The first step in the analysis of any material is to draw a sample truly representing the bulk, which is often by no means easy, while failure to accomplish it is probably the cause of more errors and disputes than any inaccuracy of the method of analysis itself. In very many cases, chemists are blamed for discrepancies which really exist in the samples supplied to them. The chemists of the International Association only hold themselves responsible for the accuracy of their analyses when the sampling has been done strictly according to the rules prescribed by their Association. On this account, all important samples should be drawn in the presence of a principal, or some other responsible person.
In liquid extracts, the thorough mixing of the liquid is of the greatest importance. Most extracts contain a portion of “difficultly soluble” tannins (see [p. 297]), which slowly settle to the bottom, or adhere to the sides of the cask; from which such expedients as merely rolling a full cask are quite inefficient to dislodge them. In fact nothing but taking the heads out of a sufficient number of casks, and actually stirring them with a suitable plunger, which should be specially applied to the sides and bottom, or emptying the entire contents of the casks into a tank in which the whole can be adequately mixed, is really thoroughly reliable, though at times it is necessary to be content with less satisfactory methods. In any case, when it is probable that samples must be submitted to more than one chemist, the whole should be drawn at once, thoroughly mixed and divided, and sealed in separate bottles, and in dividing a sample the same care must be taken to ensure complete mixture, as in drawing the original sample.
Solid and pasty extracts, such as quebracho, cutch and gambier, are still more difficult to sample fairly, as the outside is almost invariably much drier than the interior. Generally the only way is to select such portions as are thought fairly to represent the bulk, to chop them into moderately small pieces, mix and seal in an air-tight tin, leaving it to the chemist to draw from these the smaller sample required for analysis. Gambier is best sampled with a tubular tool like a cork-borer, designed by Mr. Kathreiner, [Fig. 63], which should be passed completely through the bale, or the cylindrical sample of gambier cannot be withdrawn. The same tool may also be used for sampling sumach in bags, if the damage to the bag is not objected to. If such a tool is not available, the only fair way to sample gambier is to cut slices completely through the bale with a clean fleshing knife. In any case it is of the utmost importance that the sample once drawn, should be mixed as rapidly as possible, and at once enclosed in an air-tight box or jar, sealed and labelled.
Fig. 63.—Kathreiner’s Sampling Tools. A, strong cross-handle; B, guard-disc; C C´, brass tube sharpened at C´; D, brass or wooden plunger.
Dry tanning materials, such as bark and valonia, require judgment in selecting samples which fairly represent bulk. If they are of a nature which do not readily separate into dust and fibre, a good method is to grind a sufficient quantity in an ordinary bark-mill, and after well mixing, to draw the sample from the ground portion. In other cases it is best to empty a sufficient number of bags one upon another in layers on a smooth floor, and to take out a section down to the floor. In such materials as valonia and divi-divi, the dust or beard is usually much stronger than the average of the pods or cups.
The same sort of precautions are required in drawing the still smaller sample required for analysis from the larger original sample, but these are sufficiently detailed in the directions of the I.A.L.T.C. given in the [Appendix]. As materials usually require finer grinding than can be managed with the mills employed in the tannery, a suitable mill must be provided, and one of the simplest, at a moderate price, is a No. 4 drug-mill made by A. Kenrick and Sons, Limited, West Bromwich, [Fig. 64]. Coffee mills are seldom strong enough for the purpose, but if nothing better is available, the sample must be thoroughly dried before grinding, and its loss of weight noted, and taken into account in calculating the analysis, care being taken that the sample after grinding is so preserved that it cannot re-absorb moisture. Valonia, myrobalans and even barks, may before grinding be broken with a flat-faced hammer, on a thick cast-iron plate, with raised edges to prevent loss from flying fragments.
Fig. 64.—Kenrick’s Drug-Mill.
Preparation of solution for analysis.—As the method of analysis only gives satisfactory results when the quantity of tanning matter in the solution is within certain limits, the International Association prescribes that it must be such as to contain between 3·5 and 4·5 grms. of tanning matter per liter, or as near as possible, on the average, to 4 grms. If, as will rarely happen, the strength of a material is quite unknown, it may be necessary to make a trial test to ascertain the quantity of substance to be used, but the following table gives the quantity with sufficient accuracy for most ordinary materials.