Silica. Silicon dioxide. SiO. 350G Mesh denotes the screen mesh it has been sieved through. The larger the number, the finer than the mesh.
The glassy substance which is the most important constituent of pottery glazes and which is an integral part of clay and many
potters' minerals. It occurs naturally as quartz rock, flint and sand. It also occurs in combination with other oxides in what are called silicates. An analysis of all the substances in the Earth's crust shows that 60% is silica
Silica is a hard glassy substance which melts at 1710°C (3110 F) to a transparent glass. This temperature is too high for a potter's kiln and so some way of melting silica at a lower temperature is used in order to make a glaze. Fluxes are mixed with the silica. They have the effect of bringing down the melting temperature to within the range of the potter's kiln. The resulting melt is a mixture of silica and other oxides which link together on cooling to form a glass. Silica is the necessary constituent of this glass.
The term 'silica' can be misleading. It is important to understand the difference between 'silica mineral', 'silicates', and 'silica glass'. Quartz is the best example of a natural mineral that is almost pure silicon dioxide (it is the most abundant mineral on planet earth). Other ceramic minerals like feldspar and clay contain some 'free silica' (accessory quartz). However these also usually contain 'silicates', that is, SiO2 chemically combined with other oxides to form crystalline minerals.
Quartz and flint are the popular silicas for glazes and are bought in powder form. Preparation of one's silica is a difficult process because it is so physically hard. Quartz is crushed and ground. Flint is calcined and then ground. There is little difference between quartz and flint in a glaze because free silica in a glaze recipe rarely exceeds 20%. Flint should melt more easily than quartz but a mixture of the two will probably be found to melt more easily than either alone because of the phenomenon of interaction. The addition of silica to a body makes the fired ware
harder and more durable. Sand may also give a pleasing texture. Silica also alters the amount by which the body contracts in cooling after firing. This contraction is
important for correct glaze fit. Silica is therefore added to a body for a number of reasons which must be balanced against one another. As a general rule, flint is to be preferred for earthenware because it converts to cristobalite more easily than quartz. Quartz or quartz sand are preferable for stoneware because they do not convert so easily to cristobalite.