Page:EB1922 - Volume 31.djvu/783

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Filtration. The infusion must be filtered until optically clear. No correction for absorption is needed for the Berkefeld candle, or for S. and S.-sgo paper if a sufficient quantity (250-300 c.c.) is re- jected before measuring the quantity for evaporation, and the solu- tion may be passed through repeatedly to obtain a clear filtrate. If other methods of filtration are employed the average correction necessary must be determined in the following manner: About 500 c.c. of the same or a similar tanning solution is filtered perfectly clear, and, after thorough mixing, 50 c.c. is evaporated to determine " total soluble No. I." A further portion is now filtered in the exact method for which the correction is required (time of contact and volume rejected being kept as constant as possible) and 50 c.c. is evaporated to determine " total soluble No. 2." The difference between No. I and No. 2 is the correction sought, which must be added to the weight of the total solubles found in analysis.

An alternative method of determining correction, which is equally accurate and often more convenient, is to filter a portion of the tan- ning solution through the Berkefeld candle till optically clear, which can generally be accomplished by rejecting 300 or 400 c.c. and re- turning the remaining filtrate repeatedly; and at the same time to evaporate 50 c.c. of the clear nitrate, obtained by the method for which correction is required, when the difference between the residues will be the correction sought. (It is obvious that an average correction must be obtained from at least five determinations.) It will be found that this is approximately constant for all materials, and amounts in the case of S. and S.-6O5, 150 c.c. being rejected, to about 5 mgm. per 50 c.c., and where 2 grams of kaolin are em- ployed in addition, to "j\ mgm. The kaolin must be previously washed with 75 c.c. of the same liquor, which is allowed to stand fifteen minutes and then poured off. Paper 605 has a special absorp- tion for the yellow colouring matter in sulphited extracts.

Hide powder must be of a woolly texture, thoroughly delimed, preferably with hydrochloric acid, and must not require more than 5 c.c. or less than 2-5 c.c. of decinormal NaOH or KOH to produce a permanent pink colour with phenolphthalein on 6J grams of the dry powder suspended in water. If the acidity does not fall within these limits it must be corrected by soaking the powder before chroming for 20 minutes in 10 to 12 times its weight of water, to which the requisite calculated quantity of standard alkali or acid has been added. The hide powder must not swell in chroming to such an extent as to render difficult the necessary squeezing to 70- 75% of water, and must be sufficiently free from soluble organic matter to render it possible in the ordinary washing to reduce the total solubles in a blank experiment with distilled water below 5 mgm. per 100 c.c. The powder when sent out from the maker must not contain more than 14 % of moisture, and must be sent out in air- tight tins.

The detannization must be carried out in the following manner: The moisture in the air-dried powder is determined, and the quantity equal to 6-5 grams actual dry hide powder is calculated, which will be practically constant if the powder be kept in an airtight vessel. Any multiple of this quantity is taken according to the number of analyses to be made, and wet back with approximately 10 times its weight of distilled water. Very woolly powders require slightly more than 10 times their weight of water; a powder may be con- sidered " woolly " if it cannot be poured like sand from a beaker. Two grams per 100 of dry powder of crystallized chromic chloride, CrCl 3 6H 2 O, is now dissolved in water and made basic with 0-6 gram of Na2COs by the gradual addition of 11-25 c - c ' f normal NazCOs, thus making the salt correspond to the formula Cr-Cls (OH) 3. This solution is added to the powder, and the whole churned slowly for one hour. In laboratories where analyses are continually being made, it is more convenient to employ a 10% stock solution, made by dissolving 100 grams of CrCls6H 2 O in a little distilled water in a litre flask and very slowly adding a solution containing 30 grams of anhydrous sodium carbonate, with constant stirring, finally making up to the mark with distilled water and well mixing. Of this solution 20 c.c. per 100 grams or 1-3 per 6-5 grams of dry powder should be used.

At the end of one hour the powder is squeezed in linen to free it as far as possible from the residual liquor, and washed and squeezed repeatedly with distilled water, until, on addition to 50 c.c. of the filtrate of one drop of 10 % K 2 CrO4, and four drops of decinormal silver nitrate, a brick-red colour appears. Four or five squeezings are usually sufficient. Such a filtrate cannot contain more than o.ooi gram of NaCl in 50 c.c.

