Tanning extracting

Any vegetable tanning extract used commercially is a complex mixture of related substances. The individual tanning properties of the extracts have been extensively studied and are weU known in the industry. 

Gerb-anlage, /. tannery, -auszug, m. tanning extract. 

Barkotneier Scale—This scale is used essentially in the tanning and tanning-extract industry. Water equals zero. Each scale degree equals a change of 0.001 in specific gravity. The following formula applies ... 

Tanning extracts are aqueous decoctions of the preceding and other prime materials, conveniently concentrated and sometimes purified. 

The prime materials may likewise be subjected to chemical examination, which is the only means applicable to the recognition of tanning extracts this is carried out on an aqueous decoction of the raw material or on a conveniently diluted solution of the extract. 

Tanning extracts are liquids, more or less dense according to the concentration, or solids which are brown with either a green or red tinge or more or less pale yellowish brown (purified or decolorised) they have an astringent taste and are more or less completely soluble in water. Their dilute... 

Detection of Sulphur Dioxide.—Sulphurous anhydride, which is often added to tanning extracts, either as such or more often as sulphites, to enhance their keeping qualities, may be detected by treating 10 grams of the extract in a beaker with 20 c.c. of hydrochloric acid and 20 c.c. of water, a piece of pure zinc being added and the beaker covered with a clock-glass under which is suspended a strip of lead acetate paper if the extract contains sulphurous anhydride, the paper becomes brown or assumes a blackish tint with metallic reflection. If the paper remains white for 15 minutes, absence of sulphurous acid may be assumed. 

Extraneous Substances.—Besides being replaced or mixed with other extracts, tanning extracts may be adulterated with extraneous substances, such as sugars (glucose, molasses), dextrin, sulphite cellulose liquors (lignorosin) or mineral salts (e.g., sodium sulphate). 

With tanning extracts adulterated with cellulosic extracts, the indirect method does not, however, give reliable results, since cellulosic extracts contain non-tannins which are fixed by the hide powder. In such a case the tannin should be determined by the direct method, since these non-tannins have no appreciable action on permanganate the difference between the results of the two methods will give an approximate indication of the extent of the adulteration, if this is at all marked. 

Dyeing extracts may be mixed or adulterated with tanning extracts and may also be adulterated with artificial organic dyes or -various inert substances, either organic (molasses, glucose, dextrin, starch, glue) or inorganic (sodium sulphate and other soluble salts, and insoluble substances). 

The qualitative test may be confirmed by a quantitative determination of the tannin substances, which occur in far larger proportions in tanning extracts. 

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