470

DYEING AND CALICO PRINTING.

and lime of good quality, slaked just before use, and sifted. The readtion ofthese substances upon each other takes place in the cold, but is greatly aidedby a moderate elevation of temperature. It is not possible to give precisepradtical instructions as regards the proportionate quantities of each of thesematerials to be employed, because these vary in almost every dye-works,and in many instances they are trade secrets. When the vat is to be used forplain dyeing, it is usual to take the quantities of lime and copperas in excessof what is indicated by theory, because by proceeding thus there is formed anexcess of hydrated protoxide of iron, and enough lime is left to serve thepurpose of again reducing and re-dissolving any indigo which becomesoxidised and insoluble. This proceeding is greatly promoted by stirring upthe contents of the vat, as is usual after some pieces have been dyed. Thefollowing examples may serve the purpose of exhibiting the proportions of theingredients employed by dyers in setting vats :—Indigo, I part; crystallisedsulphate of iron, 3 parts ; carefully slaked lime, 3 parts. Some dyers preferto employ a larger quantity of lime than of sulphate of iron, and make themixture—Indigo, 2 parts; crystallised sulphate of iron, 5'5 parts; lime,6-5 parts.

When white indigo is intended to be used for printing, the following mixtureanswers the purpose:—Indigo, 1 part; sulphate of iron, 2 parts; lime, 3 parts;but these proportions do not answer well for dyeing.

We stated, upon the authority of Berzelius, that an excess of lime isinjutious, and tends to occasion a considerable loss of colouring matter,because the white indigo can—while combining with lime in a higher pro-portion—form an insoluble basic body, which, by settling to the bottom of thevat along with the rest of the sediment, becomes lost for the purpose ofdyeing. Although the corredtness of this statement need not be doubted, hehas overrated its pradtical bearing, because the pradtical experience of M. E.Schlumberger, who purposely studied this point on the large scale, does notagree with this statement of the eminent Swedish savant. It often happensthat in pradtice on the large scale matters differ from laboratory experiments,and Berzelius is quite corredt in recommending to avoid an excess of lime andsulphate of protoxide of iron, because it increases the bulk of the sedimentwithout any real advantage.

As regards the method of making the mixture, this also differs in differentdye-works. The most rational plan is to mix the indigo in a pasty state withthe milk of lime, and next to add gradually, and while the contents of the vatare being stirred, the solution of copperas. By following this plan, the whiteindigo, while being gradually formed, is at once dissolved by the excess oflime : but when the quantity of substances is rather large this method isdifficult of execution, on account of the contents of the vat becoming verythick and pasty, and difficult to stir at first, and it is therefore preferable tomix the indigo with the solution of the green vitriol, and to add the milk oflime next gradually. The operation, to be successful, requires only a smallquantity of water; the milk of lime should be used freshly prepared and stillwarm, owing to the heat developed while the quick-lime is being slaked, andthe green vitriol is best dissolved in hot, or even in boiling, water. After themixture has been made it is left quietly standing for several hours, care beingtaken to stir the liquid occasionally. As soon as the liquid becomes an olive-