A Text-book of Tanning / A treatise on the conversion of skins into leather, both practical and theoretical.

by H. R. Procter

CHEMICAL ANALYSIS FOR THE TANNERY

Reagent.	Myrabolanes.	Divi-divi.	Valonia.	Oak Bark.	Chestnut wood
Boiled with equal volume of sulphuric acid (1 vol. to 9 vol. water).	Pale deposit (eliagic acid) on cooling.	Pale deposit (eliagic acid) on cooling.	Slight pale deposit.	Slight pale deposit or turbidity on cooling.	Slight red deposit on cooling.
Bromine water.	No pp.	No pp.	No pp.	Pale pp.	No pp.
Dilute ferric chloride.	Blue-black pp.	Dark blue pp.	Blue-black pp.	Bluish black pp.	Blue-black pp.
Add ammonia.	Brown pp.	Dark red pp.	Red brown pp.	Red brown pp.	Dull red pp.
Sol. tartar emetic.	No pp.	Faint clouding.	No pp.	No pp.	Slight clouding.
Add ammonic chloride.	Light pp.	Dense pp.	Pale pp.	Whitish pp.	Pale pp.
Copper sulphate.	Faint clouding.	Slight green pp.	No pp.	Slight pp.	No pp.
Add ammonia.	Dense dark pp.	Dense dark pp.	Dark reddish pp.	Brown pp.	Dark brown pp.
Lime-water.	Yellow pp. turning greenish.	Yellow pp. turning purple.	Yellow pp. turning red-purple.	Brown pp.	Purplish brown pp.
Ammon. molybdate in nitric acid.	Dirty yellow pp.	Dark greenish pp.	Dark greenish pp.	Greenish pp.	Dirty green pp.
With sodic sulphide exposed to air on a tile.	Yellow colour.	Yellow colour.	Turns purpulish red.	Turns red.	Reddish pp.
Add concentrated sulphuric acid to 1 drop infusion.	Yellow colour.	Intense crimson.	Deep yellow.	Deep red pp. on dilution.	Dark brown.
Lead Nitrate	Light yellow pp.	Dark yellow pp.	Pale pp.	Brown pp.	Brown pp.
Cobalt Acetate	Buff pp.	Buff pink pp.	Dirty pink pp.	Ditto.	Dirty yellow pp.
Manganese acetate.	Yellow pp.	Yellow pp.	Dirty yellow pp.	Ditto.	Grey pp.
Uranium acetate.	Dark red colour.	Dark red colour.	Dark red colour.	Dark brown colour.	Dark red colour.
Ammoniacal picric acid sol.	No pp.	No pp.	Brown pp.	No pp.	No pp.
Potassic dichromate.	Brown pp.	Brown pp.	Brown pp.	Brown pp.	Brown pp.

CHEMICAL ANALYSIS FOR THE TANNERY — Continued.

Hungarian Larch (Extract).	Hemlock (Extract).	Mimosa bark.	Cutch (Pegu).	Gambier (Cuba).	Gallotannic Acid, 1 per cent.
Yellow Flocculent deposit separates quickly.	Abundant red flocculent deposit.	Heavy red depositon cooling.	Light red depositon cooling.	Reddish depositon cooling.	Usually some pale deposit.
Yellow pp.	Yellow pp.	Yellow pp.	Yellow pp.	Yellow pp.	No pp.
Dull brown pp.	Dirty green pp.	Full brown pp.	Green-black pp.	Intense green colour.	Blue-black pp.
Dull red pp.	Reddened pp.	Purple colour.	Dark red pp.	Reddened pp.	Reddened pp.
No pp.	No pp.	White pp.	No pp.	No pp.	No pp.
Pale pp.	Slight pale pp.	Dense white pp.	Pale pp.	Faint clouding.	White pp.
Slight cloud.	Pale pp.	Slight pp.	Dense pp.	No pp.	No pp.
Deep blue coloration.	Dark green coloration.	Deep red pp.	Deep violet coloration.	Dark green coloration.	Brown pp.
Dirty brown pp.	Brown pp.	Slight reddish pp.	Slight cloud soluble in excess.	No pp.	Pale pp. turns blue.
Slight clouding.	Slight pp.	Brown pp.	Ditto.	Ditto.	Yellow colour.
No change.	No change.	Turns red.	Slight reddening.	No change.	No change.
Dark brown or crimson.	Intense crimson.	Intense purple-red.	Deep red no pp. on dilution.	Dark brown or crimson.	Yellow.
Pale pp.	Pale pp.	Clouding.	No pp.	Faint clouding.	White pp.
Purplish pp.	Purple pp.	Brown pp.	Brown pp.	No pp.	Purple pp.
Slight clouding.	Slight pp.	No pp.	No pp.	Ditto.	White pp.
Slight darkening.	Light brown pp.	Dark red colour.	Dark red colour.	Dark red colour.	Crimson colour. Brown pp.
No pp.	Clouding.	No pp.	No pp.	No pp.	No pp.
Ditto.	Brown pp. slowly formed.	Brown pp.	Brown colour.	Brown pp. slowly formed.	Brown pp.

