All 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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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. While the effect might be considered decorative in some cases, this is generally considered a glaze fault. It is a common problem in high boron glazes, the higher the CaO the worse it is. It is about the chemistry of the glaze (even though some might like to blame the frit).

If possible, try cooling faster (of course there is a risk for dunting with this approach). That gives the crystals less time to grow. Or, by firing tests it might be possible to isolate the temperature range where it happens and just cool quickly through that.

The most effective approach is to reduce the CaO. As long as it is not excessive the crystals should not form. Even if the glaze has plenty of boron.

If the glaze is melting well it will likely tolerate the addition of more Al₂O₃. Increasing it without changing anything else impedes the growth of crystals (by stiffening the melt thereby denying the molecules the mobility they need to form the preferred crystalline arrangement). There are added benefits to this also (e.g. better hardness, lower expansion).

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 duplicate the recipe to view it side-by-side with the original. Adjust ingredients and note the impact on the chemistry. It may be as simple as changing some or all of an existing frit for another frit (or mix of frits) having higher Al₂O₃ and a mix of fluxes having a lower percentage of CaO. A common way to add Al₂O₃ is to increase the kaolin and reduce the silica (to re-match the 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 will affect other properties (like color response, thermal expansion, melting temperature, hardness and durability, surface character, matteness or gloss). Since the root problem is with two oxides the best approach is to focus on them.

Boron blue can also be used as a decorative effect, especially on low-temperature ware. To encourage it just do the opposite of everything mentioned above.

Related Information

Boron blue in low fire transparent glazes

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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 perfect storm to create boron-blue clouding at low fire

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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 grow best 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 a lot lower, KNaO fluxing is significantly 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?

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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 ceramic glazes lack Al2O3?

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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.

Why would a low fire transparent require four frits?

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To get the needed chemistry to avoid boron blue clouding (calcium borate crystals). The one on the right clouds, the other does not. Why? Differences in the chemistry (as seen in my account at insight-live.com). G2931K, on the left, has greater Al₂O₃ (which impedes the growth of crystals), lower CaO (starves their growth) and more boron (for better melting). There is actually no practical way to adjust the recipe on the right (by supplying MgO with talc and fiddling with frit percentages) to achieve this. Frit 3124 lacks Na₂O and B₂O₃. 3134 has excessive CaO and almost zero Al₂O₃. Talc does not melt well enough. But Frit 3249 supplies the needed MgO and has lots of B₂O₃ and low CaO. And Frit 3110 has low CaO and supplies the needed Na₂O.

A severe case of boron blue creates opalescence

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So-called opal or opalescent glazes are commonly ones having a combination of boron blue coupled with plenty of titanium or rutile to enhance the effect. However such glazes bring along significant "baggage". For example, 6-7% titanium, which, on cooling in the kiln, would matte or crystallize many glazes, can be tolerated because of the high melt fluidity. Unfortunately runny glazes bring issues for production. And, glazes of this type commonly have a high percentage of feldspar (to source the needed KNaO), unfortunately that means they will craze. Plus, the high melt fluidity needed for the effect requires a low percentage of Al₂O₃. That has other implications. First, hardness of the fired glaze is affected. Second, since Al₂O₃ is being contributed by the feldspar, there is no room left in the recipe for kaolin or ball clay (which normally sources the Al₂O₃). In the past, Gerstley Borate, a very active melter that is also plastic was an ideal solution. Unfortunately, Gillespie borate, the recommended substitute, does not have the same suspending properties. One solution is to source the KNaO from Ferro Frit 3110, using glaze chemistry. It has almost no Al₂O₃ so significant kaolin can be added to source it. However, that still does not fix the issue of crazing. An alternate solution is to find another melt fluid lower expansion base glaze and add the titanium to that.

Inbound Photo Links

Example of variegation by thickness-induced boron blue

A pottery glaze so reactive it can eat through a firebrick. The fix struck boron-blue gold!

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