Borax

Borax is sodium tetraborate decahydrate (Na2B4O7 10H2O), a soft white crystalline salt mined from ancient lake beds, most famously in Death Valley, California and in Turkey. It has been used in metallurgy and glassmaking for over 4,000 years, with written records of borax trade along the Silk Road from Tibet to Persia dating to the 8th century.

In ceramics, borax supplies two of the most useful glaze oxides simultaneously: boron oxide (B2O3), one of the strongest low-temperature fluxes available, and sodium oxide (Na2O), another powerful flux. Together they dramatically lower the melting point of a glaze, making borax the key ingredient in many low-fire and raku glazes.

The challenge with raw borax is that it is water-soluble. If added to a glaze slurry, it dissolves into the water and migrates unpredictably during drying, causing uneven surfaces and firing defects. For this reason, borax is usually calcined (pre-heated to drive off the water) before use, or incorporated into pre-melted frits where it becomes insoluble.

Many modern potters use boron frits instead of raw borax to avoid these solubility issues, but borax remains essential for glass making, raku glazes, and traditional historical recipes.

Typical usage: 10 to 30 percent in low-fire raw glazes (calcined). 5 to 15 percent in raku and crackle glazes. Rarely used uncalcined in modern practice.

Safety note: Borax powder is a mild irritant. Wear a dust mask when handling.