A frit is a glaze material that has already been melted into a glass, ground back to a powder and sold in a bag. Fritting exists to solve two problems that raw materials cause: solubility, which makes a glaze change as it dries, and toxicity, which makes some materials unsafe to handle raw.
What fritting fixes
Soluble materials such as borax and soda ash dissolve in the glaze bucket and migrate into the bisque and to the surface as the glaze dries, so the fired glaze is not the recipe you weighed. Melting them into a glass first makes them insoluble, so they stay suspended and stay put. That is the single biggest reason modern recipes are full of frits where old ones are full of raw materials.
The toxicity argument
Lead frits exist for the same reason: the lead is locked into a glass rather than present as a soluble compound, which changes how it behaves if handled or ingested. Most studios today avoid lead altogether and use boron frits, and this site takes no position on any glaze being food safe except by quoting a manufacturer's data sheet for a specific product and firing.
Choosing between frits
Frits are sold by number and each has a published analysis: how much boron, sodium, calcium and silica it carries. A recipe naming a frit is naming that analysis, and substituting a different number without adjusting the rest of the recipe changes the glaze. Where a recipe names a frit your supplier does not stock, the published analyses are what you compare to find the nearest equivalent.
Cost and handling
Frits cost more per kilogram than the raw materials they replace, because someone has already melted them. They are still fine powders and carry the same weighing precautions: a ventilated space, a fitted respirator, wet cleaning. Kept dry they last indefinitely.
Questions people ask about frit
What is a frit?
A glaze material that has been melted into a glass and ground to a powder, which makes soluble materials insoluble and stabilises the recipe.
Can I substitute one frit for another?
Only by comparing published analyses. Frits differ in boron, sodium, calcium and silica, and swapping numbers changes the glaze.
Why do modern recipes use so many frits?
Because they solve solubility. A raw soluble flux migrates as the glaze dries, so the fired result is not the recipe that was weighed.