BURNING BRtCKS. 379 



iv. Pure Clay. 



I KNO\V not that pure clay is liable to fufion, 

 T>y any intcnfity of heat, unlcfs perhaps by that 

 of the burning glafs. D'Arcet found it to un- 

 dergo no change in the heat .of a porcelain fur- 

 nace. In the lire, however it becomes fo hard 

 tn to give fire with ftccl. This is owing to the 

 incrcafc of its denlity, for it IbfcValmofl one half 

 of its bulk. 



Pure clay is hot Aifiblc with quicklime, in a- 

 ny proportion. But the addition of even the 

 fmalleft quantity of filiceous matter brings the 

 niafs to iufion. And the fufion takes place very 

 readily if to one part of pure clay, and one of 

 lime, two or three parts offiliceous earth be ad- 

 ded ; a larger proportion offiliceous matter is un- 

 favourable to the fulion of the mixture ; and 

 the addition of five parts renders it almoll infu- 

 lible. A mixture of equal parts of clay and lime 

 fullers wne half lefs diminution in bulk than the 

 fame quantity of pure clay would fuller. But, 

 if in the mixture, the clay be only in the pro- 

 portion of one to five, or one to fix, it produces 

 Icarce any alteration on the character of the, 

 lime. Clay is not fufiblc with pure qurirtz; 

 but, according to Pott, it melts without great 

 difficulty with tluor mineral. Fcldfpath, or 

 il luting ipar, often fufes l>y it felt* in the lire, 



suut 



