Fig. 20.

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Several modifications of the Clark process have been introduced, in which the precipitation is carried on continuously instead of intermittently. The most important of these is the Porter-Clark, in which one portion of the water to be softened flows through an agitator containing excess of lime, with which it forms saturated lime-water, which is passed slowly up a cylinder where it deposits the excess of suspended lime. The clear lime-water so produced is mixed with a fresh portion of the water to be softened in a second cylinder also provided with an agitator, the proportion of the two liquids being regulated by cocks. The carbonate of lime is at once precipitated, and is removed by passage through a filter press. This process is in successful operation on a considerable scale at Messrs. Hodgsons’ tannery at Beverley.

Several other forms of filter have also been employed with success, and also methods in which the treated water traverses tanks with sloping partitions on which the carbonate of lime is deposited. The latter plan was originally patented in France by Gaillet-Huet, and has been introduced into England by Stanhope.

So far as is yet known, from the tanner’s point of view, it is hardly necessary to make any distinction between lime and magnesia, either or both of which may be considered simply as “hardness.” A hard water probably softens dried hides more slowly than a purer water, though it is possible that the observed difference in the time required 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 for unhairing, a certain waste occurs from temporary hardness which may render it advisable to add a little lime. It is in washing the hides free from lime that the influence of hard water is first distinctly felt. If limy 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.” 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.

Unfortunately it is not the grain alone which is injured by the use of hard water for washing the hides, but on coming into the liquors the precipitated bases combine with the acids and tannins, forming compounds which oxidise and darken when exposed to the air, and which are the commonest causes of stains and markings on all descriptions of leather. Even when goods are drenched or bated before tanning the injury is not prevented, since the weak organic acids which are capable of removing the lime (as such) from the hide have little effect on the precipitated carbonate, which can only be dissolved by the use of stronger acids. It must be noted that the same injurious effect on limed goods is produced by free carbonic acid, which may be present even in soft waters.

When temporarily hard waters are employed for leaching tanning materials, the carbonic acid is displaced by the tannins, which form compounds similar to those just mentioned, which are incapable of tanning, and darken and discolour when exposed to the air. Though the amount of lime present in a liter of even the hardest water is very small, yet in the aggregate of thousands of gallons used weekly in a good-sized yard it amounts to something very considerable, and as the molecular weight of tannins is very high, the quantity destroyed is many times that of the lime present. This loss can be prevented (a) by the addition of sufficient mineral acid to convert the temporary into permanent hardness, (b) by the use of oxalic acid, which precipitates the whole of the lime as oxalate, or, (c) best of all, by softening the water by suitable treatment before use. Each part of temporary hardness reckoned as CaCO3 (L.I.L.B., p. 19), requires 1·26 parts of crystallised oxalic acid or 0·98 parts of H2SO4, or say one part of ordinary oil of vitriol of sp. gr. 1·840 per 100,000 parts of water.

As the lime and magnesia of temporarily-hard water is thrown down by boiling, it is deposited in steam boilers as a soft precipitate, much of which can be blown out by suitable sludging; but if oils or fats obtain access to the boiler, a soft, bulky, adherent deposit is formed, keeping the water from the plates, which may become red hot, and lead to collapse or explosion. This effect is not produced by mineral oils, which, on the contrary, tend to prevent adherence of scale to the plates, and as suitable mineral oils are not only cheaper, but much less injurious to the working parts of steam engines than animal or vegetable oils or tallow, they should always be used in preference for cylinder purposes.

Water which is temporarily hard owing to calcium and magnesium carbonates, is unsuitable for dyeing, as the carbonates react with basic dyes, precipitating the colour-base, and so rendering a part of the dye useless. Further, as this precipitate is deposited on the skins it causes uneven dyeing and gives rise to spots and streaks. In dyeing with basic dyes, therefore, it is advisable to add sufficient acetic acid to the water before use to exactly neutralise the carbonates present. Of course this treatment is quite unnecessary when acid dyes are employed, as acid is usually added with the dye, and with dyewoods the presence of a little calcium salt is advantageous.

As each “degree” of total hardness represents a soap-destroying power of at least 2 oz. of soap per 100 gallons of water, allowance must be made in making up “fat-liquors” with soap and oil for the loss of soap due to its precipitation by the mineral matter in the water. The sticky lime-soaps are apt to adhere to the leather and interfere with glazing; so that it is much better to employ a soft water.