522 ALUMINUM. 



tinated by partial fusion, it becomes a strong solid mass, which 

 adheres to the tongue, and absorbs water with avidity. To- 

 render it impermeable by that liquid, it is covered with a vitre- 

 ous matter, which is fused at a high temperature, and forms an 

 insoluble glaze or varnish upon its surface. But the interior 

 mass of ordinary pottery has always an earthy fracture, and 

 presents no visible trace of fusion. 



When an addition is made to the clay, of some compound, 

 which softens or fuses at the temperature at which the earthen- 

 ware is fired, such as felspar in powder, then the clay is agglu- 

 tinated by the fusible ingredient, and the mass is rendered semi- 

 transparent, in the same manner as paper that has imbibed 

 melted wax remains translucent after the latter has fixed. The 

 accidental presence of lime, potash, protoxide of iron, or any 

 similar base in the clay, may produce the same effect by forming 

 a fusible silicate diffused through the clay in excess. Such is 

 the constitution of porcelain, and of brown salt-glaze ware of 

 which soda-water bottles are made, which is indeed a sort of 

 porcelain. When these kinds of ware are covered by a fusible 

 material, similar to that which has entered into the composition 

 of their body, and a second time fired, they acquire a vitreous 

 coating. Their fracture is vitreous and not earthy, the broken 

 surface does not adhere to the tongue, and the mass has much 

 greater solidity and strength than the former kinds of earthen- 

 ware. In combining the ingredients of porcelain, an excess of 

 the fusible ingredient is to be avoided, as it may cause the ves- 

 sels to soften so much in the kiln as to lose their shape, or even 

 to run down into a glass ; while on the other hand if the vitrifi- 

 able constituent is in too small a proportion, the heat of the 

 furnace may be inadequate to soften the mass, and to aggluti- 

 nate it completely. 



Porcelain. The mode of fabricating porcelain, which has 

 been known for a long time to the Chinese, was discovered in 

 1706 by Bottcher, at Dresden, where the art was first practised 

 in Europe, and published afterwards by Reaumur. The mate- 

 rials employed are (1) a fine and pure clay, prepared by leviga- 

 tion from mouldering granite or other disintegrated felspathic 

 rocks. In China it is called kaolin. That consumed in the 

 great potteries of Staffordshire is prepared in Cornwall, and 

 known as China clay. A comparison of compact and disinte- 

 grated felspar, shows that by the solvent action of water, the 



