286 The Art of Dyeing. 



article of trade, is termed also ferro-prussiate of potash, 

 and is employed for producing light blue colours upon 

 calico, usually termed chemical blue.* 



This blue is nothing else than a Berlin or Paris blue, 

 intimately united with the calico, and is formed when the 

 calico impregnated with the iron mordant is dipped in a 

 solution of prussiate of potash. 



In order that this salt may exhibit a sure and rapid action, 

 sulphuric acid must be added. When it does not dye 

 without this addition, it proceeds from the mordanted 

 cotton having been previously washed in hot water. 



The proportion of sulphuric acid is important. If a small 

 quantity only is used, the blue does not adhere to the 

 cotton, but is soluble in water, and is washed off by the 

 rinsing. On a white ground it spreads and gives it a 

 light blue colour. The best proportion is, 2 lbs. prussiate 

 of potash, and 1 lb. sulphuric acid. 



The solutions of both in water must be mixed while cold. 

 If this mixture is allowed to stand long it is destroyed ; the 

 prussic acid is disengaged, and a blue precipitate of Berlin 

 blue falls to the bottom. No more should be prepared, 

 therefore, than is required. 



If a piece of cotton dyed with chemical blue be dipped in 

 chlorine water or chloride of lime, it not only remains un- 

 changed, but rather gains lustre. The same happens in 

 the light of the sun. This property makes this colour of 

 great importance in the manufacture of madder-purple, 

 where only such dye? can be used as withstand the action 

 of chloride of lime, and the light of the sun. Indigo and 

 logwood colours want this power. Ferro-prussiate of 

 potash is not poisonous. When it is heated with sulphuric 

 acid solution, the vapour disengaged is, however, delete- 

 rious, as it contains prussic acid. 



When large quantities of prussiate of potash are poured 

 from one vessel into another, a powder is formed, which 

 smells of prussic acid, and is very troublesome to the nose 

 and throat. 



If a solution of prussiate of potash is mixed with a solu- 

 tion of green vitriol, a white precipitate is produced which 

 becomes light blue in the air. A very strong solution of 

 chloride of lime heightens its lustre and makes it finer. 



* See a specimen, Records i. 324. — Edit. 



