Monday, 5 March 2012

Sands for Texture

Firing on sand can give an even stippled texture. It can also provide immediate, free form textures and shallow shapes. You can use different sized grains for variations in texture size. So it can be a quick, responsive medium to give textures and shallow shape to the glass.

Although you can fire on sand, you must use a separator. When I fire directly onto the sand for texture, I dust alumina hydrate over the sand. People who watch me dust it through a sock have a laugh, but it does work to provide a fine layer of separator, so that any sand picked up by the finished glass can be removed as it is not fused to the glass. Sometimes it takes a bit of cleaning effort.

There are a number of sands that you can use, although all require a surface separator.

Silica sand is the kind of sand found on a beach and is the most commonly used sand. It is the most commonly used sand because of its great abundance and low cost. Its disadvantages are high thermal expansion and low thermal conductivity which requires caution in annealing the glass.

Olivine sand is a mixture of orthosilicates of iron and magnesium from the mineral dunite. Its main advantage is that it is free from silica, although a separator is still required. Other advantages include a low thermal expansion, high thermal conductivity, and high fusion point. Finally, it is safer to use than silica.

Chromite sand is a form of magnesium aluminium. Its advantages are a low percentage of silica, a very high fusion point, and a very high thermal conductivity. Its disadvantage is its cost.

Zircon sand is a compound of approximately two-thirds zircon oxide and one-third silica. It has the highest fusion point of all the refactory sands, a very low thermal expansion, and a high thermal conductivity. However, it is expensive and not easily available

Chamotte sand is made from previously fired clay. It has a relatively high fusion point and has low thermal expansion. It is the second cheapest sand, however it is still twice as expensive as silica sand. Its disadvantages are coarse grains.