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This high-boron high-CaOcone 04 glaze is generating calcium-borate crystals during cool down (called boron-blue). This is a common problem and a reason to control the boron levels in transparent glazes; use just enough to melt it well. If more melt fluidity is needed, decrease the percentage of CaO in favor of a lower melting oxide, that will certainly help. There is a positive: For opaque glazes, this effect can actually enable the use of less opacifier.
The middle front mug is glazed with an 85:15 lead bisilicate:kaolin mix, the G3971 recipe. It is an absolutely "knock your socks off" crystal-clear hyper-glossy surface that transmits the terra cotta color beautifully regardless of whether the clay is smooth or coarse or the glaze thick or thin (this one was applied as a brushing glaze in three coats on L215). My lead testing kit passes it with no detectable lead release. The other pieces are done using brush-on versions of boron-based clear glazes (commercial and made from a recipe). At almost any thickness and whether on L215 or L4170B clouding occurs. The worst one is a commercial three-coater on the right, the best is G1916W (it has 2% added iron as a fining agent for the micro-bubbles). My terra cotta plan: Glaze the inside functional surfaces with that and the outsides with the leaded one (and using a kiln exhaust system).
Glossary |
Boron Blue
Boron blue is a glaze fault involving the crystallization of calcium borate. It can be solved using glaze chemistry. |
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Glossary |
Crystallization
Ceramic glazes form crystals on cooling if the chemistry is right and the rate of cool is slow enough to permit molecular movement to the preferred orientation. |
Glossary |
Opacity
Opacity of ceramics glazes is normally achieved by adding an opacifier like tin oxide or zircon. However, there are chemical profiles that can turn transparent glazes milky and make it cheaper to opacify them. |
Glossary |
Reactive Glazes
In ceramics, reactive glazes have variegated surfaces that are a product of more melt fluidity and the presence of opacifiers, crystallizers and phase changers. |
Glossary |
Transparent Glazes
Every glossy ceramic glaze is actually a base transparent with added opacifiers and colorants. So understand how to make a good transparent, then build other glazes on it. |
Oxides | B2O3 - Boric Oxide |
Oxides | CaO - Calcium Oxide, Calcia |
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