids at once melt and disappear. The fatty acids are at once removed by a drop of benzene or petroleum spirit; but unaffected by water, while with water-soluble salts the reverse occurs.
The second form of spueing is of a much more troublesome character, and makes its first appearance as minute spots or pimples of resinous matter, raised above the surface of the leather, which if removed, generally reappear, and which may become so bad as to form a sticky resinous coating over the whole surface. The exsuded matter consists of the oxidised products of oxidisable oils, but the cause of its appearance is not always easy to explain. The currier generally attributes it to adulterated oils, and it must be admitted that some oils almost invariably produce it, but it appears occasionally when only the purest and absolutely genuine cod-oil has been used. It can only be produced from drying or semi-drying oils, which include all the ordinary fish oils and most of the vegetable seed oils, but can never arise from tallow or stearine, from mineral oils or vaseline, or from genuine non-drying oils, such as tallow, neatsfoot, sperm, or mineral oils, nor, probably, from rosin oil. It is favoured by causes which promote the oxidation of oils, such as moist heat with limited access of air, and by the presence of oxygen-carriers, such as iron-salts in blacks, and possibly also by the presence of free acids. A large amount of free fatty acid in the oils themselves is suspicious, not only because the free acids oxidise more freely than the neutral fats, but because their presence is an evidence of the tendency to rancidity and change in the oil. It is also said to be caused by previous mildewing of the leather, and certainly often occurs where the grain has been rendered porous by bacterial action in the soaks, limes, or bates, probably from the greater quantity of oil absorbed by these parts. While it is easy to say which oils may possibly spue, there is no known chemical test which will foretell whether a given sample is likely to do so under ordinary conditions. Eitner[169] states that seal oil extracted at a low temperature is very liable to spue, but that when heated for a considerable time to a temperature of 250°-290° C. it darkens in colour and loses the tendency. This is probably true of many other marine oils; and may be one cause of the frequent trouble with modern oils, many of which, especially the lighter coloured kinds, are extracted by steam at a temperature below boiling point. It is very probable that one effect of heating to a considerable temperature is to dehydrate and separate albuminous or gelatinous matters which are present in the fresh oils, and which probably increase their tendency to decomposition. Many of these substances separate as “foots” from oils during long storing, and such old oils are said to be less liable to spue than those of recent manufacture.
[169] Gerber, 1880, p. 243.
If oxidisable oils are used upon leather, they “dry” upon the fibre, and if a sufficiency of non-drying constituents are not present at the same time, the leather will ultimately become hard, and may even crack from hardening of the fibre. Mineral oils are not liable in this way to form a hard coating on the fibre, but as they are slightly volatile, though of very high boiling point, they may ultimately evaporate, and leave the leather insufficiently nourished. From their low surface-tension, they have great powers of capillary penetration, as is witnessed by the way that lamp oils “creep” over the surface of the lamp, but they have less affinity for water than the more oxidisable oils, and probably do not combine so intimately with the leather-fibre. They are probably better used in combination with other greases than alone. The admixture of solid paraffin with stuffing greases has the tendency to make the leather feel less greasy and drier than it otherwise would; and crude turpentine and rosin are said to have a still greater effect in this direction.
The water which is required for satisfactory stuffing may in some cases be introduced into the stuffing grease as well as into the leather. The effect of dégras is largely due to the water with which it is intimately mixed, and when dégras or sod-oil is deprived of that which it naturally contains, by heating it to too high a temperature, either before or after its mixture in a stuffing grease, its efficacy is greatly lessened.
Fat-liquoring ([pp. 217], [239]) may be considered a special case of stuffing, in which the oil is very perfectly emulsified with a large quantity of water. In this way, very considerable quantities of oil may be introduced into leather without giving it the least greasy feel. Egg-yolk contains about 30 per cent. of an oil chemically very like olive, but with a larger proportion of palmitin, and may be considered as a very perfect natural fat-liquor, containing also some albumen which serves as “nourishment” for the leather. If a means of emulsifying olive, lard, or tallow oil (with the addition of a little palm oil) with albuminous matter as perfectly as in the egg could be discovered, the problem of an egg-yolk substitute would in all probability be solved. Milk and cream are also natural fat-liquors.