Chemical Composition.--In the natural or raw state each wool fibre is surrounded by a considerable amount of foreign matter, so that in treating of its chemical constitution it is necessary to distinguish between pure wool and the raw fibre. The incrusting substance is technically known as "Yolk," or "Suint," and is principally composed of a kind of natural soap, consisting of the potash salts of certain fatty acids, together with some fats which are incapable of saponification.
The amount of yolk present upon different samples of wool varies greatly, the finer varieties containing, as a rule, a larger proportion than the coarser, and less valuable sorts.
The variation in the relative amount of pure fibres and yolk is well shown in the following analyses which, however, do not by any means represent extreme cases.
ANALYSES OF RAW MERINO WOOL. DRIED AT 100° C.
| No. 1. | No. 2. | |||
| Moisture | 6·26 | 10·4 | ||
| Yolk | 47·30 | 27·0 | ||
| Pure Wool | 30·31 | 59·5 | ||
| Dirt | 11·13 | 3·1 | ||
| ------- | ------- | |||
| 100·00 | 100·00 |
Yolk consists very largely of two complex substances which have been termed wool perspiration and wool fat. The former is composed of the potash salts of fatty acids, principally oleic and stearic acids; the latter of the neutral carbohydrate, cholesterine, with other similar bodies. The wool perspiration may be removed by a simple washing with water, and on the Continent forms a valuable source of potash salts, since the ash after ignition contains 70 to 90 per cent. of potassium carbonate. The wool fat is insoluble in water, but dissolves readily in ether, benzene, carbon disulphide, etc.
It is also removed from the wool by a treatment with alkali, and it is not easy to explain the action in the case, since the wool fat is not a glyceride, and will not form a soap, but is probably emulsified by the wool perspiration.
Chemical Composition of the Pure Fibre.--The following analyses of purified and dried wool fibre indicate its percentage composition:--
Carbon 50·5 per cent. 50·8 per cent.
Hydrogen 6·8 " 7·2 "
Nitrogen 16·8 " 18·5 "
Oxygen 20·5 " 21·2 "
Sulphur 5·4 " 2·3 "
------- -------
100·00 100·00
It is sometimes stated that wool fibre consists of a definite substance, keratine, but this view cannot now be admitted, since wool appears to be composed of a mixture or combination of several very complex substances. It is possible and even probable that the outer epidermal scales have a somewhat different composition to the bulk of the fibre, but whether that is the case or not is not known with any degree of certainty, this much can be asserted, that wool is not a simple definite chemical compound.