WATER AS USED IN TANNING.

Water, as obtained from rivers, wells, or water companies, contains a variety of impurities which affect its use in tanning, but of which in most cases the precise influence is very imperfectly known. These may be classified into (1) merely suspended matters, such as clay and mud, and sometimes animal or vegetable organisms such as infusoria; (2) dissolved mineral matters, which consist mostly of lime and magnesia salts and which make the water hard; (3) and organic dissolved impurities, such as the brown colour of peat water and the putrefying animal matters of sewage contamination.

Mud is always objectionable. It frequently contains organic slime and organisms which encourage the putrefaction of hides put in it to wash or soften. It also almost invariably contains iron as one of its constituents, and hence stains leather, and gives bad coloured liquors. It is not easily got rid of by filtration, as large filter-beds are expensive and difficult to keep in order, and much space is required to clear water by subsidence. Some filter easily cleaned offers the best chance of success. The Pulsometer Company supply such a filter, consisting of sponge tightly packed below a perforated piston. To cleanse the filter a stream of water is passed the reverse way, and the piston raised, and worked up and down, either by hand or power, so as to loosen and knead the sponge. The Atkins "water scrubber," in which sand may be used as a filtering medium, seems also well adapted for the purpose. If lime be precipitated by Clark's, or other process, it usually carries down the mud with it.

Rain water and the water of streams in mountain districts of hard igneous rock are generally nearly free from mineral constituents. This is the case with the Glasgow water from Loch Katrine, and the Thirlmere water which is to supply Manchester. Such water, if cold enough, and free from mud and organic impurity, is the best for almost every purpose in tanning. Most river water, however, and all spring water, is contaminated with mineral matter which it has dissolved out of the soil and rocks through which it has flowed. The principal of these mineral constituents are lime and magnesia. These occur both as sulphates and chlorides, and as hydric carbonates, or "bicarbonates." The sulphates and chlorides constitute "permanent" hardness, while that due to bicarbonates is called "temporary," from the fact that on boiling, half the carbonic acid is driven off, and the lime or magnesia is deposited as an insoluble neutral carbonate, thus softening the water. Any water which can be softened in this way by boiling may also be softened by the addition of a suitable quantity of lime, thus:—

This is Clark's process, and the chalk may either be separated by subsidence, which quickly takes place, or by a special filter (Porter-Clark). Thus the Bristol water, which from determinations by Mr. W. N. Evans, contains considerable temporary hardness and but little of permanent, may be almost completely softened by Clark's method. (For method of determining hardness and quantity of lime required, see p. 97).

The lime and magnesia constituting permanent hardness may be removed by the addition of sodic carbonate (soda ash or crystals); but this is expensive on a large scale, and as an equivalent quantity of sodic sulphate or chloride is left in the water, it is for most purposes of questionable advantage, though in some cases useful for the feed water of boilers. When employed for this purpose, the water should if possible be softened and settled before using, instead of adding the soda in the boiler itself, as is generally done. Soda is the active ingredient of many boiler compositions. For preventing furring, most tanning materials or even waste tan-liquors are very effective, and the danger of any corrosive action is lessened by the addition of a portion of soda ash. So far as is yet known, from the tanning point of view, it is hardly necessary to make any distinction between lime and magnesia, which may be considered simply as "hardness." A hard water probably softens dried hides more slowly, though it is possible that the observed difference may be due in many cases to the lower temperature of wells from which hard water is generally derived. In the actual limes, the hardness of the water can have no appreciable influence, though if sodium sulphide be used alone, a certain waste occurs from temporary hardness, which may render it advisable to add a little lime. It is in the washing of the hides from lime that the influence is first distinctly felt. If limey goods, after unhairing, are placed in a water with much temporary hardness, the same action occurs as in Clark's water softening process, and chalk is deposited in the surface of the hides, making them harsh and apt to "frize" or roughen the grain in scudding, and causing bad colour by combining with the organic acids of the liquors. The common, but not wholly satisfactory, expedient is to add a little lime, or better, a few pailfuls of lime liquor to the water before putting in the hides. The best plan is to use a properly softened water. Permanent hardness is not injurious in this way.

