449 
Copperas . 
is produced, for most of the sulphur and acid already form- 
ed is dissipated and also the iron becomes too much oxy- 
dated to yield the crystallizable salt. Hence it is dangerous 
and prejudicial to make too large heaps of pyrites or to 
put it up into stacks however preserved from the weather. 
Some moisture is also necessary to vitriolization, but too 
much of it keeps the pyrites too cold, and the process is 
languid. The iron added during the boiling is certainly 
useful, both as saturating the acid and encreaing thereby 
the yield of the salt, and also as precipitating by its supe- 
rior affinity any copper which may arise from the admix- 
ture of copper pyrites, and also undergo vitriolization. In 
some manufactures however the admixture of a small por- 
tion of sulphat of copper is even an advantage, as in the 
dying of hats. ( Aikin’s Diet. I. 614) 
Second method. In copper works, the sulphuretted 
ore of copper is roasted in ovens to drive off part of the 
sulphur. The ore is then washed. But as in the pro- 
cess of desulphuration, much of the sulphur is acidified 
by the oxygen of the air that supports the combustion, a 
sulphat of copper, or blue vitriol, blue copperas, is pro- 
duced. When the ore is washed in water, that water 
dissolves the blue vitriol, from which the copper is pre- 
cipitated by immersing bars and plates of iron in the so- 
lution. The copper thus precipitated is nearly in a me- 
tallic state ; and when no more copper falls down, the 
iron is taken out, the precipitate collected for fusion, and 
the liquor which now contains sulphat of iron is eva- 
porated to obtain that salt, which crystallizes in the usual 
way. 
Third method. Oil of vitriol (concentrated sulphuric 
acid) is mixed with six times its bulk of water. Refuse 
iron is immersed in it till the liquor is saturated, and will 
take up no more iron. The clear liquor is gently evapo^ 
rated and left to crystallize* One hundred parts by 
