DYEING. 188 



tartar is added. The cloth is then put in and kept 

 till it is saturated. A double decomposition takes 

 place ; the nitro-muriatic acid combines with the 

 potash of the tartar, while the tartareous acid dis- 

 solves the oxide of tin. When tartar is used, 

 therefore, in any considerable quantity, -the mor- 

 dant is not a nitre-muriate, but a tartrate of tin. 



The murio-sulphate of tin, produced by dissolving 

 tin in muriatic acid, combined with about one- 

 fourth of its weight of oil of vitriol, is also a valuable 

 mordant, and is preferable to the last for some pur- 

 poses ; it is also less expensive. 



The oxide of iron is also a very useful mordant, 

 a"nd all kinds of cloth have a strong affinity for it. 

 The N permanency of the iron spots on linen and 

 cotton is a sufficient proof of this. Iron, as a mor- 

 dant, is used in different states. Wool is dyed 

 generally by means of the sulphate of iron, which 

 may also be used for cotton. Acetate of iron, pre- 

 pared by dissolving iron in vinegar, sour beer, &c., 

 is preferable for some purposes. The pyro-lig- 

 neous acid, which differs from the acetic only in 

 having in combination a certain quantity of empy- 

 reumatic oil, is at present preferred to the sulphuric, 

 o'r acetic. 



The astringent principle, or tannin, is also em- 

 ployed as a mordant, and has a strong affinity for 

 cloth, and also for colouring matters. An infusion 

 of nut-galls, sumach, oak bark, or any other sub- 

 stance containing tannin, is made in water, and the 

 cloth is kept in it till it has absorbed a sufficient 

 quantity of tannin. Silk has so strong an affinity 

 for tannin, that manufacturers sometimes employ 

 this circumstance to increase the weight of their silk. 



A compound mordant is sometimes produced by 

 impregnating the cloth first with oil, then with the 



N 4 ' 



