The production of porcelain for electrical insulation — II

This article deals with ceramic raw materials, chiefly clays, quartz and feldspar. CLAYS. The physical properties of clays are of interest. Plasticity is one of the principal properties of clays. The cause of plasticity is probably to be sought in the dispersed nature of the fine clay particles which have the properties of a suspension colloid. Small amounts of alkalies, when added to clay suspensions, cause a marked increase in fluidity. In other words a mass of plastic clay, by adding these alkalies, can actually be converted into a fluid capable of being poured into molds. This application is of great importance commercially since it enables us to form clay articles of a very homogeneous structure without the use of excessive amounts of water and without the high drying shrinkage which would accompany the use of much water. The reactions which take place in firing clay are described. Firing ranges of clays can be determined by the drop in porosity and increase in shrinkage as the firing temperature increases. Continued firing of a clay after it has vitrified produces softening of the mass and produces internal changes including the formation of sillimanite (Al
2
O
3
, SiO
2
) crystals. Continued firing releases gases which produce a porous or vesicular structure known as overfiring. The longer the firing range of a clay the better it is suited for commercial purposes. QUARTZ. Ground quartz which is practically pure silica, (SiO
2
) in porcelain, serves as a refractory skeleton which increases the stiffness of the mass during firing. Quartz undergoes molecular changes at different temperatures. There are eight forms. Each change is accompanied by a definite volume change. The specific volume starting with the lowest form, increases continuously and abruptly at the inversion points until a final total increase in volume of about 20 per cent is reached when the quartz is fused. FELDSPARS. Used for porcelain contain potash and soda as their fluxing elements. They fuse to glasses at about 1230 deg. cent. (2246 deg. fahr.) Their chemical compositions approximate Na KO Al
2
O
3
6Si O
2
. They are used as glass forming fluxes in porcelain to cement and unite the other ingredients. Other minerals are also used: — 1. Those which replace feldspar such as alkaline earths. 2. Those which replace quartz and which are preferably constant in volume such as sillimanite, zircon, etc. Porcelains are now being used in which natural and synthetic sillimanite replace quartz, and alkaline earths replace alkalies.