As few architects have specially studied the construction of tanneries, and in most cases much of the arrangement depends on the knowledge of the tanner himself, a short chapter on the subject will not be out of place.

In the selection of a site, a clay or loamy soil is to be preferred to a gravelly or sandy one, as lessening the liability to leakage, and waste of liquor. Perhaps, however, the first consideration of all is the possibility of drainage and disposal of effluent waste liquors and washing waters, since it is now rarely possible to run these, without previous treatment, into a river or stream. Some information is given in Chapter XXVIII. on the methods of partial purification which are available to the tanner, but these are always costly and troublesome, and the possibility of running direct into a sewerage system, or a tidal river is of great advantage. Under the Public Health Act, authorities are bound to receive manufacturing effluents into their sewers if the latter are of sufficient capacity, and the effluents not such as either to damage the sewers, or interfere with the processes of purification adopted by the authority. This act is in many districts practically superseded by special legislation, but tanners’ effluents are generally received into sewers if freed from solid matter. When mixed with other sewage, they do not interfere with irrigation or bacterial treatment. In selecting a site within a sewered district, regard must be had to the possibility of causing a nuisance to the neighbourhood by foul smells. Really injurious smells should not be caused by a properly conducted tannery, but it is difficult to avoid odour, and a single badly disposed neighbour may cause infinite trouble and expense.

Another important consideration is the water supply, since for the large quantities used in a tannery, town water is generally very expensive. With regard to quality and impurities of water information may be found in Chapter X.; but, as a general rule, the softer and purer the supply the better. It is also of great advantage when the source is at such a level that the water can flow into the tan-yard, or at least into the beam-house, without pumping. Filtration too, when needed, is much facilitated by a sufficient head of water.

Commercial facilities, such as nearness to markets and sources of supply of raw materials, and the availability of rail and water carriage are of an importance at least equal to the points already considered, but hardly come within the scope of this work.

The site chosen, the next question is the arrangement of the buildings. It is very doubtful, where ground is not inordinately expensive, whether it is wise to erect drying-sheds over the pits. In case of fire, very serious damage is done to liquor and leather by the heat and burning timber. If the turret form of drier be decided on, strong foundations are required, and the ground-floor or basement is occupied with heating apparatus; if fan-drying, no lofty buildings are needed, and the drying rooms are conveniently placed over the finishing and currying shops; and, on the other hand, the tan-house may be easily and cheaply covered with slated roofs, with nearly vertical sections of glass, to the north if possible, like a weaving-shed, through which sufficient light for convenient work and cleanliness is admitted. The direct rays of the sun should be avoided, but in the writer’s opinion the balance of advantage is largely in favour of a liberal supply of light. Iron roofs are unsuitable, since the moisture condenses on, and rusts them; and particles of oxide fall into the liquors, and cause iron-stains.

Good ventilation along the ridge of the roof should be provided, wherever there is any steam or hot liquor used; or the condensed moisture soon leads to decay.

In arranging the general plan of the buildings, much depends on local circumstances; but as far as possible, they must be so arranged that the hides and leather work straight forward from one department to another with as little wheeling or carrying as possible; that the buildings where power is used be near to the engine so as to avoid long transmissions, which are very wasteful of power; and that the different buildings be so isolated as to diminish the risk of the whole being destroyed in case of fire.

A chapter on the construction and maintenance of tanneries and leather works would be incomplete if it did not refer to the very important question of Fire Insurance.[185] To an extent this may be regarded as a fixed charge against any business, very much in the same way as local and imperial rates. It is not, however, to be lost sight of, that to some considerable extent the amount of insurance premium is regulated by the insured himself. If a man conducts his business in unsuitable and badly constructed buildings; if attention is not paid to some of the elementary hazards connected with a fire outbreak; he must not blame the insurance companies for the demand of what he considers an excessive premium. If this faulty construction and imperfect equipment of buildings pertain to any considerable extent throughout a given trade where the process is more or less hazardous, it is futile to appeal to insurance companies, which, after all, are merely commercial and not charitable institutions, for a reduction in the rates. The only standard to guide the company is the loss-ratio, and given a high loss-ratio, there must be a corresponding premium paid.

