chapter xiii.

Quite recently butyric acid, which is now being manufactured on a commercial scale by the process of Dr. Effront (Brussels), has come into use for deliming.[10] and is likely to become a useful acid, as it can be manufactured cheaply. According to Parker, it dissolves less skin substance than formic, acetic or lactic acids, which observation is confirmed by Thuau. The quantity required per 100 lb. of ordinary washed sheep skin is about 1/2 lb. of the 80 per cent. acid. It is used in exactly the same way as other acids, i.e. it should be added in small quantities, suitably diluted, at successive intervals of time.

[10] See also L’Acide Butyrique dans la Tannerie, Urbain J. Thuau, Le Cuir, Aug. 1910. Also Collegium, 1910, pp. 347, 363.

Acid salts are also used for deliming, and Procter suggested sodium bisulphate NaHSO_{4} (“Principles,” p. 155). More recently, sodium bisulphite (NaHSO_{3}) has been recommended;[11] the skins are tumbled for half an hour with a dilute solution, which combines with the lime according to the following equation--

(1) Ca(OH)_{2} + NaHSO_{3} = CaSO_{3} + NaOH + H_{2}O

if sulphides are present the reaction is--

(2) Na_{2}S + 2NaHSO_{3} = H_{2}S + 2 Na_{2}SO_{3}

[11] Le déchaulage des Peaux en tripe, Ettore Guisiana (Turin), Coll. 1910, p. 14.

After this treatment the theoretical quantity of hydrochloric acid, diluted with plenty of water, is run in through the axle of the tumbler, and the skins run another half hour. The reactions are--

(1) CaSO_{3} + 2HCl = CaCl_{2} + SO_{2} + H_{2}O

(2) Na_{2}SO_{3} + 2HCl = 2NaCl + SO_{2} + H_{2}O

The sulphurous acid evolved in this way slightly swells the skins, and also removes stains due to the alkaline polysulphides; it is also said to remove salt stains.

A more interesting process, introduced by Dr. Gr. Eberle[12] as a bating process, but which is really a deliming process, consists in using organic acids in the form of their anhydrides, lactones, or lactides. During the process these anhydrides, lactones, or lactides, are gradually decomposed with formation of the free acids, which in a nascent condition unite directly with the lime in the skins. The anhydrides of acetic, propionic, butyric and lactic acid have been tried, the lactone of γ-oxybutyric acid

CH_{2} CH_{2} \ O, and the lactide of lactic acid. / CH_{2} CO

[12] Verfahren zum Beizen von Haüten, Ledertechnische Rundschau, No. 24, 1910. Coll. 1910, p. 372.

After washing and deliming, the goods, warmed to a temperature of 90° to 100° F., are now measured in the truck above described, and put as quickly as possible into the bating paddle. This is constructed to carry two trucks. The figures (Figs. 3, 4 and 5) show a modern paddle adapted for puering light skins, such as goat, sheep, splits (grains), either with dung or with an artificial bate.

The inside dimensions are: length, 4 ft. 6 in.; width, 4 ft.; depth, 2 ft. 6 in. A copper pipe of two coils is provided at the back, through which steam is passed for maintaining the temperature of the wheel--the goods are prevented from coming in contact with this pipe by a perforated board. Another steam pipe, jointed so that it may be turned in and out of the paddle, is provided for heating up the wheel to the required temperature before beginning the bating. This pipe is not shown in the drawing. The thermometer can be read while the paddle is in motion. A suitable speed to keep the goods in motion is twenty revolutions per minute. The whole wheel is covered in, for the purpose of preserving the heat, and also to exclude light and air, which are prejudicial to the action of the bacteria. Such a paddle, or wheel, has a capacity of 200 gallons liquor, or 900 litres, and will bate 25 dozens of average grains, weighing in the wet state 1100 lb. or 500 kilos.

The dog dung, which is usually obtained from hunting kennels, and of which the composition is given in Chapter II., is simply diluted with water in the paddle;[13] five buckets, weighing about 165 lb., are required for the above quantity of goods.[14]

[13] When the puer is dirty it should be diluted with water and strained through a bag, or the diluted puer may be put into a tub and the heavy grit and dirt allowed to fall to the bottom of the vessel. Puer from hunting kennels is usually clean enough to use without this process.

