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After a customer experienced blistering with a transparent glaze on a terra cotta body, suspicion was raised that the batch of frit of bad. Ferro even admitted there was an issue. But we decided to test, sampling about 20 of the 50 lb bags and combining that to make these tests. Upper left is a GLFL test, if compares the melt flow of an old batch of frit 3195 with the questioned one, fired at 1750F. Although the questioned batch does run a little more, a more important difference is in the bubble development (look closer), perhaps there is some fluorine contamination. A GBMF test compares the two samples on the bottom left (10 gram balls melt downward onto a sample tile). Again, the questioned batch is melting and bubbling a little more. We made two glazes: Cone 6 GA6-B (top right) contains 20% of the frit (the left one uses an old frit batch). The cone 05 G1916QL1 glaze (bottom right, it contains 60% of the frit) uses the questioned frit and works well on both the white and red bodies. So, although it might not be working for some, we determined that the frit is OK! Although it is possible that only certain bags of the batch were bad, that seems unlikely given the way frits are made, all bags in a batch should be the same.
Materials |
Ferro Frit 3195
A commonly used boron frit, it is a balanced glaze all along at cone 06-02 (with the addition of 10-15% kaolin). Not fully glossy. |
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