There is, however—thanks to modern science—a method available whereby the great bulk of fires may be checked in their inception; an appliance, automatic in its operation, and of proved efficiency. This appliance is known as the sprinkler. A system of water-pipes is fixed under the ceilings of the building to be protected, to which are attached sprinkling jets at suitable intervals, each of which is closed by a valve held in place by a joint of fusible metal, which gives way if the temperature rises beyond a given point. There are two or three recognised patterns approved by the Fire Offices Committee after patient investigation and practical test. These appliances have now been at work for something like fifteen years in this country. One of the first trades to recognise their utility was that of the cotton-spinner. At one time serious fires in the cotton trade were of frequent occurrence. Now—owing to the efficient fire appliances—while fires may be as frequent in their inception as formerly, they are stopped at such a stage as to prevent any considerable loss. The consequence has been that the cotton-spinner, at one time the owner of a highly-rated risk, and one which few companies cared to insure, is now in the position of having his business eagerly sought after, and large discounts offered him off the charges he was once called upon to pay.

More important still is the consideration to him that his business is not so liable to be interfered with or stopped as the result of fire. There are, it is estimated, at the present moment, no less a proportion than 90 per cent of the cotton-spinners whose premises are protected by sprinkler installations.

Other hazardous risks such as corn-millers’, woollen and worsted manufacturers’, saw-millers’, engineers’, are adopting these appliances freely, and it is a matter of surprise that so very few tanneries or currying shops—so far as I have been able to learn, not more than twelve—have done the same. The consequence is that the loss-ratio in tannery risks still retains its unenviable notoriety: the rates for fire insurance have risen considerably, and as a result the tanners’ profits are correspondingly less. Considering the extent and importance of many of the tannery risks throughout Great Britain, one can only express surprise that these appliances have been so little adopted.

The construction of a new tannery demands serious attention from an insurance standpoint. The boiler-house should be a detached building; the grinding of bark and myrobalans should be conducted in buildings isolated from the general works; in fact no better advice could be given to a tanner, either in the construction of new premises, or the rearrangement and remodelling of old, than to consult an experienced insurance man, whether official or broker, as to the best means of constructing and arranging to secure the most favourable terms.

Another point which should be provided for, and which is often overlooked, is the feasibility of future extension without serious changes of arrangement. It may be taken as a probability of the future, even if it be not already a fact, that small tanneries cannot be made to pay, and that if a business succeeds, its extension will prove desirable; and in an ill-planned yard this may involve either entire reconstruction of a very expensive and inconvenient sort, or the separation of new departments, so as to involve serious increase of carrying. A good arrangement is that of a long front building serving to connect the whole, behind which the various departments are erected at right angles leaving room for extension backwards as required.

As regards the first of these conditions, if the various soaks, limes, bates, and handlers are well arranged, it is hardly necessary to do more than draw the goods from one pit into the next throughout the whole of the process. To, and from the layers, the goods must generally be carried or wheeled. In the sheds, if it be a sole-leather tannery, the butts should first come into turrets or open sheds for the rough drying; then into a room sheltered from draughts to temper for striking. The striking machines or beams should be in an adjoining room, or immediately below; then a small shed-space for drying before rolling; next the roller room; and then the warm stove for drying off. If two of the latter can be provided to be used alternately, it will allow the goods to be aired off without taking down, and they may then be immediately handed or lowered into the warehouse, without fear of over-drying, which is sometimes difficult to avoid where leather must be taken direct out of the hot drying-room. The same principles are easily applied in yards for lighter leathers.

