Glazes must be completely melted to be functional, hard and strong. Many are not. This compares two glazes to make the difference clear.
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:
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.
Cone 6 glazes require boron (from frits or gerstley borate) or zinc to melt (the one on the left has neither). Materials like feldspar, dolomite, whiting and talc just do not melt well at all at cone 6. If a glaze is not properly melted then it is not hard and strong and resistant to leaching and cutlery marking.
Textbook glaze recipes often are junk. Unquestioned use of them demonstrates our willingness to accept something ridiculous just because it is in a book or on a web page. We need use common sense when looking at glaze recipes for cone 6 to determine if they are for real or impostors (the glaze on the left is actually really not a glaze, it is a porcelain). If there is no frit, no zinc, no gerstley borate, then a red this-likely-does-not-melt light is flashing.
Glazes with high feldspar or nepheline syenite (40% is really high) are likely to craze. If this one does not craze it is simply because it has not melt enough for the sodium to impose its high thermal expansion on the glass.
A cone 6 glaze with 30% clay (in this case ball clay and kaolin) and no boron is certainly not going to melt well because clay is refractory.
True matte glazes have high alumina and low silica (which this has) but they also need to melt well to form a glass (which this does not). So this is just a matte because it is not melted.
If a white-firing glaze has no zircon or tin oxide, then how can it be white? Something is wrong. In this case it is white simply because it is not melted.
Since this does not melt, evaluating it on the basis of its chemistry would be invalid. Glaze chemistry theory depends on the glaze being melted to a homogeneous glass. Thus, in addition to knowing all about the chemistry, you need to have common sense on the recipe level also.
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!
Look at recipes before wasting time and money on them. Are they serious? This is a cone 6 GLFL test to compare melt-flow 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
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).
What Determines a Glaze's Firing Temperature? By understanding how glazes melt and materials and chemistry interplay to determine behavior and temperature of melting and testing degree of melt you control the melting temperature of your glazes.
Why Textbook Glazes Are So Difficult The trade is glaze recipes has spawned generations of potters going up blind alleys trying recipes that don't work and living with ones that are much more trouble than they are worth. It is time to leave this behind and take control.
Trafficking in Glaze Recipes The trade is glaze recipes has spawned generations of potters going up blind alleys trying recipes that don't work and living with ones that are much more trouble than they are worth. It is time to leave this behind and take control.
Matte Glaze Random material mixes that melt well overwhelmingly want to be glossy, creating a matte glaze that is also functional is not an easy task.
Glaze Recipes Stop! Think! Do not get addicted to the trafficking in online glaze recipes. Learn how they work. Understand them. Then make your own or adjust/adapt what you find online.