Jsy [ 175 J 



lions as betbre given; and we should coutiuue tlius to add more alum, 

 in proportion to the sillA which are to be impregnated therewith, until 

 the bath begins to have a bad smell; it is then to be drawn off, after 

 steeping in it the silks intended for deep or dark colors, as brown, 

 chestnut, &c. &c. and thrown away, the long trough (barque) cleaned, 

 and a new bath to be got ready. 



The silk must be steeped in the alum water whilst it is cold; if it 

 ivere warm, the silk would lose its lustre. 



Observations on Mum. 



As alum has been emphatically termed the "soul of dying," it is 

 of immense importance to use it in a state of purity; on this account 

 the following remarks are given. 



It is often combined with a portion of iron, which defeats the inten- 

 tion of the dier. To free it from this metal, Dr. Cooper* directs to 

 *' dissolve it in boiling water, and expose for a week or a fortnight in 

 shallow vessels to the air. The iron will gradually be oxyded, and 

 separate in the form of rust. Filter the solution, evaporate the water, 

 and re-crystallize it." M. Vitalis directs to dissolve the alum, and 

 crystalizeit: pour off the liquor that remains, and dry the crystals on 

 absorbent paper. 



The presence of iron in alum is easily discovered by dissolving a 

 small portion of this salt in distilled water, or in rain water, and add- 

 ing a few drops of a solution of prussiate of potash. If the alum con- 

 tains iron, a blue precipitate will immediately take place, which will 

 be more or less deep, in proportion to the quantity of iron contained 

 in the alum. 



Alum varies much in its composition; sometimes it is an acid sul- 

 phate of alumine and potash: at others, in place of the latter, ammonia 

 is found; and finally, both potash and ammonia enter into the com- 

 pound, but although thus variously formed, it is equally proper for 

 dying.* The regular form of alum is in octoedral crystals; and when 

 thus found, it is a definite compound. 



Inquiry has been made of the writer, as to the proper quantity of 

 alkali to neutralize the excess in alum of sulphuric acid, which is inju- 

 rious to colors. No positive quantity can be prescribed. The only 

 certain mode to neutralize the acid, is to add a solution of alkali, until 

 the earth of alum begins to precipitate. We may then be suro that the 

 acid is fully neutralized. 



The LidigO'Blue Vat. 



For eight pounds of indigo, take six pounds of the best potash, from 

 three to four ounces of madder, for each pound of potash, and eight 

 pounds of bran, watered several times, in order to carry off its flour. 

 When washed, press it to carry off most of its water, and place it at 



* Treatise on Dying, page 25, Philadelphia, 1815. 



t Vitalis, Cours Elementairn de Teinture, page 107 — Paris, 18*?*. 