[14] The quantities of puer used vary considerably in different works. At Trent Bridge, 10 to 15 lb. of wet puer from hunting kennels are used per 100 lb. of wet pelt. In a German works, 15 kilos of fairly dry dog dung are used per 100 kilos of pelt (Blösse). In an American works, two buckets (one bushel) wet dog dung were used to puer 10 dozens of 8 lb. calf skins. This is equal to about 62 lb. of puer to 100 lb. of skin, which seems an excessive quantity. In an Italian works, two kilos dry dung were used to 20 skins of sheep, equivalent to about four kilos per 100. In an English tannery, 100 hides for harness leather required four bushels by measure of pigeon dung, costing 1_s._ 6_d._ per bushel (one bushel equals eight gallons).

The amount used depends on the state of the goods, the prevailing weather, and, further, on the quality of the puer.[15] When the temperature is maintained at 40° C., ordinary limed goods, which have been washed, come down in from one to three hours.[16]

[15] A German firm, who supply dry dog and pigeon dung for bating purposes, recommend that the material should be spread in a warm room at 30° C., and moistened up to 80 per cent. of its weight with an infusion of oat straw, and continuously turned over until completely softened. Other materials may be used for the fermenting fluid in place of the oat straw infusion, such as broth made from waste fleshings, whey, or even a simple inorganic solution, such as is used for cultivating bacteria, consisting of--

2 parts potassium phosphate. 1 part magnesium sulphate. 1 part calcium nitrate. 1·5 part calcium chloride. 100 parts water.

This procedure is neither more nor less than an application of the principles of bacterial cultivation and propagation, shown to be practically useful in the bating process by Professor H. Becker, of Frankfort, and by the Author in various publications (see Chapter VII.).

[16] There is considerable variation in the length of time required to bring down the skins. Lamb states that half an hour is frequently sufficient, but at Trent Bridge 2–3 hours are generally required. The causes of the variation are not altogether clear, but depend partly on the previous history of the goods (fellmongering, liming, etc.), and partly on the water used.

During puering, the lime soaps in the skin are decomposed, and the fat set free--in an estimation of the fat, 0·151 per cent. on the wet skin was found before puering, and 2·48 per cent. after puering. The fat thus set free can be removed to a great extent by scudding; but in modern practice this is not pushed to extremes, as the grain is liable to be damaged by excessive scudding. The fat is afterwards removed from the dry leather by means of benzine or other solvent, employed in a suitable apparatus.

*Scudding.*--When the goods are “down,” they are ready for scudding. This operation in the case of grains is performed either by hand over the beam, or by means of a special scudding machine.

The scudding machine figured (Fig. 6) was invented by Sir John Turney in 1880,[17] and is the only type of machine which will safely scud split skins. It will readily be understood that the usual type of scudding machine, in which a spiral knife is used, would tear the delicate structure of the grain. The Turney scudding machine works with hard, circular brushes, which revolve against a wooden roller carrying the skin. At the same time a continuous stream of water, at a temperature of 110° F., is sprayed upon the skins from a perforated pipe.

[17] The original Turney scudding machine, was invented in 1871, and patented in that year. The specifications are numbered 1351, and 3310, 1871. It was also patented in America. Specification No. 131,480, dated September 17, 1872. Improvements in apparatus for cleaning and scouring hides and skins.

After scudding, the skins are washed in cold water for about ten minutes. This checks the action of the puer, and also washes away any loose dirt. They are then ready to pass on to the drench (Chapter IX.), or to the pickling process, or direct to the tan liquor, according to the kind of leather required.

*The Bating of Hides for Harness or Dressing Purposes.*--The hides intended for harness leather, or for dressing purposes, are usually bated with hen or pigeon manure. The exact process varies in different tanyards, being dependent upon the condition of the pelt prior to bating, as to whether it is bated directly after fleshing or receives a preliminary deliming. The process is also dependent upon the amount of bating action required for the specific purpose prior to tanning, but the following three methods are in actual use, and may be taken as typical methods:--

_Harness Backs._--About a hundredweight of hen manure is put into a suitable tub or vat capable of holding about fifty gallons; some thirty gallons of water, at a temperature of about 100° F., is poured on to the manure, and the whole is thoroughly stirred up by means of a wooden stick or plunger, working the manure about in order to mix it to a consistence of thin soup. The vat is now stored in a warm place, so that the temperature is kept about 70° F.--it is customary to store this in the boiler-house, or some similar warm room. It should be stirred two or three times a day, until it begins to show signs of working; two days is generally sufficient for this.

