A frit is a glaze material made by first melting a mixture of raw materials together into a glass, then rapidly quenching and grinding it into a fine powder, rather than adding the original raw materials to a glaze recipe directly.
A frit manufacturer melts a chosen combination of raw materials in a furnace, the same basic chemistry that happens later in a kiln, but under controlled industrial conditions. The molten glass is then quenched, usually poured into water, which shatters it into small fragments as it cools too fast to form a stable crystal structure; those fragments are ball-milled into a fine powder, the frit itself, ready to be weighed into a glaze recipe like any other raw material. Whatever oxide chemistry the original raw materials contributed is now locked into a stable glass, already reacted, rather than waiting to react for the first time inside a kiln.
Fritting changes a material's behavior in two useful ways. It can make an otherwise hazardous or water-soluble raw ingredient safe to handle and stable in a wet glaze suspension: historically, this was the standard way to make lead usable in a glaze at all, since fritted lead is bound into a glass network and resists leaching far better than raw lead compounds do (see glaze's health and safety section for why raw lead glazes are avoided on functional ware even so). It can also pre-combine materials that would otherwise be difficult to melt together evenly from raw powders alone, or that would dissolve unpredictably in a glaze bucket before ever reaching the kiln, such as some boron sources.
Commercial frits are standardized, reproducible products, typically sold under a manufacturer's own product code (Ferro's frit numbers are a common reference point), each formulated to a known, published oxide analysis. That consistency is part of why many glaze recipes call for a specific frit by name or number rather than a raw mineral: it lets a recipe rely on the same oxide contribution batch after batch, in a way a variable natural mineral cannot fully guarantee. Two widely referenced examples illustrate how differently two frits can be formulated for different jobs: Ferro Frit 3134, a general-purpose boron flux with no alumina content, common in earthenware glazes and usable in small amounts in stoneware ones; and Ferro Frit 3124, a more alumina-rich, more chemically balanced frit that needs only a modest kaolin addition to form a complete low-fire glaze on its own.