To lessen loss of power in transmission, the engine should be near the centre of the main range of buildings, with perhaps the grinding machinery on one side, and the leather finishing on the other; but this would be rather liable to increase the fire-risk. A very good plan would be to have the engine-house in the centre as suggested, but separated from the buildings on each side by brick gables; and with the boiler-house behind it, and under a separate roof, say of corrugated iron. If it be impossible to have the engine near its work, it is in most cases better to employ a separate high-pressure engine, which may be within a glass partition, and will work all day with scarcely any attention. The loss of power in carrying steam for moderate distances through sufficiently large and well-clothed pipes is much smaller than that of long lines of shafting. The writer has known cases where fully half the indicated power of the engine was consumed in friction of the engine, shafting and belts. High-pressure engines are as a rule to be preferred to condensing for tannery use, since the waste steam can generally be employed for heating, and both the first cost and that of maintenance are smaller. Where much fuel is used, it is quite worth while to have the cylinders indicated occasionally, both running light, and driving the machinery; much information is gained in this way as to the power spent on the various machines, and very frequently large economy is effected by proper adjustment of the valves. To work economically, an engine should be of ample power for all it has to do; and adjusted to its work, not by lowering the pressure of steam, or by checking it at the throttle-valve, but by setting the slide-valves to cut off as early in the stroke as may be. As to how early this is possible, an indicator-diagram will at once give information. If the whole of the waste steam can be used profitably for heating purposes, economy in the working of the engine is of little consequence, but, otherwise, it is very injudicious, for the sake of a little saving in first cost, to put in an old or inferior engine, which has to be dearly paid for in waste of fuel. In the choice of an engine, the advice of an expert engineer is desirable, since many engines which are mechanically well made, are uneconomical through the faults of a rule-of-thumb design. In this respect the English engine-builder is frequently inferior to his better trained continental competitor.

In place of using small steam engines to distribute power, electric driving deserves consideration. For long drives the loss of power is much less than that of shafting, and by concentrating the whole production of the power in one large and well-constructed engine, the cost per horse-power can be much reduced. While large and well-constructed engines may develop 1 horse-power at a cost in coal of 1¹/₂ lb. per hour, it is not uncommon to use 12 lb. for the same output. In tanneries, however, the power used bears a much less proportion to total expenses than it does in the textile and many other trades. The first cost of electric driving is somewhat high. Motors of the “armoured” or iron-cased type must be used in all positions where they are subject to wet or dust. It must be borne in mind that an electric motor will not start against a heavy load, as it only develops its full power at a high speed, and if it receive the full pressure of the current before this is attained, its coils will probably be burnt out, unless saved by the melting of its safety-fuse. A similar danger is incurred, if the motor is brought up by overloading while the current is on. It is therefore generally necessary to connect a motor with its work by a belt which is only brought on to the working pulley when its full speed is attained.

In some cases the use of gas-engines is convenient and economical; for though gas from town-supplies is an expensive fuel, the best gas engines give a higher mechanical efficiency than steam-engines, and they work with very little attention.

In arranging shafting, moderate speeds, say 100-150 revolutions per minute, should be chosen for main lines, and when higher speeds are necessary, they should be got by light and well balanced counter-shafts, with wrought iron or wooden pulleys. (Cp. p. 452.) In calculating speeds, it must be remembered that they vary inversely as the size of the pulleys. Thus a 3-feet pulley running at 100 revolutions will drive a 2-foot pulley at 150 revolutions, and a 12-inch one at 300. Of course the higher its speed, the more power any shaft will transmit, but increased friction and wear and tear soon limit this advantage. The velocity of a belt in feet per minute is obtained by multiplying the number of revolutions per minute by the girth of the pulley in feet or by its diameter multiplied by 3¹/₇, or more accurately, 3·1416.

Pulleys should always be of ample breadth for the power they have to transmit; and it is more economical, both in power and cost, to use broad single belting than the same strength in double. If the pulley will not take a belt broad enough for the work it has to do, a second belt may be made to run on the top of the first, as suggested by Mr. J. Tullis, and will do its share of the work. Belts should be washed occasionally with soap and tepid water, and oiled with castor or neatsfoot oil; but if of sufficient breadth, should not require the use of rosin, or adhesive materials, to make them grip the pulley. Chrome-leather belts should be kept thoroughly oiled. They have a much greater adhesion than vegetable tannages, and this is increased by oiling. Good chrome belting is much stronger than bark-tanned; and is unaffected by damp or steam, but generally stretches somewhat more. Makers of machines often err in constructing their driving pulleys too small both in breadth and diameter.

