REFSAN BORIC OXIDE AMORPH CYRSTAL (R7138)

BORIC OXIDE (B₃O₄)

B₂O₃ is a low melting glass of low thermal expansion and surface tension. It is an extremely useful oxide, indispensable in many industries and applications. With B₂O₃ in a glaze you can make it melt at almost any common kiln temperature you want, and get a brilliant finish that will not craze. Na₂O melts as well or better than B₂O₃, but unlike B₂O₃, its high thermal expansion limits it to much lower percentages.

Below cone 10, additions of B₂O₃ are almost always needed to make glazes melt well. Other fluxes, like ZnO, will melt glazes at cone 6, but many issues limit them to only certain types of glazes. The lower you go the more boron is needed. A melt fluidity checker is the best way to determine if the glaze is melting enough (and not too much). At cone 06, 0.5 molar parts of B₂O₃ are usually needed, whereas at cone 6, only around 0.1 to 0.2 are required. Reactive glazes often contain much more boron (e.g. 0.4-0.5 at cone 6), these formulations have issues with running and contributes to various defects. Almost all frits for low and medium temperatures contain boron as their melting mechanism.

Its low expansion makes it valuable in preventing crazing. However, each glaze recipe tends to have an optimum amount above which the effect is can be reversed and crazing increase (typically 10-14%). This effect is due to the loss of elasticity associated with excess B₂O₃. Predicting the expansion of high boron glazes can thus be misleading due to this factor.