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Frit manufacturers face a daunting challenge: Converting carbonates, hydrates, borates, and nitrate materials, which off-gas during firing, into a homogeneous melt, quenching to a glass of zero LOI and then grinding that back into a consistent powder. You face a challenge also.
Shown here is a GLFL test that reveals a combination of improper mixing and inadequate smelting. This melt fluidity test compares the previous good shipment, L, with the latest faulty one, S. S appears to have not been fully reacted during manufacture (as exhibited by the foam blanket). But there’s more: a glaze containing 50% of this had inadequate melting. That could be chemical drift or inadequate batch mixing. The latter seems most likely because S was from a full two-pallet batch (the likely capacity of their smelter), from which a dozen bags were tested. These showed the most typical results at the top of the first pallet and the worst at the bottom of the second.
A bad batch of frit can ruin a lot of ware. Buy the largest amount you can afford and do the GBMF test is almost as good as these. Character changes in the melted balls put up the red flags you need to see.

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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 was 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! This being said, this test assumes all the bags in the frit batch are be the same, a melt flow test could be done on every bag to be sure.
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