A pit is now prepared, with the necessary amount of water; and, by the aid of a steam pipe, or other similar means, the temperature of the water is raised to 70°, and the contents of the bate tub are now added. This should be poured into the pit through a sack, or a canvas filter, so as to keep back stones, solids, feathers, and other extraneous matters. The pit is now thoroughly plunged, and the goods entered; these should be handled up three to four times a day for two days. At the end of two days the goods are usually scudded, by working the back on an ordinary unhairing beam with an unhairing knife. They then go into a fresh pit containing a new solution of bate made up in the same manner, and, when sufficiently bated, which generally takes about three days, the goods are removed from the pit, scudded again by working them over with a slate scudding tool, when they are ready for tanning. Some tanners give them a bath of boracic acid prior to their going into the tanning liquor; this has the effect of materially brightening the colour. The second pack of harness backs go into the same liquor--which must be re-warmed to a temperature of 70°--in which they are handled as before for about two days, and are finished off in a new liquor made as above described.

It will be understood, therefore, that in a tanyard, working regularly, the pack of backs go first into an old bate for two days, and are then finished off in a freshly made one; so that each pack gets two days in an old pit, and one or more days in the new. The bate in the tub may also have a second water, or be strengthened.

_Dressing Hides._--For these goods a somewhat stronger bating effect is required, and pigeon manure is therefore frequently used. The preparation of the bating liquor is as described above, both as regards quantity and temperature of the water and time of maceration.

The whole process may be carried out exactly as described for harness backs, but the following is an illustration of another method in practical use. Imagine four ordinary bating pits, worked in a round. The first pit is very old, having had three packs of hides through it; the second having had two packs, and the third one pack; and the fourth is made new. The goods, after unhairing and fleshing, and washing, are put into the first pit quite cold, are handled in this three times on the first day, and afterwards put into the second pit, the temperature of the second pit having been previously raised to 65°--the cold pelt going into this pit reduces the temperature to about 60°. They are handled in this pit twice, and remain overnight; and the third day they go into the third pit, the temperature of which is raised to 70° before the goods go in. They are raised, scudded thoroughly on the grain, and new liquor is made from new bating liquor prepared as described above; the goods are now entered, handled at least twice a day, and, if insufficiently bated, are left overnight. By the next day they will probably be sufficiently “down” to permit of them going on to the scudding beam and thence on to tanning.

In the event of any hide being insufficiently bated--in other words, if the “bone” has not completely disappeared in the inner part of the hide--they are returned once more for three or four hours, the temperature of the pit being raised to 70° with a steam coil.

By this system, which is known as a system of “sets,” it will be noticed that the pits are worked in a round of four; the first pit, or oldest, being used more or less as a wash pit, the actual bating taking place in the second, third and fourth pits. No pit has more than four packs passing through it, and the temperature never rises above 70°, or, at the outside, 75°. The reason of the comparatively low temperature is due to the fact that in pit bating, if the temperature is raised above 75°, the grain of the hide becomes affected before the internal “bone” is sufficiently soft, which results in a tender grain, which considerably affects the resulting staining or dyeing which the leather usually receives before it is put on the market.

_The Third Process_.--This process is suitable for either harness or dressing hides, and differs from the above-described process only in the fact that instead of bating the hides in pits, a latticed drum is used in order to keep the hides in constant motion. This consists of a large pit, in which a latticed drum revolves two-thirds immersed in the liquor. The bate liquor is made up as usual, and the temperature of the liquor raised to 75° or 80° F.; the hides are then placed in the latticed drum, which revolves at from four to six revolutions per minute, and, if the goods are put in the drum in the morning, and if the temperature is kept at about 70° F., the hides are generally sufficiently bated by evening. They may then be taken out of the drum, scudded, and left suspended in boracic acid overnight, and can then be taken to the liquor next morning.

Where there is considerable motion, even the heaviest hides will stand a temperature of 75°, and even 80°. Some tanners who use the latticed drum method, do not place their hides in the drum until the afternoon; they are drummed for about an hour in the cold bate, and left overnight; next morning the temperature of the bate is raised to 75°, and the drum started; they are then removed from the drum towards the afternoon, when sufficiently “down” to admit of them passing forward into the next process.

The above are three typical methods in common use in England, but many modifications of these processes exist. Everything depends upon the amount of lime in the hide, on the number of haulings or handlings given to the goods, and the temperature or the conditions under which the hides are bated, so that no hard-and-fast lines can be laid down. Much also depends upon the method of liming used, prior to the bating. It must be borne in mind that the bating of hides, and even of kips, differs materially from the bating of goat and sheep skins, as one has to deal with double or even treble the substance. If the bate is worked at a temperature of even 70° or 75°, there is the danger of the grain being seriously affected before the bate has penetrated to the interior of the pelt, unless the goods are kept in constant motion.

Instead of a latticed drum, large paddles are used in some factories. If the pit is large enough the paddle causes both hides and liquor to revolve, and thus gives the required movement necessary for a regular and even “bating effect” to be obtained.