500 On the Preparation of Crayons for Drawingf 



foften the compofilion. Inftead of reddle the other red ox- 

 ydes of iron may be ufed, fuch as colcothar of vitriol, &c. ; 

 and in that cafe they flioukl be chofen foft to the touch, and 

 of a Hvely colour; for thofe ufed in commerce are often 

 mixed with too ninch clay, which gives them a dull yellowini 

 caft that ought to be avoided. I attempted to incorporate 

 thefe fubftances with the whites of eggs and the albumen of 

 blood; but crayons compofed in this manner were not good. 



The beft reddle, in Iranps, fliould be fele6led and grouiKl 

 with pure water on a marble flab, as is done in the prepa- 

 ration of colours for painting ; taking care to moiften it as 

 much as is neceffary to make the grinding ftone glide, and 

 to employ as httle water as poffible. When it is intended to 

 prepare a large quantity of this fubftance, this operation be- 

 comes very difficult and expeufive : in that cafe a different 

 metliod mud be ufcd. The reddle muft be pounded and 

 £fLed through a fine fieve, then diluted with a large quantity 

 of water in a trough; where, after it has been well ftirr^d 

 round, it fliould be left a few minutes to fettle, iu order that 

 the grofler particles may precipitate themfelvcs to the bot- 

 tom. The water, which is ftrongly impregnated with the finer 

 particles, is then poured off, and fuffered to fettle for twenty- 

 four hours. The clear water on the top is then poured oiT 

 again, and a very fine fedimeut will thus be obtained, which 

 muft be pounded and wauicd once more. The fedimcnt of 

 the firfl; wafliing mu'l be treated iu the like manner, and the 

 procefs is repeated until the whole be reduced to the utmoft 

 linenefs.. 



The gum., fize, or foap, dcftincd to give the crayon the 



neceflary degree of fcliviity, mult be diflblved feparately. 



Thefe folutions muft be carefully mixed with the pounded 



reddle, and the watery particles muft be evaporated by being 



expofed to the fim or to the heat of a gentle fire, taking care 



to turn the pafte often till it has acquired aeonfirtence fome- 



vviiat harder than butter : the crayons are then to be formed 



m the mourds. 



The 



