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A Textbook Cone 6 Matte Glaze With Problems

Section: Glazes, Subsection: Formulation

Description

Glazes must be completely melted to be functional, hard and strong. Many are not. This compares two glazes to make the difference clear.

Article Text

The picture below is a flow tester that was fired at cone 6. The tester itself is Plainsman M370. The glaze on the right is G1214W, a typical cone 6 transparent used by potters. It is fluxed by a boron sourcing frit. The glaze on the left is typical of what many use to achieve a matte. The recipe is:

Nepheline Syenite40.00
Dolomite15.00
Whiting10.00
Ball Clay20.00
China Clay10.00
Silica5.00

But this glaze is not flowing at all whereas the boron-fluxed glaze has run right to the bottom (and it is not even considered a highly fluid glaze). This simple comparison teaches us many things about glazes and even ourselves. Here are some of them.

Understanding your glazes is so much better, especially when it comes to dealing with their problems (and this one would have plenty!).

The next time I buy a bunch of materials to test an undocumented online recipe slap me!

The next time I buy a bunch of materials to test an undocumented online recipe slap me!

Look at recipes before wasting time and money on them. Are they serious? This is a cone 6 melt flow comparison between a matte recipe, found online at a respected website, and a well-fluxed glossy glaze we use often. Yes, it is matte. But why? Because it is not melted! Matte glazes used on functional surfaces need to melt well, they should flow like a glossy glaze. How does that happen? This recipe has 40% nepheline syenite. Plus lots of dolomite and calcium carbonate. These are powerful fluxes, but at cone 10, not cone 6! To melt a cone 6 glaze boron, zinc or lithia are needed. Boron is by far the most common and best general purpose melter for potters (it comes in frits and gerstley borate, colemanite or ulexite; industry uses more boron, zinc and lithia frits). The lesson: Look at recipes before trying them.

A functional matte cone 6 glaze should melt as well as a glossy

A functional matte cone 6 glaze should melt as well as a glossy

True functional mattes have fluid melts, like glossy glazes. They need this in order to develop a hard, non-scratching durable glass. The mechanism of the matte on the right is high Al2O3 (G1214Z), it is actually melting more than the glossy glaze on the left (G1214W).

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By Tony Hansen




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