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Material Level Changes

• Adding silica: This works if the glaze can tolerate it and still melt as well and surface character is not affected. In fact it makes no sense not have have as much silica in a glaze as it will tolerate. However be aware that the amount of silica you may have to add may be a lot more than you think. For example, to move the 20x5 cone 6 transparent from 7.0 to 6.5 requires the addition of 17% silica. There is no way this could be tolerated and still maintain a good melt. Therefore it seems logical that silica additions alone are reserved for fine tuning thermal expansion, not solving crazing.
• Add a low expansion frit: Ferro Frit 3249 is a good example, its thermal expansion is around 4.0. However to move the 20x5 cone 6 transparent from 7.0 to 6.5 requires the addition of 21% of this frit. This amount amount of frit doubles the boron and cuts the silica.
• Substitute a low expansion frit: Replacing the Frit 3134 with Frit 3249 causes a drastic change in the overall oxide balance in the recipe, however substituting half of the the frit takes the expansion down to 6.5.
• Use a higher expansion clay body (usually one with a higher quartz content).

Oxide Level Changes

For these solutions chemistry is needed to juggle the amounts of materials to achieve the desired oxide change.

• Add Al2O3: Like adding silica, it can change the surface character of the glaze. However even if you removed all of the silica and replaced it with kaolin this hardly moves the calculated thermal expansion of the 20x5 glaze.
• Add low expansion boron: Doubling the boron in the 20x5 recipe only takes the expansion down to 6.8, not much of a decrease. However increased boron will help to dissolve more silica and alumina which will help to further reduce expansion. However not as much as we need.
• Substitute lower expansion CaO or SrO for high expansion Na2O and K2O: Removing half of the 0.2 molar equivalents of KNaO in the 20x5 recipe takes the expansion down to 6.4. This is a remarkably large change for a comparatively small formula change. However to achieve this change required major surgery on the recipe, it will not likely melt nearly as well because the feldspar is gone, silica and wollastonite are much higher. Wollastonite just does not melt well at cone 6.
• Introduce low expansion Li2O at the expense of another flux: Adding more than 2% or so will change the recipes fired appearance and this does not reduce expansion by much.
• Introduce low expansion MgO: This is the jackpot! By replacing half of the KNaO with MgO the expansion drops to 6.15. This is incredible. However the accomplish this in the recipe using 5% talc as an MgO source requires removing the feldspar, increasing the kaolin by 10%, the silica by 8%. There is no way this is going to melt even though the chemistry says it should. What could we do? The answer is the MgO low expansion frit mentioned above. This frit sources all the oxides that are hard to find good melting suppliers for at cone 6 (CaO, MgO, Al2O3, SiO2) plus lots of boron.

Although many of the above strategies do not have a large reducing effect on glaze expansion, in combination it is true that they can add up. I have found that crazing glazes invariably have other deficiencies (e.g. too glossy, too matte, tendency to pinhole or blister, crystallize, cloud up, settle in the bucket, knife mark, dissolve in acids, etc). It is thus best to pick a strategy that reduces crazing and at the same time addresses other problems. For example if the glaze is melting very well and is glossy then it likely will tolerate more silica. Likewise, if the glaze is melting well but is too glossy, you can add alumina, it will at least help reduce expansion and gloss plus increase hardness. If a glaze is not melting quite well enough then the addition of boron will reduce expansion and melting temperature. If a glaze is settling in the bucket and melts well, it will likely tolerate both alumina and silica additions, that means you can increase kaolin that glaze glaze will suspend better. All of these will help, but I repeat that a crazing glaze needs major work, even a combination of these will likely be enough to fix the problem.

Na2O, K2O Must Go

All the crazing glazes I can remember had one thing in common: high sodium and potassium (from feldspar, nepheline syenite or frits). When you consider increasing sodium and potassium has up to 10 times the effect on expansion as does reducing silica, it only makes sense that replacing part of their content with a flux of the lowest possible expansion will have the maximum effect. Substituting MgO for KNaO imparts the maximum effect. However you could also substitute KNaO for a mixture of CaO, ZnO, Li2O, SrO also.

There are some possible problem with increasing MgO in glaze recipes. First, MgO can contribute a different color response for some metal oxides. However I have not found this to be a big concern in most cases (except for the important exception of Chrome Tin pink colors, they do not like MgO). As noted, the main raw sources of MgO, talc and dolomite are viable in high fire glazes, but they do not melt to release their oxides at lower temperatures (in addition both are rather gaseous and can contribute to glaze imperfections at lower temperatures). Frits like Fusion F69 or Ferro 3249 are the perfect solution for middle temperatures like cone 6. They release no gases and melt well. If your glaze already contains a material contributing boron (and almost all middle fire glazes do) these frit are ideal candidates to help you reduce thermal expansion dramatically.

Consider glaze G1214W Cone 6 transparent:

This glaze is an adjustment to improve the earlier 20x5 recipe G1214M. However this glaze does tend to craze on clay bodies having a lower quartz content. In addition, its CaO content is lower and it does not work well with Chrome-Tin colors. I have attempted various strategies to reduce its thermal expansion and they have met with limited success. The best so far involves sourcing Li2O from spodumene to replace some of the Na2O/K2O. However the glaze slurry is not as nice to use (it foams and bubbles).

However using this frit I was able to use less total frit content, I had more flexibility to adjust the chemistry (I was able to eliminate the feldspar completely to produce a high calcia glaze), and I was able to retain the feldspar (the Frit 3134 contributed considerable Na2O and that was removed by eliminating it). The new glaze has a very similar chemistry:

remember effect of dif clay bodies
It is possible that you need to move still further to stop crazing on your clay body, especially if it is very low in silica. We suggest increasing B2O3 slightly and then increasing Al2O3. Or you may simply be able to add silica. In serious cases of crazing the magic oxide is MgO, increasing at at the expense of K2O and Na2O will dramatically reduce the expansion. MgO does not melt as well at middle temperature but we have found the diversifying the fluxes and increasing the boron a little will make room for as much MgO as you need. Here is our G1215M recipe. Although it looks very different, the chemistry is very similar. The small amount of zinc should not be a problem for most, but if it is you can leave it out and see if it still melts enough, if not add a little more frit.

Silica                 35.0
Strontium Carbonate     2.0
Zinc Oxide              2.0
Spodumene              10.0
Dolomite               11.0
Frit 3195              30.0
EPK                    15.0