PART IV.—MISCELLANEOUS TANNAGES
SECTION I.—ALUM TANNAGES
The use of alum for making pelt into leather is several centuries old. It was the first case of what are called "mineral tannages." The tannage is closely analogous in theory to the chrome tannages discussed in Part III., and if soda be added to ordinary potash alum in solution, a basic alum liquor is obtained which is quite capable of yielding a satisfactory leather, and which is thus a strict analogy of the basic chrome alum liquor described in Part III., Section II. The range of basicity which is practicable is very limited, however, and it is much more usual to use common salt with the alum instead of soda. The alum is, of course, hydrolyzed and free sulphuric acid is quickly adsorbed, whilst the colloidal solution of alumina is adsorbed also but more slowly. The adsorbed acid tends to swell the pelt and to cause it to take up the alumina less readily. The function of the salt is to repress the swelling by a pickling action. The actual result is thus partly due to the alum tannage and partly due to the temporary tannage given by the pickle. Hence such tannages are not firmly "fixed," nor is the result water-resisting, for much of the tanning material may be washed out. If, however, such leathers be stored for a time in a dry condition, the alumina becomes much more firmly fixed, owing probably to a further dehydration of the alumina gel deposited upon the fibres. The tannage is thus relatively more "irreversible," and such storage is practised in commerce for this purpose, being known as the "ageing" of the leather. It will be understood that it is possible to use too great a proportion of salt, the hygroscopic nature of which would keep the leather moist and thus interfere with a glossy finish. About one-third the weight of the alum used is usually sufficient.
All that has been said in Part III. as to the empty nature of the chrome tannage is equally applicable to the alum tannages. It is as necessary therefore to employ filling agents. A fat liquor is quite satisfactory for many purposes, but is too dark coloured and greasy for glove leather. Egg yolk is the favourite emulsion in these cases. It contains about 30 per cent. of an oil very similar to olein and in very perfect emulsion. Olive oil is also largely olein and is also used, being emulsified by the egg yolk and effectively reducing the proportion required of this expensive material. Flour is also used as a filling agent. It acts also as a whitening agent and as an emulsifier. Its use enables the tanner to obtain the required fullness without so much greasiness. Thus softness and fullness may be obtained, and yet a glossy finish be possible. It will be clear that the more flour is used, the more oil may also be used.
The materials mentioned, viz. alum, salt, flour, egg yolk and olive oil, are all mixed together into a paste with some amount of water. The goods are drummed in this paste and then dried out. This operation is known as "tawing." The goods are then "aged" for several weeks and finished as required.
The manufacture of "glove kid" from lambskins and kid skins is the most typical example of alum tannage. Lambskins are unwoolled very usually by painting the flesh with a mixture of lime and sodium sulphide. There must not be too much of the latter on account of its tendency to give harshness, a fatal defect in glove kid. The addition of calcium chloride is desirable, and the skins, which should be pulled as soon as possible, should be quickly placed in soft water or weak lime. For kidskins a set of lime liquors may be used, and in preference to sodium sulphide red arsenic is employed. About one per cent. realgar on the weight of the lime is used, but more often larger quantities are preferred, even up to 6 per cent. The liming is thus shortened to 4 or 5 days. Fresh lime liquors are sometimes used without any sulphides. Another method is to place the skins in a paste of lime to which realgar has been added in slaking. In any method it is necessary to saponify or emulsify the grease on the grain, or difficulties occur in dyeing and finishing.
Skins which are to be tawed for glove kid are both puered and drenched. They are heavily puered at 70° F. for 3 hours, or even longer for the heavier skins. After scudding they are drenched with 10 per cent. bran and some pea meal at 95° F. for a few hours only.
In preparing the tawing paste, the flour should be mixed with tepid water; the egg yolk should also be diluted with tepid water slightly, and strained if necessary, and then added to the flour. The oil is then carefully mixed in. The alum and salt are dissolved separately at 110° F. and added to the flour and oil. The tawing paste should be used at about 105° F. For every hundred medium-sized lambskins there will be required: 10 lbs. flour in 2½ gallons water, 1 quart preserved egg yolk, 3¾ lbs. alum and 1¼ lbs. salt. The skins are drummed in this for an hour or so and dried out on poles rapidly, but not with great heat. This is essential to get "stretch." They are next wet back, staked, dried and staked again. They are then "aged."
To wet back for dyeing and finishing the skins are drawn through warm water and then drummed in water at 95° F. for 15 minutes to wet evenly and thoroughly. This liquor, which contains much of the tawing material, is run off and replaced by the dye solution, e.g. fustic or turmeric, with which the goods are drummed for half an hour. Iron, chrome or copper salts may be used for saddening. After this "bottom" colour is obtained, a coal tar colour is added for "topping" and the drumming continued until the required shade is obtained. The excess liquor is now run off, and the materials lost in soaking are replaced by drumming further with egg yolk and salt for 15 minutes. This is known as "re-egging." Blacks are obtained with logwood and iron. After re-egging, the skins are dried out and staked. They are "seasoned" with a weak emulsion of soap and oil, dried, oiled lightly with linseed oil, ironed, re-oiled and finally brushed. Whites are undyed, and 10 lbs. French chalk per 100 skins is used in re-egging.