QUANTITATIVE DETERMINATION.

Many processes have been proposed for the quantitative estimation of tannins, but it cannot be said that any method yet known is wholly satisfactory. The oldest, that of Sir H. Davy, recently improved by Stoddart and others, consists in precipitating with gelatin, and drying and weighing the precipitate. This is almost impossible to filter off as directed by Davy; but by the use of a little alum, and by pouring hot water on the precipitate, it becomes curdled into a mass which may be washed by decantation. As the precipitate contains varying quantities of tannin, according to the strength of solution employed; as it is soluble in excess of gelatin solution, and as it is almost if not quite impossible to wash it free from gelatin and alum, the method can hardly lay claim to much accuracy. A somewhat better one consists in the employment of a standard solution of gelatin with a little alum, determining the end of the reaction by filtering off a portion and ascertaining if another drop of the reagent produces a further precipitate. This method is very tedious, the end reaction is difficult to hit, the standard solution is very unstable, it is inapplicable to gambier and cutch because the mixture will not filter clear, and its results are irregular, probably from the power of tannin to combine with various proportions of gelatin. A plan, which has a seductive appearance of simplicity, is that of Hammer; he takes the sp. gr. of the infusion, then absorbs the tannin with slightly moistened hide-raspings, again takes the sp. gr., and from the difference calculates the percentage of tannin, a difference of 5 per cent. of tannin corresponding to one of 1·020 sp. gr. (20° barkometer). Unfortunately the hide is more or less soluble in the liquor, and absorbs acids other than tannic with considerable energy; the moistening of the raspings introduces an error, and the smallness of the quantity to be measured makes a slight error completely vitiate the results. With extreme care, due corrections for temperature, for the water introduced with the raspings, and for their solubility, and by substituting evaporation of the infusions to dryness for mere calculation from their sp. gr., the method is useful as giving almost the only information obtainable as to the actual weight of tannin in any material capable of being absorbed by hide. It is, however, only suitable for use as a check on easier and more rapid methods, such as Löwenthal's, which give accurate relative results, but no information as to absolute weight of unknown tannins. A modification of Hammer's method has been introduced by Müntz and Ramspacher, in which the liquor whence the tannin is to be removed is forced through a piece of raw hide by pressure. This method, except that it is more rapid, has all the evils of Hammer's in an intensified form, and gives such variable results as to be quite useless in practice. A set of very careful determinations of one sample of sumach gave results ranging from 18 to 28 per cent., and similar variations occurred when the experiment was repeated with valonia. Wagner's method by precipitation with a standard solution of cinchonine and magenta has proved wholly unreliable.

Gerland's method with a volumetric solution of tartar emetic, used in presence of ammonic chloride, gives constant results with sumachs, ²/₃ of those given by permanganate and Neubauer's equivalent. Tartar emetic does not precipitate the tannins of cutch and gambier. Fleck's, by precipitation with copper acetate, and subsequent washing with ammonic carbonate and gravimetric estimation, either of the tannate dried at 212° F. (100° C.), or of the copper oxide left on ignition; and Carpene's, by precipitation with ammoniacal zinc acetate, and subsequent estimation with permanganate and indigo, though giving fairly accurate results on some tannins, are only of limited application. They may therefore be passed over, as well as Jean's method with a volumetric solution of iodine in presence of sodic carbonate, and Allen's method with lead acetate, which are tedious and difficult, and present no advantage over Löwenthal's improved process. This last is easy of execution, constant in results, and universally applicable. Before proceeding to describe it in detail, it may be well to give some hints as to the best modes of sampling and preparing tanning materials for analysis, since this is often more difficult and tedious than the actual analysis.