The horse-power which a belt is capable of transmitting obviously varies extremely with circumstances, but may be approximately calculated by the formula a.v66000, where a is the area of contact of the belt with the smallest pulley, and v its velocity in feet per minute. Another rule is, that at a velocity of 1000 feet per minute, each inch of breadth of belt should transmit 2¹/₂ horse-power on metal pulleys, or 5 on wooden ones, on which the adhesion is greater. Adhesion may also be increased by covering the pulleys with leather or indiarubber. Both rules assume that the belt is of ample strength. One horse-power would be transmitted by a belt running 1000 feet per minute with a pull of 33 lb. A good single belt should not break with a much less stress than 1000 lb. per inch of breadth, and should stand about ¹/₁₀ as much as a working stress.

The following table gives the experimental breaking stresses and extensions of some leathers. It may be noted that 1 square inch sectional area is equal to a belt 4 inches wide × ¹/₄ inch thick; and that kilos per cm² × 14·22 = lb. per inch².

Breaking Stresses of Leather.[186]

Good English tanned belting leather breaks at from 4500 to 5500 lb. per sq. inch sectional area.

Over-tanned leathers are less tough, whether of vegetable or mineral tannage, than those somewhat lightly tanned, and the tensile strength of leather varies considerably with the part of the hide from which it is taken, that from approximately over the kidneys being the strongest. Even thick and tough leather is easily torn if a cut or nick is once started, and all holes used in jointing belts should be carefully rounded. Glucose, and the use of acid in bleaching both lessen the toughness of belts, and they may also be rendered tender by the heat evolved in slipping on a pulley.

Countershafting and high-speed machinery, such as disintegrators, striking machines of the Priestman type, etc., should run without material jar or vibration. If this occurs, it is generally a sign that the running part is not equally balanced. In this case the shaft or spindle must be taken out of its bearings, and supported on two exactly horizontal straight-edges, on which it will roll till the heaviest part is downwards; and weight must then be taken off or added till it will lie in any position. In this way the writer has had to add fully 2 lb. of iron to balance the drum of a striking machine before equilibrium was secured, and a most troublesome vibration prevented. Of course all machinery should be supported as solidly as possible; and if circumstances permit, most machines are better on a ground floor. In placing bark mills, however, it is frequently convenient to fix them at a higher level, so that the ground material may be sent down shoots by its own weight to the required places. An alternative plan is to set the mill on the ground over a pit, and to raise the ground material with a bucket-elevator. This may be done successfully by letting the material fall directly from the mill into the buckets; but otherwise it must be thrown in with a shovel, as buckets will not pick up ground bark, even from a hopper; and in any case such elevators are apt to be troublesome. In a grinding plant designed by the writer, the unground material was filled on the basement floor into an iron barrow, which was wheeled into an iron sling working between upright guide-rails like a hoist. On pulling a brake line, the barrow was raised to the top of the building, and its contents were tipped into a large hopper, after which the barrow righted itself, and descended for another load. In the bottom of the hopper was a sliding shover, which forced the material on to vibrating screens, by which it was guided either into a disintegrator, or crusher-rolls, at pleasure. Both these discharged through iron spouts into large hoppers on the outside of a brick gable, from which powdery materials like myrobalans and valonia could be run direct into barrows or trucks. It is very desirable that such hoppers should be separated from the main building by a fireproof partition. Fires may occur from hard substances getting into disintegrators along with the bark, etc. and if this occur with a dry and dusty tanning material, it is not unlikely that it may result in an explosion such as sometimes happens in flour mills, in which the fire is rapidly conveyed along spouts, and into chambers filled with dusty air. Insurance companies generally charge an extra rate for disintegrators, and it is very desirable to keep the mill-house structurally apart from other buildings, either by actual separation or by the introduction of brick gables dividing the roofs. On the whole, however, mills of the coffee-mill type are probably quite as dangerous as disintegrators; since if they become partially choked, the heat caused by friction is very great.

