58 The Art of Dyeing, 



IRON ALUM. 



Iron alum contains no alum, but derives this name from 

 its crystalline form which resembles that of alum, and in 

 consequence of the similarity of its chemical constitution 

 with alum. When the alumina is taken out of the alum, 

 and oxide of iron substituted for it, we obtain iron alum. 

 It may be formed by mixing 78 lbs of red oxide of iron, 117 

 lbs sulphuric acid combined with it, both dissolved in water, 

 and 87 lbs sulphate of potash added while boiling, and then 

 allowing the iron alum to crystallize. 



Iron alum, made on a large scale, has*a clear amethyst 

 colour ; subsequently it becomes covered with a yellowish 

 white crust which does not injure it. It is very soluble in 

 water. The solution is yellow coloured and undergoes de- 

 composition by boiling, while oxide of iron separates. 



The solution of iron alum is employed by itself for the 

 production of a great many colourless grounds, variously 

 shaded according to the strength of the solution. This so- 

 lution acts differently from the alum ; for the stronger the 

 solution, the more iron alum the calico takes up. A weak 

 solution of 1 lb iron alum in 60 water with quercitron, 

 gives a pale straw colour, while with 10 lbs iron alum and 

 60 water, the colour produced is comparatively very dark. 

 In order to obtain such shades of an equable nature, it is 

 necessary to impregnate the calico with the mordant, press 

 it and rinse it as described. Unless very carefully dried, 

 more of the mordant collects on the edges and faults of the 

 calico than on other places, and produces inequalities. 

 This is prevented by rinsing it before drying. 



Solutions of iron alum may be kept for a long time without 

 losing their properties. The air does not produce any in- 

 jurious eflfect, as occurs with the solution of sulphate of 

 iron. 



Acetate of iron. — When vinegar or pyroligneous acid is 

 poured upon heated iron, and left in contact with it for a 

 month, a mordant of acetate of iron is formed, which when 

 thickened with gum or starch is very useful for printing ; 

 for mordanting colourless grounds it does not answer so 

 well. 



This mordant is most readily produced by decomposing 

 iron alum with sugar of lead. The two following formula? 

 are in most general use. 



