CONSTRUCTION AND MAINTENANCE OF TANNERIES.

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 water supply, since for manufacturing purposes town water is generally very expensive. With regard to quality and impurities of water, information may be found on p. 83; 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 as to 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.

Of scarcely less importance than the water supply is the drainage of the yard. It not unfrequently happens that tanneries are prohibited from discharging their refuse liquors, limes, and soaks into rivers and watercourses, and it is sometimes a matter of extreme difficulty to find any other way of getting rid of them. In default of an outlet, recourse must be had to precipitation and filtration, but this is a costly expedient, and in fixing a site for a new yard it is far better to provide against such a possible contingency. Should, however, such means become necessary, it may be borne in mind that limes and liquors in great measure mutually precipitate each other, and that if all the various refuse is run into one tank, mixed, and settled, much is accomplished in the direction of purification. The further treatment of the effluent water must be determined by its nature and composition.

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; and, on the other hand, the tan-house may be easily and cheaply covered with slated roofs, with 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.

As regards 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.

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 these 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.

To fulfil the second condition named, the engine should be at 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 contrary to the third requirement. 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. Figs. 61, 62, from Eitner's book on American Tanning, show the arrangement of a sole-leather tannery in the United States. 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. In arranging shafting, moderate speeds, say 100-150 rev. per min., should be chosen for main lines, and when higher speeds are necessary, they should be got up by light and well-balanced counter-shafts, with wrought-iron pulleys. In calculating speeds, it must be remembered that they vary inversely as the size of the pulleys. Thus a 3 ft. pulley running at 100 rev., will drive a 2 ft. one at 150 rev., and a 12 in. 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 min. 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, and will do its share of the work. Belts should be washed occasionally with soap and tepid water, and oiled with cod-oil; but if of sufficient breadth, should not require the use of rosin, or adhesive materials, to make them grip the pulley. 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

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 ft. per min. 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 india-rubber. Both rules assume that the belt is of ample strength. One horse-power would be transmitted by a belt running 1000 ft. per min. with a pull of 33 lb. A good single belt should not break with a much less strain than 1000 lb. per inch of breadth, and should stand about ¹/₁₀ as much as a working strain.

Countershafting and high-speed machinery, such as disintegrators, striking machines of the Priestman type, &c., 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 must be taken out of its bearings, and supported on two exactly horizontal straight-edges, when it will roll till the heaviest part is downwards; and weight must be taken off or added till it will lie in any position. In this way, the writer had recently to add fully 2 lb. of iron to 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 in the top of a building, 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-floor, 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 often troublesome. In a grinding plant designed by the writer, the unground material is filled on the basement floor into an iron barrow, which may be wheeled into an iron bow working between upright guide-rails. On pulling a brake-line, the barrow is raised to the top of the building, and its contents are tipped into a large hopper, after which the barrow rights itself, and descends for another load. In the bottom of the hopper is a sliding shover, which forces the material on to vibrating screens, by which it is guided either into a disintegrator, or crusher rolls, at pleasure. Both these discharge through iron spouts into large hoppers on the outside of a brick gable, from which, powdery materials like myrabolanes and valonia, can 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. The writer is glad to say, he does not know of a case of fire from disintegrators grinding tanning materials, but he is informed that a Carter's disintegrator employed in grinding bones in a manure works has repeatedly set fire to the flannel bag into which the dust was allowed to escape. If this were to occur with a dry and dusty tanning material, it is not unlikely that it might result in an explosion such as sometimes happens in flour-mills from a similar cause. On the whole, however, mills of the coffee-mill type are probably more dangerous than disintegrators; since if they become partially choked, the heat caused by friction is very great.

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, their mixture with cotton-waste and other porous vegetable materials is not spontaneously combustible, while vegetable and animal oils occasionally are. 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. 60), and the latter corrode the valve faces and seatings, and in combination with temporary hardness in the boilers form a very dangerous porous deposit, which often leads to overheating of the tubes.

Next to the machinery, the pits demand special consideration. The chapter on the subject in Mr. 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. 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, being still in use. 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.

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 from defective workmanship in the first case, but those made on the last-named plan, which is that adopted from very early times, have 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."

If bricks be used, great care must be taken that the cement is not merely laid so as to fill the joints towards the two surfaces of the wall, as is the habit of modern bricklayers, but actually floated into all the joints so as to make the wall a solid mass; or leaks can hardly be avoided. Cement pits are very good, and, though not particularly cheap in material, which must be of the best, are readily made by intelligent labourers under good supervision. The first step is to lay a level floor of good concrete, in which glazed pipes for emptying the pits may be embedded; care being also taken that all joints in these are thoroughly tight, since future repairs are impossible. The next step is to make frames, the exact length and breadth of the pits required, and perhaps 15 in. deep. These are arranged on the floor where the pits are to be, and the intervening spaces are filled with concrete of perhaps 1 of cement to 3 or 4 of crushed stone or brick. Rough stones and bricks may also be bedded in the concrete as the work goes on, to help to fill up. After the first layer has set, the frames may be raised and a second added, and so on. The work is generally finished by floating over it, while still damp, a little pure cement, to give a smooth surface. Before using, the cement should be tried on a small scale, to be sure that it does not discolour leather or liquors, and the pits should always be seasoned with old or cheap liquor before actual use.

If possible, both latches and handler-pits should be provided with plugs and underground pipes, communicating with a liquor-well some feet below their levels. Glazed fire-clay is very suitable both for pipes and plug-holes, which should be in the pit corners. Some means should also be provided for the ready clearing of the pipes when choked with tanning materials. A good plan is to let each line of pipes end in a liquor-well large enough for a man to go down. As it is almost impossible to make plugs fit without occasional leakage, it is not well to run pits with very different strengths of liquors to one well, but the layers, handlers, and different sets of leaches should each have their own, so as to avoid mixture. A good means of clearing pipes consists in a series of iron rods 3-4 ft. long, connected by hooks fitting into double eyes, as shown in Fig. 63. It is obvious that in a narrow pipe or drain, these cannot become disconnected.

It is, as Schultz points out, of questionable advantage to lay wooden troughs for supplying liquor to each pit under the alleys, since it is almost impossible to preserve them from decay; but the same objection would not apply to glazed pipes, well clayed or cemented. A very good and cheap plan in practice, is to let the liquor-pump, or a raised liquor-cistern, discharge into a large and quite horizontal trough raised 5 or 6 feet above the level of the yard, and provided with plug-holes at intervals, under which short troughs may be set to run the liquor into the various pits.