207 The attraction of alumine for the greater num ber of colouring substances may also be shewn by di rect intent. If a solution of colouring matter be mixed with a solution of alum, a precipitation general ly takes place, consisting of the colouring particles com bined with alumine. The precipitate is known in the arts under the name of lac or lake, and is more readily formed when an alkali is added to detach the acid, and leave the alumine at greater liberty to unite with the colouring matter. Au excess of alkali ought not to be added, however, because alkalies dissolve the greater number of the lakes. M. Thenard has shewn that alu mine may even be intimately mixed with several me tallic colouring matters, as prussiate of iron, oxide of cobalt. sec.
208. The metallic oxides have also a strong attrac tion liar colouring matters, and when they are presented to the latter in combination with an acid, they frequent lyquit the solvent, and arc precipitated like the alu mine in combination with the colouring matter. This class of bodies has also the property of combining with animal substances, and of thus serving as a bond of union between these and colouring particles. But not only have the oxides a strong attraction for colouring particles, and animal matters, but their solutions in acids possess properties which render them no less fit to be employed as mordants ; and indeed it is generally in this state that they arc used in dyeing.
209. In order that a substance may be fit for being employed as a mordant, it is not always sufficient that it should possess an affinity to the colouring principle and the stuff ; it must also be perfectly white, other wise its colour, mixing with that of the colouring prin ciple, would produce an intermediate or mixed colour. In some cases, indeed, this may be an advantage, and a substance may be employed both as a colouring mat ter, and a mordant. Thus the oxide of iron, which, if it were employed alone, would produce a nankeen or buff colour, communicates to cotton a violet with mad der red ; and in like manner, the oxide of tin not only fixes, but greatly exalts several colouring matters. Alordants ought also to be little liable to change, by the action of air and water, the two agents to w;.ich dyed stuffs are most frequently exposed. The metallic oxides, from their disposition to attract additional por tions of oxygen from the atmosphere, and on that ac count to stiffer changes themselves, as well as to affect the shades of colouring matters, are less valuable as mor dants than they would otherwise be.
210. Neutral salts, particularly nitre and muriate of soda, (common salt) act as mordants, and modify co lours ; but the nature of their action has been little at tended to, and is imperfectly understood. Berthollet found that the muriate of soda was contained in the precipitates produced from some kinds of colouring matter, and that these precipitates retained a consider able degree of solubility ; he conceives that a small part of the salt attaches itself to the colouring principle and the stuff. Salts with calcareous bases also modify colours; but as the changes to which they give rise may be produced by the addition of a small quantity of lime, it is probable that they suffer a partial decomposi tion, and that when they are used in dyeing, a little of the calcareous base enters into combination with the co louring particles and the stuff.
211. The two mordants most extensively employed at present, are salts of alumine and tin. The former is prepared, according to Berthollet, by dissolving in eight pounds of hot water, three pounds of alum, and one pound of acetate of lead, (suga• of lead,) to which two ounces of potash, and afterwards two ounces of powdered chalk, arc added. The alum is decomposed by the acetate of lead, because the oxide of lead com bines with the sulphuric acid of the alum, and forms an insoluble compound, which is therefore precipitated, while the alumine is attracted by the disengaged ace tous acid, and forms the acetate of alumine. The chalk and the potash are added to neutralize the excess of the sulphuric acid, and thus to assist the operation of the acetic acid. The substitution of the acetic acid for the sulphuric, in the preparation of the aluminous mor dant, is attended with several advantages. The acetate of alumine being much more soluble than alum, the same quantity of solvent will hold suspended a greater quan tity of the former than the latter, and thus present the mordant in a more concentrated state to the stuff; while, on account of its great solubility, it will not be apt to crystallize when mixed with starch or gum, io prepare it for being applied to the block, in topical dyeing. The alumine is also retained less powerfully in a state of combination by the acetic than by the sul phuric acid, and is therefore at greater liberty to attach itself to the stuff and the colouring particles. Lastly, the acetic acid being volatile, gradually separates itself from the earthy basis, and thus leaves the alumine to act with full energy as a mordant, without affecting the colour of the colouring matter.