as Ohje£ls of the Art of Dyings i^c. 375 



colouring matter j but the two firft have defefts 

 which render them lefs eligible* Jhe calx of 

 zinc may perhaps be ufefully employed. But 

 ihould firft be thoroughly purified from the iron 

 which it generally contains. 



Thefe all form bafes for the more brilliant reds 

 and yellows. For the laft, the calx of copper is 

 alfo employed, having a ftrong attradion for 

 the colouring matter of weld. Salts containing 

 iron furnilh a bafts, which, with the aftringent 

 matter of vegetables, produces a black dye. 



When alum is ufed to fupply a bafis for the 

 dying of wool, it is the praftice to join with it, 

 either crude tartar or its purified cryftals, in the 

 proportion of five ounces of alum, to one of 

 tartar. This laft fubftance we know confifts of 

 an alkaline vegetable fait, fuperfaturated with a 

 peculiar acid, which bears its name. The 

 fuperabundant vitriolic acid of the alum, will 

 decompofe a part of the tartar, by attaching 

 itfelf to the alkali j and thus the quantity of free 

 tartarious acid is increafed ; which has no 

 properties injurious to the cloth. 



In the aluming of filk, no tartar is employed ; 

 for the filk, in the previous preparation, being 

 impregnated with alkaline or foapy matter, the 

 fuperabundant acid will be neutralifed by it. In 

 this procefs, when the tubs have been long ufed, 

 a very confiderable incruftation' is formed on 

 their fides ; which, the dyers, finding no injury 

 B B 4 from. 