The hardness of water, and the dissolved carbonic acid which it contains, are, together with its temperature, the principal factors which determine whether a hide will plump or fall in it. Almost the only accurate investigation of this point has been made by W. Eitner ('Der Gerber,' iii. 183). He placed pieces of hide, unhaired by sweating, and quite flat and fallen, in water for 4 days at a temperature of 46° F. (8° C.), with the following results:—

The peculiarities which were shown by the hide pieces on removal from the water were maintained throughout the tanning, which was conducted in imitation of the German method, the hide being swollen and coloured through in weak birch-bark liquors, made with distilled water and acidified in each case with equal quantities of lactic acid, and finally laid away till tanned in a mixture of oak bark and valonia. No. 6, from magnesia sulphate, was the best; then No. 2; No. 3 was less good, but all the pieces from 1 to 6 were firm, close, and of good substance and texture, No. 1 having swelled well in the sour liquor. On the other hand, 7 and 8 scarcely swelled in liquor, but remained flat throughout, and were looser, thinner, and of finer fibre. From this experiment it is clear that while sulphates and carbonates exert a favourable influence on plumping, chlorides do the reverse, not only not plumping themselves, but placing the hides in an unfavourable condition for the plumping action of acids in the liquors. These experiments are quite borne out by the writer's experience in practice. The water at the Lowlights Tannery, which in dry weather is mostly obtained from beds of what was originally sea-sand, and which consequently contains a very abnormal proportion of chlorides (up to 68 pts. NaCl per 100,000), requires special and very careful management to make thick leather, notwithstanding its containing a considerable quantity of calcium and magnesium sulphates. These facts also indicate the importance of the thorough removal of salt from hides intended for sole-leather. Plumping is not a desirable thing in leather intended for dressing purposes, and it is possible that the use of a small percentage of salt in the liquors or wash waters might enable bating to be dispensed with. Like a bate, salt would dissolve a small proportion of hide substance (see p. 19). There is no practicable means of removing chlorides from water, but Eitner suggests the addition of a small quantity of sulphuric acid to water containing much temporary hardness (bicarbonates), by which it is converted into permanent (sulphates), which, as we have seen, plumps better. For this purpose about 2·8 oz. of ordinary English vitriol (sp. gr. 1·490) per 100 cub. ft. of water is required for each part of lime Ca(OH)₂ per 100,000 (see p. 97 for testing of water). A simpler guide is to add enough to purple, but not to redden litmus paper, even after moving it about in the water for some minutes. The acid must of course be well mixed by plunging. It must be borne in mind that Eitner's experiment was on sweated hides, and that with limed hide, which is kept plump by the dissolved lime retained in the hide, the conditions are different, and different results as regards carbonic acid and bicarbonates would probably be obtained. Both these would convert the lime in the hide into chalk, which is both insoluble and inert, and the hide would probably fall, at any rate till the lime was completely carbonated, while hides would remain plumpest in waters most free from substances capable of neutralising lime. One of the waters most effective in plumping limed hides is that of the river at Lincoln. Its hardness and contents in chlorine is, as compared with Lowlights water in dry weather,

Both waters have a considerable quantity of organic matter, and both owe their hardness in part to magnesia. From this we might conclude, what may be À priori expected, that the softer the water, the plumper limed hides remain in it. I am informed, however, by Mr. S. L. Evans, that in the Dartmoor water, which is very soft, but peaty, hides fall rapidly. In this case the colouring matters of the peat, which are of the nature of very weak acids, probably neutralise the lime. It may also be remarked, that wherever the conditions of putrefaction or decaying organic matter is present, hides rapidly fall, for the same reasons as they do in a bate.

While the injurious effect of bicarbonates on limed hide is matter of common experience, their influence on liquors and tanning is not so well understood. It is certain that they neutralise and combine with the organic acids of the liquors, and probably with some species of tannin, and as 1 part per 100,000 amounts to 1 oz. per 100 cub. ft., the acid required to neutralise a very hard water amounts to something considerable. It is well known that hard waters make bad tea, and the influence of hardness on the extraction of tannin is a subject well worthy of investigation, and which the writer hopes to examine.

On dyeing, at least as regards dye-woods, the influence of bicarbonates is distinctly favourable, and this is also stated to be true of woad, cochineal, and indigo-carmine.

Beside lime and magnesia salts, water may contain sulphates and chlorides of soda and potash; but not carbonates of these bases in presence of permanent hardness. In soft waters carbonates are sometimes present, and form carbonate of lime in limed hides. Hides are said to soften rapidly in such water. Alkaline sulphates are not known to have any injurious action, and chlorides have already been spoken of. Iron may be present in solution as bicarbonate, but not in any other form in presence of bicarbonate of lime. It is removed completely with the temporary hardness by Clark's process, or boiling. Iron is much more common merely in suspension, as mud, but is always objectionable. Most waters contain a little silicic acid and alumina, and some few considerable quantities. Such waters are said to harden leather, but the writer knows of no case where they are in use in England; and their occurrence is comparatively rare.

For comparison, analyses of a few spring and river waters are given on p. 89.

Analyses of various Waters.