Part I., Section VIII. Calfskins were also used for very light upper
work, in which they were not so heavily greased in finishing, but rather dyed and finished as a light leather. In this direction, however, the vegetable tannage has been almost completely superseded by the mineral tannages, first by "calf kid," an alumed leather (Part IV., Section I.), and afterwards by the now popular chrome tannage of "box calf," "willow calf," "glacé calf," "dull calf," etc. (Part III., Section III.). In this section, therefore, we have only to consider calfskins as used to make a vegetable-tanned light leather, such as may be employed in bookbinding and in the manufacture of fancy goods. For these purposes the skins receive a mellow liming of 2-1/2 - 3 weeks. No sulphide need be employed, as the goods are soon fit to unhair. In such a mellow liming it is important that the bacterial activity is not too prominent, and hence it becomes advantageous to work the liming systematically in the form of a round of pits. To avoid over-plumping in the newest limes some old liquor is used in making up a new pit, and its bacterial activity is reduced by adding it to the new caustic lime whilst slaking. Thus for a pack of 200-250 skins, 14-16 stone of lime may be slaked with about 30 gallons of old lime, and the pit filled up with water. If it be necessary to shorten the process and to use sulphide, this should be added only to the tail liquors of the round, and with it should be added, if possible, some calcium chloride to reduce the harshness of the soda. The skins should be puered thoroughly to obtain the necessary softness, bate-shaved if desirable, and drenched with 8 per cent. of bran overnight.
In tanning for fancy work and for dark colours, the goods are coloured off and evenly struck through with sumach liquors, and then tanned further with liquors made from oak bark, myrabolans or chestnut extract. The methods are very closely similar to those used for goatskins and sealskins (Part II., Sections II. and III.), and need not be described in further detail. The tannage is finished off in sumach. For bookbinding work, however, a pure sumach tannage is given, using liquor slightly warm (70° F.). Paddle tannages are common, but for bookbinding the bag or bottle tannage is often preferred. The skins are sewn together in pairs, grain outwards, and nearly filled with warm sumach infusion, just as described for goatskins. They are then handled in old sumach liquors for about 3 days, and piled to drain and press. At this stage the bag is cut open, the goods worked on the flesh, and the tannage is completed with separated skins in newer sumach liquors, handling at least once a day for 4-5 days, as necessary.
In finishing there is the usual variety, but a plain ungrained finish is most typical, as the smooth and fine grain of the young animal lends itself to this type of finish better than the skins of goat and seal, and gives a better quality leather than those from the sheep. The crust skins are wet back with water at about 110° F., and, if necessary, sammed and shaved. Sumaching follows, the operation being carried out in a drum for 1-2 hours. The skins are then well struck out. Striking and setting should always be thorough for a plain finish, and this case forms no exception. Dyeing follows next, the paddle being often preferred to the drum, which is liable to work up a grain. The dyed skins are placed in cold water for a while and again well struck out. They are often nailed on boards to samm, and are then set out, lightly oiled with linseed oil and dried out in a cool shed. Seasoning follows, with milk and water only. The operation may be done with either brush or sponge, after which the goods are piled grain to grain and flesh to flesh to regulate. They may be next perched to soften and fluffed if desired. After top seasoning with milk, water and albumin the skins are hung up for a while, piled to regulate and brushed, first lightly and then more vigorously. They may be then oiled very lightly and dried out in a cool stove to ensure a soft leather.
REFERENCE.
Bennett, "Manufacture of Leather," pp. 55, 84, 105, 201, 207, 303.
SECTION VI.--JAPANNED AND ENAMELLED LEATHERS
The leathers which receive a japanned or enamelled finish are usually vegetable tannages, and so may be discussed at this stage. They are popularly known as "patent" leather, but for no obvious reason. The chief object is to obtain a leather with an exceedingly bright and permanent gloss or polish, and this is attained by coating the leather several times with suitable varnishes. The great difficulties are to prevent the varnish cracking when the leather is bent or in use, and to prevent it peeling off from the leather. Almost all classes of vegetable tannage are japanned and enamelled. Hides are split and enamelled for carriage, motor car and upholstery leathers, and enamelled calf, seal and sheep skins are used for boot uppers, toe caps, dress shoes, slippers, ladies' and children's belts, hat leathers, and so on. Broadly speaking, a japanned leather is a smooth finish and is usually black, whilst an enamelled leather is a grain finish with a grain pattern worked up, and more often in colours. Hence japanned leathers are often made from flesh splits or leathers with a damaged grain. It is in any case advantageous to buff the grain lightly, for this permits the varnishes to sink rather deeper and get a firmer grip, and avoids the too sudden transition from phase to phase which is one cause of stripping or peeling. Many flesh splits, however, are printed or embossed to give an artificial grain and are then enamelled, which tends to fix the embossed pattern.
