408 Fahrication of Crayons. 



whofe orifice is equal to the fize of tliefe crayons. The pieces when mouWed in this 

 manner, are left to dry. This deficcation ought to be performed flowly, and in the 

 (hade, to prcven* cracks, which would be formed, if this precaution were not attended 

 to. 



When the pieces are dryed, they are to be divided into lengths of two inches {ex five 

 centimetres) each ; the edges are rounded off and they are roughly pointed. They mull 

 then be fcraped to remove a hard external furface which they acquire during their 

 drying, and would prevent their marking. 



A flight coating of oil muft be fmeared over the wooden moulds, to prevent the 

 tafte from adhering to the fides. 



Gum arable and ifinglafs are to be preferably ufed. The gum and foap may be dif- 

 folved in water ; but the ifinglafs muft be firft cut into fmall pieces, afterwards put int© 

 warm water and diflblv&d on a water-bath. Thcfe folutions muft be fufficiently 

 liquid to pafs through a hair-feive, and leave their impurities behind. 



.It is not without difHculty that the paft'e incorporates with the folutlon of glue. 

 Both muft be warm, and the mixture muft be made over the fire at the boiling water 

 heat. 



The pafte muft be well mixed together before it is put into the moulds, in order that 

 it may be uniformly incorporated with. the folution, and leave no hard parts. It 

 would be better to work it with the painter's muller, and to grind it for a ftiort time on 

 the ftone, before it is put into the moulds. 



It is only in the crayons which contain gum that foap can be admitted. None of the 

 experiments in which glue and foap have been ufed together have ever fucceeded; and 

 it muft be fo, becaufe the excefs of the alcali of the foap afting on tlie gelatine deftroys 

 its adhefive quality. 



As thofe crayons, into the corporation of which foap enters, are found to aftbrd a 

 darker fhade, it feems that this combination takes a portion of the oxigen from the red 

 oxide of iron, and renders it brown by bringing it nearer to the ftate of the martial 

 ethiops. I have obferved that all the partes prepared with the oxide of iron, even if 

 with mere water, become brown on their exterior furface during the drying. This efFcfl: 

 takes place in a more evident manner when they are expofed to the fun ; and it appears 

 to arife from the light taking a portion of the oxigen to the oxide of iron. I fhall 

 hereafter return to the chemical properties of thefe kinds of preparations, but my pre- 

 fent purpofe is only to difcribe thofe proceffes of fabrication in which I have conftantly 

 fucceeded, in order that they may be repeated with fuccefs. 



The crayons compofed after thefe direftions have all the good qualities which can be 

 required ; they do not coft one fourth of the price of thofe in the fliops ; but it maft be 

 noticed that their compofitiou requires great exaftnefs in the quantity prefcribed, be- 

 caufe 



