These reactions take place within the fibre, after the hair has been impregnated with the solution of the neutral salt, and when the compound has been rendered completely basic, in other words has reached the form of the hydroxide, it is supposed to combine with the acid groups contained in the hair substance, forming thus some complex, insoluble organic compound of the metal within the hair. According to some authorities the mordant is supposed to be present in the hair simply as the hydroxide, being tenaciously held by some physical means. The facts seem to indicate, however, that the metal is actually combined in some chemical way with the hair. For, if the mordant were present as hydroxide, then on white hair it would show the color of the hydroxide, which it does not. The same facts obtain with regard to other metals.
In order for the hair to be properly mordanted, it is necessary that the metallic compound which is taken up by the hair be held in such a manner that the mordant cannot be removed by water or even dilute acids or alkalies. Salts which dissociate too readily produce mordants which are only superficially precipitated on the hair and subsequently come off. Usually some substance is added to the solution of the salt to cause slower and more even dissociation of the salt, so that the hair substance can be quite saturated with the metallic compound before any insoluble precipitate is formed. Dilute sulphuric acid, organic acids like acetic and lactic, and cream of tartar are used to facilitate the uniform absorption of the mordant salt by the hair.
When the skins are mordanted before dyeing, they are immersed for 6 to 24 hours in a solution containing 1 to 20 grams of the metallic salts per liter of water, together with the corresponding quantity of the assistant chemical. The skins should be so entered into the mordant solution that the hair is uniformly in contact with the solution, and all the skins so that they are acted upon alike. Machinery such as is used for killing is suitable for mordanting also. The duration of the mordanting, and the concentration of the solutions are varied according to the depth of shade required, and also according to the nature of the dye to be employed. By suitably combining several mordants a considerable range of colors can be obtained with a single dye.
The various chemicals used as mordants are essentially the same no matter for which class of dyes they are used, there being only slight differences in the concentrations of the solutions, the manner of application of the mordants being practically the same. It is interesting to note that with the exception of chromium compounds, which are of comparatively recent adoption as mordants, all the chemicals now used for mordants were employed by the earliest masters of the art of fur dyeing. While some of the formulas used by those dyers display a lack of appreciation of the true action and function of the mordanting chemicals, yet it is quite remarkable that they chose, in spite of their limited knowledge of chemical processes and phenomena, just those materials which do act as mordants if properly applied. The most important metallic compounds for mordanting furs at the present time are salts of aluminum, iron (ferrous), copper, tin and chromium (as well as chromates and bichromates). The compounds of the metals with organic acids such as acetic acid are preferable, being more easily dissociated, and also leaving in solution an acid which is less injurious to the fur than a mineral acid. However, sulphates and other salts of the metals are also used extensively, inasmuch as they are cheaper than the organic salts.
Aluminum Mordants
Chief among the aluminum mordants are the various kinds of alum, which is a double sulphate of aluminum and an alkali such as sodium, potassium or ammonium. All these salts except that of sodium, form large, colorless, octahedral crystals, and are soluble in about 10 parts of cold water, and 1⁄4 part of hot water. Sodium alum is even more easily soluble, but on account of the difficulty of obtaining it in crystalline form, it is little used. The common commercial alum is the potassium aluminum sulphate.
Recently, aluminum sulphate has to a large extent replaced alum for mordanting purposes, because it can be obtained very cheaply in pure form, and it contains a greater amount of active aluminum compound than does alum. Only the iron-free salt, however, may be used for the needs of fur dyeing.
Aluminum acetate also finds extensive application as a mordant in fur dyeing, and while somewhat more expensive than the alum or aluminum sulphate, it has the advantage over these compounds of being combined with an organic acid, which is preferable when the action on the hair and leather is considered. Aluminum acetate can be obtained in the market in the form of a solution of 10 degrees Beaumé, but can also be prepared very easily as follows:
| 665 | grams pure aluminum sulphate, or |
| 948 | grams potassium alum, are dissolved in |
| 1 | liter of hot water. |
| 1137 | grams of lead acetate (sugar of lead) are also dissolved in |
| 1 | liter of hot water. |
The two solutions are mixed, and thoroughly stirred. A heavy white precipitate forms, which is filtered off, and discarded after the solution has cooled. The aluminum acetate is contained in the filtrate, and the solution is brought to a density of 10 degrees Beaumé by the addition of water, if necessary, and is preserved for use in this form.