In America, the fire-risk from mills is often lessened or prevented by the introduction of a jet of steam into the chamber or spout by which the mill discharges, but this is only permissible if the tanning material is conveyed at once to the leaches or yard.

The use of chain-conveyors for handling tanning material both wet and dry is practically universal in America, though comparatively rare in England. Various forms are used, the most common consisting of a chain of square links of malleable cast iron which hook into each other, so that a broken link can be immediately replaced (see p. 325). At intervals special links are inserted, which can be had of various patterns, for the attachment of scrapers or buckets. The endless chain runs in a trough of rectangular or V-shaped section, and is driven by a toothed wheel, over which it runs like a belt. In some cases the returning half of the chain can be utilised to bring back the spent tan on its way to the boiler house. For dry materials, cotton or leather belts with short wooden cross-laths attached, may often be used satisfactorily in place of the chain.

For lubricating purposes, mineral oils of high density are not more dangerous than animal or vegetable, but rather the reverse; as, though they are possibly more inflammable, and make more smoke, their mixture with cotton-waste and other porous materials is not spontaneously combustible, as those of vegetable and animal oils occasionally are. The danger of spontaneous combustion is very considerable when heaps of leather shavings or cuttings containing fish-oils are allowed to accumulate in warm workshops, and, especially near steam-pipes. Heavy mineral oils should always be used as cylinder-oils in high-pressure engines, in preference to other oils or tallow, since they are not decomposed by steam, and do no harm if blown into the feed-water, but serve to loosen and prevent scale and deposit. Ordinary oils and tallow, on the other hand, when submitted to the action of high-pressure steam, are separated into glycerin and fatty acids (see p. 351), and the latter corrode the valve faces and seatings, and are liable with “temporary hard” waters to form a very dangerous porous deposit in the boilers, which often leads to overheating of the tubes.

Next to the machinery, the pits demand special consideration. The chapter on the subject in the late Mr. Jackson Schultz’s book on ‘Leather Manufacture,’ is well worth attentive study as giving American practice on the subject.

The old-fashioned method of sinking pits is to make them of wood, and carefully puddle them round with clay, which should be well worked up before use. It is of no use to throw it in in lumps and attempt to puddle it between the pits, which will not be made tight, but probably displaced by the pressure. Such pits, if made of good pine and kept in constant use, are very durable, some of the original pits at Lowlights Tannery, constructed in 1765, having been in use till 1889. Loam mixed with water to the consistence of thin mortar may also be employed, the pits being filled up with water, to keep them steady, at the same rate as the loam is run in. Probably the best materials for pit-sides are the large Yorkshire flagstones. Where these are not attainable, very durable pits may be made of brick, either built with Lias lime, and pointed with Portland cement, or built entirely with the latter. Common lime cannot be used, as it spoils both liquors and leather; and even cements with too large a percentage of lime are unsatisfactory. Brick and common mortar are, however, suitable for lime-pits, and for these Mr. C. E. Parker’s plan of constructing the bottom of cement, the ends and sloping hearth of brick, and the sides of 3-inch planks bolted together is also very satisfactory (Fig. 96).

The writer has constructed wooden pits in two ways. In the one case, after making the excavation, beams were laid in a well-puddled bed of clay; on these a floor of strong tongued and grooved deals was laid, and on this the pits were constructed of similar wood to the floor, and puddled round with clay. In the second case the pits were built like large boxes above ground, and when finished, lowered on to a bed of clay prepared for them, and then puddled both around and between. It may have been due to defective workmanship in the first case, but those made on the last-named plan, which is that adopted from very early times, certainly proved the tightest and most satisfactory. Mr. Schultz describes a plan as the Buffalo method, in which a floor is laid as just described, and grooves cut with a plane for the reception of the sides, which are formed of perpendicular planks, each end and side being finally tightened up by the insertion of a “wedge plank.” Owing to the perpendicular position of the side-planks such pits would be difficult to repair in the common case of decay at the top.

Fig. 96.—Mr. C. E. Parker’s construction of Lime-Pits.