The powder is then squeezed to contain 70-75 % of water, and the whole weighed. The quantity Q containing 6-5 grams dry hide is thus found, weighed out, and added immediately to 100 c.c. of the unfiltered tannin infusion along with (26-5 Q) of distilled water. The whole is corked up and agitated for 15 minutes in a rotating bottle at not less than 60 revolutions per minute. It is then squeezed immediately through linen, one gram of kaolin added to the filtrate, stirred and filtered through a folded filter of sufficient size to hold the entire filtrate, returning until clear, and 60 c.c. of the filtrate is evaporated and reckoned as 50 c.c., or the residue of 50 c.c. is multiplied by six-fifths. The non-tannin filtrate must give no tur- bidity with a drop of I % gelatine, 10 % salt solution. The kaolin may be used by mixing it with the hide powder in the shaking bottle. The analysis of used liquors and spent tans must be made

by the same methods employed for fresh tanning materials. The liquors or infusions, being diluted, are concentrated by boiling in vacua, or in a vessel so closed as to restrict access of air, until the tanning matter is, if possible, between 3-5 and 4-5 grams per litre, but in no case beyond a concentration of 10 grams per litre of total solids, and the weight of hide powder must not vary from 6-5 grams. The results must be reported as shown by the direct estimation, but it is desirable that, in addition, efforts should be made, by de- termination of acids in the original solution and in the non-tannin residues, to ascertain the amount of lactic and other non-volatile acids absorbed by the hide powder and hence returned as " tanning matters." In the case of used tans it must be clearly stated in the report whether the calculation is on the sample with moisture as received, or upon some arbitrarily assumed percentage of water; and in that of used liquors whether the percentage given refers to weight or to grams per 100 c.c., and in both cases the specific gravity shall be reported.

Processes Preparatory to Tanning. Considerable advances have been made in the methods of preparation of the hides and skins for tanning, particularly in the liming process. The bene- ficial influence of the addition of sodium sulphide to lime in the depilation of skins and hides, in the direction of accelerating the process and effecting the desired result as regards loosening of the hair and epidermis, has long been well known. During recent years the employment of gradually increasing strengths of this addition to the liming process has been more largely practised, with the result that the time required for the depila- tion of various goods has been materially shortened. Particu- larly in the manufacture of calf, sheep and goat skins into chrome tanned leather, the use of this depilatory has assumed great importance. Dependent upon the characteristics required in the resulting leather, the percentage of sodium sulphide employed varies between i% and 5% of the soaked weight of skins. When dealing with somewhat common sun-dried or dry- salted skins, it is now general practice to use a very strong solution, with a view to reducing the time of the liming process to two or three days, whereas previously two or three weeks was not considered unduly prolonged.

Improvements have been made in the carrying-out of the liming process in the direction of applying mechanical labour- saving devices.

In order to obtain greater uniformity than is the case in the finished leather resulting from the older-fashioned method of laying the goods in a saturated solution of lime in a pit for varying periods, several manufacturers have, with a view to saving time and labour, installed special mechanical apparatus for the purpose of agitating the lime liquor. In the case of heavy goods, e.g. hides, the goods are either suspended in the lime-pit and the liquor agitated by means of a screw propeller or small paddle-wheel working in the bottom of the pit ; or the hides are suspended from a wooden or steel framework placed slightly below the surface of the liquor in the pit, and the necessary movement imparted by rocking the frame by mechanical power. A further method installed in several large work's consists in keeping the lime liquors in a state of agitation by blow- ing compressed air into the bottom of the pit, thereby causing the liquor to be continually in a state of motion during the period of irfimersipn of the goods.

With a view to minimizing the labour involved in handling hides or skins when liming by the pit method, liming in cages is now practised. In this method the goods are placed in a large wooden cage contained in the ordinary lime-pit; the cage and its contents can be bodily removed by means of a travelling crane working overhead. The goods are placed in the cage on one of the pits and are then transferred, as and when required, from one lime solution to another by attaching same to the travelling crane, withdrawing the cage containing the pack of hides and skins. The removal of the goods through the whole of the series of pits, and the transference of the pack to the unhairing and fleshing machines without the em- ployment of any hand-labour for handling the goods, may thus be accomplished by this means.

In the liming of light skins, sheep and goats, for example, the liming process has been considerably accelerated by liming in paddle- wheels instead of in pits; the goods being paddled several times daily during the period of their immersion, thus eliminating the labour involved in the older-fashioned method of withdrawing each individ- ual skin by hand from the pit and then replacing for the purpose of altering the position in the solution.

Puering and Bating. Great improvements have been made in the operations of puering and bating by substituting solutions containing pancreatic enzymes for the ordinary bird-excrement and dog-ma- nure infusion used for depleting skins after the liming process. Enzymes of pancreas absorbed in wood sawdust, or vegetable meal, mixed with varying proportions of ammonium chloride or boracic