Almost any method of preparing dressing hides for upper or bag work will yield a suitable leather for enamelling and japanning (see Part I., Section VIII.; and Section IX.). If anything the liming should be somewhat longer and mellower in order to eliminate grease, as the natural grease of the hide causes the stripping of some varnishes. In finishing it is important to obtain even substance, or the varnish is liable to crack. Hides are soaked and sammed in, and often split. Sometimes they are split twice, giving grain, middle and flesh, the two former being enamelled and the last japanned. Other goods are shaved very smooth. The goods should be next thoroughly scoured and stoned to get as much "stretch" as possible removed. They are often sumached, washed in warm water, slicked out again and sammed. They are then lightly buffed on the grain, and after oiling lightly are thoroughly set out and dried. Embossing or printing for enamels is done before the goods are quite dry. Considerable difference of opinion obtains as to the best oil to use in the above oiling. Linseed oil is widely preferred as being most likely to agree with varnishes made from linseed oil. Some manufacturers of japans do not dislike the use of mineral oil, but strongly object to cod oil, tallow or other stuffing greases as tending to cause the varnish to strip or peel. Other manufacturers, on the other hand, will not have leather with mineral oil in it, and indicate that nothing but cod oil should be used. In all probability these various preferences are determined by the nature of the varnish, which differs widely in various parts of the globe.
In this country the varnishes are made largely from linseed oil by boiling it with "driers." This oil contains much triglyceride of an unsaturated relative of stearic acid. The double bonds are very susceptible to oxidation with the production of resinous bodies of unknown constitution. This phenomenon is known as "drying the oil," and has been extensively used in the manufacture of linoleums. The driers are either oxidizing agents or oxygen carriers, such as litharge, Prussian blue, raw umber, manganese dioxide, manganese borate, and "resinate." Prussian blue is most preferred for British japans, as it always materially assists the attainment of the desired black colour. The exact details of the boiling, and the manufacture of the varnishes is still largely the trade secret of the master japanners, and differs indeed for the various stages of japanning. The varnish for the earlier coats is boiled longer, and the drying carried further, than in the case of the later coats. This is partly to obtain a product of such stiffness that it will not penetrate the leather. The driers and the pigments should be finely powdered and thoroughly mixed in. The boiling takes several days when at a low temperature, but if done in 24 hours the temperature may be up to 570° F. In the later coats driers are often not used, and the product is often mixed with copal varnish, pyroxylin varnish, etc., which greatly help in obtaining smoothness and gloss. Turpentine, petroleum spirit and other solvents are also used to thin the varnishes. Before boiling, the oil is often purified by a preliminary heating with nitric acid, rose spirit and other oxidizing agents, which precipitate impurities and thereby assist in obtaining a bright gloss.
Before the application of the varnishes, the leather is first dried thoroughly in a stretched condition. This is accomplished by nailing down on boards which fit like movable shelves into a "stove," a closed chamber heated by steam pipes. The temperature of the stove varies widely in different factories, from 140°-200° F., according to the nature of the varnishes. The first coat of warm and rather stiff japan is laid over the hot leather in a warm room, being spread over first by hand, then by a serrated slicker, and then again smoothed by hand. The goods are then put into the stove for several hours to dry. When dry the surface is pumiced and brushed and a second coat applied in a similar manner, but with increased care. This is repeated with finer japans until the desired result is obtained. Brushes are used to apply the later coats. Up to seven coats may be applied for the production of a smooth japan--three coats of ground japan, two coats of thinner japan, and two coats of finishing varnish.
After the stoving is complete, the product is given a few days under ordinary atmospheric conditions to permit the reabsorption of moisture to the usual extent. Enamelled leathers are then grained to develop the pattern.
REFERENCE.
Bennett, "Manufacture of Leather," p. 380.