For the purpose of liming, the quality of the lime used is of the utmost importance, the milk of lime being frequently quite valueless by reason of having become carbonated or a bad quality of quick lime having been originally employed. It should be borne in mind that only the hydrate of lime which is present in solution in lime water is of use, whereas in milk of lime so much carbonate and other impurities may be present that the liquid, though thick, may be quite useless. The value of a lime should always be tested by determining the amount of real calcium hydroxide, Ca(OH)2, contained in it. The operation according to S. Rideal, is conducted as follows: Water free from carbonic acid is first prepared by boiling distilled water for half an hour in a strong, round-bottomed Bohemian or Jena flask. While steam is still issuing, the flask is removed for an instant, closed by a well-fitting greased cork or a rubber stopper, and allowed to cool. When the temperature has somewhat fallen, the cooling may be cautiously accelerated by dipping into a pail of warm water, then transferring to the cold stream from a tap. The water may be preserved in the flask or, preferably, a number of bottles with vase-lined stoppers should be filled quite full and retained for use.
From the sample of lime, well mixed, a small portion (about 0.25 gramme) should be accurately and rapidly weighed, placed in a wide-mouthed, stoppered bottle holding about 300 cubic centimeters, 250 Cc. of the boiled water added, and then allowed to settle. The whole of the calcium hydrate will now have dissolved. Fifty cubic centimeters of the clear liquid should now be withdrawn by a pipette, transferred to a flask, colored with an indicator—either phenol-phthalein, methyl-orange, or litmus may be used—and its alkalinity determined by running in decinormal hydrochloric or sulphuric acid from a burette till the change of color occurs. Each cubic centimeter of the acid corresponds to 0.0028 gramme of calcium oxide, or 0.0037 gramme of the hydrate, Ca(OH)2. The amount by calculation will give the percentage of real lime present in the sample. It is well to notice that any soda or potash present will equally neutralize the acid, and be returned as lime, but as these are of almost equal efficiency their presence in small quantity has no disadvantage. For special work it will be necessary to have a full analysis. As a rule the product made from limestone, or “stone lime,” is the best article in commerce, and is much more free from stones and clay than “gray lime” or “shell lime.” The best stone lime contains sometimes only ½ per cent. of impurities, and seldom more than 5 per cent., while inferior kinds of gray lime may contain as much as 50 per cent., and would be of little use in glue-making.
After removal from the lime pit, the material is placed in willow baskets or nets, and immersed in the stream to remove the greater portion of the lime, which is generally effected in a few days. It is still more effectively accomplished by placing the waste, after soaking in the willow baskets, in the wash drums. After taking it from the baskets or wash drums it is spread in the drying yard to drain and dry, the desiccation being accelerated by turning it over with a fork several times a day. While drying, the quick-lime is converted into carbonate, the latter exerting no disturbing effect in the manufacture of glue. When sufficiently dried, the material is ready for boiling, and the crude glue thus obtained can be stored for any length of time, until wanted for further manipulation.
In summer it is scarcely possible to cleanse the raw material as rapidly as it is brought to the factory, and to work it immediately without putrefaction setting in, and for this reason it would frequently be risky to purchase larger quantities of it, even if offered at very favorable terms. During the colder season of the year, drying of the cleansed raw material is such a slow operation, that in order to prevent putrefaction, recourse would have to be had to artificial heat.
These drawbacks can, however, be overcome by the use of carbolic acid, which possesses in a high degree the property of preventing putrefaction. It is quite cheap, and as but a comparatively small quantity of it is required, the additional cost need scarcely be taken into consideration, since the value of glue-stock annually destroyed by putrefaction is considerably greater than the expense for carbolic acid.
The raw material is thoroughly cleansed, and while in a moist state is gradually brought into a brick cistern or large vat, carbolic acid solution being poured over each layer, so that, when the cistern or vat is filled, it stands about an inch or two deep over the material. The latter may be left in this state until wanted.
The carbolic acid solution is prepared by dissolving 2 lbs. of carbolic acid in 1000 quarts of water; the fluid thus obtained possessing a slight odor of smoke. The washed glue-stock treated as above described with carbolic acid remains absolutely unchanged, and when wanted needs only be taken from the cistern and worked like fresh material.
In plants having no running water at their disposal and depending entirely on well water, and where the waste water has to be discharged into rivers or creeks, water containing carbolic acid should be used for all the washing operations, a fluid containing 1 to 2 parts of carbolic acid in 10,000 parts of water being sufficient for this purpose. Such an addition of carbolic acid prevents the wash-water from becoming foul.
Carbolic acid has the tendency of hardening the glue-stock and imparting its odor to the glue, and among other antiseptics, formaldehyde and boric acid have been recommended for the purpose of preventing putrefaction for a reasonable time. Formaldehyde in weak solution (1 part in 10,000 to 100,000 parts water) has been found beneficial. In this small quantity it does not harden the stock nor affect the subsequent boiling, as it is dissipated by the heat. Boric acid and its preparations, notwithstanding their low antiseptic power, are much in favor. A fluid containing 1 part boric acid in 200 parts water will have to be used.
The principal varieties of hides and leather for glue-stock may be classed as follows: