COLOUR DISCHARGE. 247 



thus. Dissolve in hot water some alum (or sulphate of aluminium), 

 and in a separate vessel some acetate of lead, so as to obtain pretty 

 strong solutions when cold ; pour the alum solution into the other 

 liquid, whereby a thick white precipitate of sulphate of lead will 

 be formed, whilst acetate of aluminium remains in solution, these 

 two compounds being formed by double decomposition. The 

 addition of the alum solution should be made a little at a time, 

 the fluid being allowed to stand for the precipitate to subside, so 

 that it can be seen whether the further addition of alum solution 

 produces more precipitate or not ; or a sample may be filtered and 

 tested by adding a little alum solution to it. When all the lead 

 is precipitated as sulphate of lead, so that no more is thrown down 

 by adding more alum solution, the whole is to be filtered. Part 

 of the filtered liquid should be diluted with, say, two or three times 

 its bulk of water, and another part with seven or eight times its 

 bulk, and the rest preserved undiluted. Each of the thin liquids 

 is to be thickened with gum arabic or dextrin, so that when a piece 

 of dry calico is written on with the thickened fluid the writing 

 does not run. 



By means of a quill-pen, brush, stencil plate, india-rubber stamp, 

 &c., letters or devices are drawn on a piece of calico, using the 

 three liquids according to fancy. The prepared cloth is hung up 

 in a warm room for twenty-four hours, and is then dipped into an 

 evaporating basin containing boiling water in which some alizarin 

 or madder extract has been dissolved, and well stirred about for 

 some minutes ; at the end of which time the writing or other 

 devices will be developed, appearing in different shades of red, 

 according to the strength of the mordanting liquor used. 



Expt. 292. To discharge Colours. Some kinds of goods are 

 treated by what is termed the method of "discharge." In this 

 case the fabric is uniformly dyed all over, and, after drying, the 

 pattern to be discharged is printed on (by wooden blocks, &c.), 

 employing solutions of chemicals so chosen as to bleach or other- 

 wise alter the colour. For example, a cambric handkerchief may 

 be dyed red with alumina mordant and alizarin, and a pattern then 

 stamped upon it from an engraved block, &c., using a weak solu- 

 tion of bleaching powder ; this will more or less bleach the spots 

 to which the liquid is applied, especially if the handkerchief be 

 " soured " by dipping into water slightly acidulated with a drop 

 or two of sulphuric or hydrochloric acid (Expt. 162) ; so that the 

 pattern will finally appear in white on a red ground. 



In similar fashion calico may be more or less completely dyed 

 black by first boiling it in water that has been previously boiled 

 with chips of logwood and strained, and then immersing it in 



