All Temperature Numbers Temperature Listing

760C (1400F)

Common frits begin melting

Common boron frits begin melting before talc, dolomite and calcium carbonate have completed gassing. A glaze containing 20% Ferro Frit 3134, for example, could reach full density (zero porosity) by less than 1800F (typical bisque temperature). Such would acts as a barrier to the escape of gases.

Related Information

Melt fluidity comparison of frits - 1400F

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Fired at 350F/hr to 1400F and held for 15 minutes. Frit 3134 is still expanding. 3602 is also starting to flow. A number of them are shrinking and densifying like a porcelain would.

A frit softens over a wide temperature range

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Raw materials often have a specific melting temperature (or they melt quickly over a narrow temperature range). We can use the GLFL test to demonstrate the development of melt fluidity between a frit and a raw material. On the left we see five flows of boron Ferro Frit 3195, across 200 degrees F. Its melting pattern is slow and continuous: It starts flowing at 1550F (although it began to turn to a glass at 1500F) and is falling off the bottom of the runway by 1750F. The Gerstley Borate (GB), on the other hand, goes from no melting at 1600F to flowing off the bottom by 1625F! But GB has a complex melting pattern, there is more to its story. Notice the flow at 1625F is not transparent, that is because the Ulexite mineral within GB has melted but its Colemanite has not. Later, at 1700F, the Colemanite melts and the glass becomes transparent. Technicians call this melting behaviour "phase transition", that does not happen with the frit.

Cone 6 glazes can seal the surface surprisingly early - melt flow balls reveal it

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These are 10 gram balls of four different common cone 6 clear glazes fired to 1800F (bisque temperature). How dense are they? I measured the porosity (by weighing, soaking, weighing again): G2934 cone 6 matte - 21%. G2926B cone 6 glossy - 0%. G2916F cone 6 glossy - 8%. G1215U cone 6 low expansion glossy - 2%. The implications: G2926B is already sealing the surface at 1800F. If the gases of decomposing organics in the body have not been fully expelled, how are they going to get through it? Pressure will build and as soon as the glaze is fluid enough, they will enter it en masse. Or, they will concentrate at discontinuities and defects in the surface and create pinholes and blisters. Clearly, ware needs to be bisque fired higher than 1800F.

LOI horse race with a surprising winner and loser

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This chart compares the decompositional off-gassing (% weight Loss on Ignition) behavior of six glaze materials (there are many more). Each was heated through the range 500-1700F at 350F/hr. It is amazing how much weight some can lose on firing. For example, 100 grams of calcium carbonate generates 44 grams of CO2! Magnesium carbonate even more. But the main message of this chart is that some late gassers overlap early melters. That can be a problem (causing glaze bubbles, blisters, pinholes). The talc in this test is an example: It did not finish gassing until 1650F. Yet many fritted and Gerstley Borate glazes have already begun melting by then.

There are two takeaways here. The biggest weight loser is not the biggest troublemaker. What matters is how gas release overlaps glaze sealing. Calcium carbonate illustrates this; its high carbonate content can still produce a clear glaze (when gases can clear ahead of any melting). And the talc, although having a small LOI, is a potential troublemaker when the glaze melt cannot effectively clear the bubbles.

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