Potash. 217 



to dry, and the cotton washed in pure water, and dyed in a 

 solution of madder or logwood, it will be found that the 

 spot produced by the oxalic acid remains white, while the 

 ground takes up the madder or logwood iron colour. The 

 oxalic acid thus dissolves the iron mordant combined with 

 the cotton thread, so that it may be washed away in water. 

 This property renders oxalic acid a very useful substance as 

 a discharger for a fine pattern with an iron ground, whicb, 

 after dyeing, appears white. 



The oxalic acid salt, which is a combination of oxalic acid 

 and potash, may also be employed ;* yet on a very dull 

 ground it acts with less certainty. 



Citric acid answers the same purpose. It is best to em- 

 ploy both acids in conjunction. 



Potash. — Most of the acids act injuriously upon cotton 

 fabric, especially on the application of heat, but potash on 

 the other hand does not produce any injury, when not al- 

 lowed to act in too great excess, or too long. Thus a piece 

 of cotton maybe allowed to hang 3 weeks in a temperature 

 of 68°, after being soaked in a solution of 1 lb of potash in 

 4 lbs. water, without any injury. 



Hence, potash may be boiled with the raw cotton to pre- 

 pare it for bleaching, without any disadvantage. For this 

 purpose, it requires to be made caustic by abstracting its 

 carbonic acid by means of lime. The proportion of lime to 

 the potash depends upon the strength of the ley, which will 

 be prepared according to the proportion of the water added. 

 To make a very strong solution of caustic potash, much lime 

 should be employed; for a weak solution, less will be re- 

 quired. A solution of potash which consists of 200 lbs. po- 

 tash in 1400 water, requires from 56 to 60 lbs. of quick lime 

 to make it caustic, while with half this quantity of water, 

 we must take double the lime to obtain the same result. 



For boiling the cotton, weak leys are employed. Hence, 

 wood ashes may be used for their preparation, when they 

 are cheap enough. For 1000 lbs. of good wood ashes, 50 

 to 60 lbs. of lime are required to render the potash contained 

 in them caustic. 



To determine if a ley is completely caustic, lime water 

 may be employed. If after the addition of the latter, tlie 



• ISiuoxalute of potash. — EblT. 



