5^> ON COLORING. 



all, or take it very faintly. Wool has the strongest affinity for 

 most coloring matter, silk the next strongest, cotton a much 

 weaker affinity, and linen the weakest of all. In order, there- 

 fore, to dye cotton or hnen, the dye-stuff should, in many cases, 

 be dissolved in a liquid for which it has a weaker affinity than 

 for the solvent employed in dyeing wool or silk. Thus we may 

 use iron dissolved in sulphuric acid to dye wool, but for cotton 

 and linen it is better dissolved in vinegar. Was it possible to 

 obtain a sufficient variety of coloring matters having a strong af- 

 finity for cloth, the art of dyeing would be exceedingly simple 

 and easy. But this is by no means the case ; if we except in- 

 digo, the dyer is scarcely possessed of a dye-stuff which yields 

 of itself a good color, sufficiently permanent to deserve the name 

 of a dye. To obviate this difficulty, some substance must be 

 employed which has a strong affinity both for the cloth and the 

 coloring matter. Substances employed for this purpose, are 

 called mordants. Those chiefly used are earth, or metals, in 

 the form of salts or in solution, tan, and oil. One of the most 

 frequently used is alum. This salt is composed of pure clay 

 (alumina) dissolved in sulphuric acid. Into a solution of alum 

 the cloth is dipped, the fibre of the cloth having a stronger 

 affinity for the clay than the sulphuric acid has, unites perma- 

 nently with it. It is then taken out, washed and dried, and will 

 be found a good deal heavier than before, although the color re- 

 mains the same, the clay, which now forms a part of it, being 

 perfectly white. The cloth may now be dyed by dipping it in a 

 solution of any coloring matter for which the clay has a strong 

 affinity. The clay and coloring matter may be united previous 

 to the immersion of the cloth, and tiie fibres will still unite 

 themselves with the compound, but not so equally and perma- 

 nently as when dipped into each of the solutions separately. 

 But the sulphuric acid has rather too strong an affinity for the 

 clay to yield it readily even to wool. Most dyers, therefore, 

 add to the solution of alum a quantity of tartar. Tartar is 

 composed of potash and an acid found in grapes and some other 

 vegetables, called tartaric acid. When solutions of alum and tar- 

 lar are mixed, the sulphuric acid quits the clay and seizes on the 



