Silica is the glass in a glaze. Every recipe contains it, usually as the largest single ingredient, and everything else in the bucket exists either to make it melt at a temperature a kiln can reach or to stop it running off the pot once it has. It is also the material behind every dust precaution on this site.
What it does in a glaze
Silica melts far above kiln temperature on its own, so it is combined with fluxes that bring the melting point down into range. The proportion of silica largely decides how hard and durable the fired glaze is and how it expands and contracts as it cools, which is why adjusting silica is one of the standard moves when a glaze crazes or shivers on a particular body.
Grades and what the names mean
Suppliers sell silica in several particle sizes, and the finer grades melt more readily and are what most glaze recipes assume. Coarser silica sand is used in bodies and as a parting material rather than in glazes. Silicon sand is a common misspelling of silica sand rather than a separate product, so a search for it lands in the same place.
In a clay body
Silica is present in every clay body as well, both naturally and as an addition, where it controls shrinkage and helps the fired body and the glaze expand at compatible rates. Adjusting body silica is a manufacturer's job rather than a studio one, but it explains why a glaze that fits one body crazes on another with a different silica level.
The dust rule, which starts here
Respirable crystalline silica is the specific hazard behind every mixing, sanding and sweeping instruction on this site, and this bag is where it is at its purest. Weigh it in a ventilated space with a fitted respirator, never dry-sweep a bench that has had it on, and wet-clean instead. OSHA 1910.1053 and the COSHH regulations bind employers rather than publishers or hobbyists, which is why the practice is written down here rather than assumed.
Questions people ask about silica sand
What does silica do in a glaze?
It is the glass former: the material that becomes glass. Fluxes lower its melting point into kiln range and alumina stops the melt running.
Is silicon sand the same as silica sand?
Yes. It is a common misspelling rather than a different product.
How dangerous is silica dust?
Respirable crystalline silica is a genuine long-term hazard and the reason every mixing and sanding instruction here specifies a fitted respirator, ventilation and wet cleaning.