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Boron Blue


Boron blue is the bluish haze or clouding in a transparent boron glaze that results from the crystallization of calcium borate in the glass matrix during cooling. This is a common problem in borate glazes, the higher the boron the worse. The more CaO and B2O3 available and the slower the kiln cools, the worse the effect will be. So first, try cooling faster (of course there is a risk for dunting with this approach).

But the most effective approach is to adjust the chemistry. Is the glaze melt fluid, does it melt really well to the point of flowing if it is thicker? Then reduce the boron (the less fluid melt will discourage crystal growth). It is best to have no more boron than is necessary to get a good melt. Is the CaO high (compared to other glazes of the same type)? Then try reducing that.

If the glaze is melting well it will likely tolerate the addition of more Al2O3. Increasing it without changing anything else may be all that is needed. There are added benefits to this also (e.g. better hardness, lower expansion). Why does this help? The Al2O3 stiffens the melt (as noted, the crystals grow in a fluid melt).

These changes are done using glaze chemistry. If you enter a recipe into your account at Insight-live.com it can show you the amount of each oxide (e.g. CaO, Al2O3, SiO2) and you can click materials to see what oxides they contribute. Then, by duplicating the recipe and viewing it side-by-side with the original you can adjust recipe ingredients and see the impact on the chemistry. It may be that changing some or all of the frit for a similar one, but of higher Al2O3 and lower B2O3, might move things in the right direction. But the most common way to add Al2O3 is to increase the kaolin and reduce the silica (because the kaolin contributes both Al2O3 and SiO2).

The most non-technical way to adjust the recipe is to simply blindly try other frits. While this might reduce the boron blue, the different chemistry they bring can affect many other properties (like color response, thermal expansion, melting temperature, hardness and durability, surface character, matteness or gloss). The problem is the balance in the chemistry and changing one or two oxides at a time is the key to effectively deal with the problem.

Boron blue can also be used as a decorative effect, especially on low temperature ware.

Boron blue in low fire transparent glazes

This high boron cone 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 a more melt fluidity is needed, decrease the percentage of CaO. For opaque glazes, this effect can actually enable the use of less opacifier.

The perfect storm to create boron-blue clouding at low fire

Two clear glazes fired in the same slow-cool kiln on the same body with the same thickness. Why is one suffering boron blue (1916Q) and the other is not? Chemistry and material sourcing. Boron blue crystals will grow when there is plenty of boron (and other power fluxes), alumina is low, adequate silica is available and cooling is slow enough to give them time to grow. In the glaze on the left B2O3 is higher, crystal-fighting Al2O3 and MgO levels are alot lower, KNaO fluxing is alot higher, it has more SiO2 and the cooling is slow. In addition, it is sourcing B2O3 from a frit making the boron even more available for crystal formation (the glaze on the right is G2931F, it sources its boron from Ulexite).

What has this low fire transparent glaze turned blue?

It is made from 85% Ferro Frit 3134, 7.5% kaolin and 7.5% silica. While not obvious from the recipe, one look at the chemistry of this (as displayed when you enter a recipe into your account at insight-live.com) will show very low Al2O3. Frit 3134 has almost no Al2O3, yet it is an essential component of functional glazes (for durability, resistance to crystallization, stability during firing). The kaolin is the only contributor of Al2O3 and there is only a little. A simple fix would be to use Ferro Frit 3124 instead, remove the silica and increase the kaolin to 15.

What happens when glazes lack Al2O3?

This happens. They are glossy, but lack thickness and body. They are also prone to boron blue clouding (micro crystallization that occurs because low alumina melts crystallize much more readily on cooling). Another problem is lack of resistance to wear and to leaching (sufficient Al2O3 in the chemistry is essential to producing a strong and durable glass). This is a good example of the need to see a glaze not just as a recipe but as a chemical formula of oxides. The latter view enables us to compare it with other common recipes and the very low Al2O3 is immediately evident. Another problem: Low clay content (this has only 7.5% kaolin) creates a slurry that is difficult to use and quickly settles hard in the bucket.

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By Tony Hansen




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