Dealing With Chrome Flashing
Close your eyes and imagine a nice pink pastel glaze. Now imagine that you wanted a nice white instead! During the ACerS convention in St. Louis I attended an eye-opening presentation by Stan Sulewski of Pfalzgraff (they are a well known porcelain table ware manufacturer in the US). After hearing what he said I came to better appreciate the synergy between chemistry and physical properties involved with glaze opacity and color. Potters absorbed in reactive and artware glazes often forget how difficult it can be to make a proper white semi-gloss food-safe glaze.
Engineers at the company faced a dilemma:
The obvious solutions of dedicating a kiln to non-chrome-bearing wear or eliminating chrome containing glazes were not feasible. Thus the objective was clear: Adjust the recipe of the tin glaze to have low metal marking and white color without pink flashing.
The first and most obvious approach of simply blending tin and zircon would address the marking problem but the pink colors of course remained because tin is so sensitive to chrome. Impossible as it may seem, they actually found an answer using ceramic chemistry.
If you have ever worked with chrome tin stains you likely know that unless the chemistry of the host glaze is right the color does not develop. Getting the color to work can be a real challenge but in this case they actually wanted to sabotage it! Among warnings on stain manufacturers chrome-tin data sheets are mentions of the detrimental effects of zinc, raw alumina, magnesia and a lack of calcia. They reasoned that it should be possible to solve this problem by making the host glaze chemistry hostile to the development of chrome tin pinks. And that is what they did.
Zinc, the most obvious choice, did kill the pink but it also imparted a yellow brown color, that would not do. The presence of adequate CaO is critical to the development of pink and MgO is detrimental. While both are fluxes a complete replacement was not practical. Glazes tolerate and usually benefit from relatively large amounts of CaO, but complete replacement with MgO or SrO (or even a mix) produces much different surfaces and less active melting.
The critical factor, as implied above, is that if calcium is not present in a threshold minimum amount chrome tin pink colors can be completely absent. Thus the answer turned out to be a compromise: An MgO/SrO mix (with more MgO) replacing much of the CaO. This preserved the surface character and killed the pink.
However the white color was compromised just a little so a final adjustment was done: a small amount of blue stain was added to brighten the white.
There you have it, ceramic chemistry to the rescue again! However the story is not quite over, they still need to adjust things to better match the thermal expansion of the new glaze with the old.
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