In ceramics, glazes are loosely classified as low, medium and high temperature. Low temperature is in the cone 06-2 range (about 1800F-2000F).
This term generally refers to glazes that mature from cone 06-04 and are not functional.
Low temperatures require the use of boron-containing materials (e.g. frits) to produce a well melted glass. While low fire glazes are not as hard as well formulated medium temperature glazes, they have the advantage of supporting a much wider range or colors (which can be very bright and vivid). Historically Gerstley Borate and Colemanite have also been used to source boron in low fire glazes. Generally, at low temperatures the major challenges are to get a well melted and well fitted glaze (the clay glaze interface is not well developed and bodies are not vitreous so glazes must have compatible thermal expansion to the body to avoid coming off). The oxides commonly regarded as fluxes at middle and high fire are often simply fillers (and even matting agents) at low temperatures, thus understanding the relationship between chemistry and physical properties is more challenging.
This is an all-fritted version of G2931F Zero3 transparent glaze. I formulated this glaze by calculating what mix of frits must be employed to supply the same chemistry of the G2931F recipe. The mug is made from the Zero3 porcelain body (fired at cone 03) with this glaze. This glaze fits both the porcelain and the Zero3 terra cotta stoneware. The clarity, gloss, fit and durability of this glaze are outstanding.
On the left is G2931J, a zinc alkali fluxed and high Si:Al ratio glaze. Those look like micro-bubbles but they are much more likely to be micro-crystals. High-zinc and high-silica is the mechanism for crystalline glazes, so it appears that is what they are. G2931K on the right has much more boron, double the Al2O3, less SiO2 and is magnesia-alkali instead of zinc-alkali. It is the product of dozens of tests to find an ultra-clear having a glassy smooth surface. This particular chemistry, although having only a 6:1 SiO2:Al2O3 ratio is ultra-gloss. In addition is has low expansion, will fast fire and the boron is not high enough to compromise the hardness.
Low Temperature Glazes Recipes
Reducing the Firing Temperature of a Glaze From Cone 10 to 6
Moving a cone 10 high temperature glaze down to cone 5-6 can require major surgery on the recipe or the transplantation of the color and surface mechanisms into a similar cone 6 base glaze.