Cooling rate drastically affects the appearance of this glaze

This is the G2934Y satin matte glaze recipe with Mason 6600 black stain (6%). The piece on the left was fired using the C6DHSC firing schedule (drop-and-hold at 2100F then 150F/hr to 1400F). The one on the right was fired using the PLC6DS schedule (drop-and-hold at 2100F then free-fall from there). The slow cooling rate gives the glaze on the left time to crystallize, creating a stony matte. My kilns are generally lightly-loaded, so free-fall firings drop pretty rapidly, producing the effect on the right. This phenomena is a characteristic of high MgO glazes (ones having significant dolomite, talc, Ferro frit 3249). To get a surface between these extremes we make this glaze using a 20:80 blend of G2934 base (which fires even more matte on slow-cool) and G2926B glossy. In our typical fast-cool this mix produces the degree-of-matteness I like.

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The same glaze fires very differently depending on kiln cooling rate

Both mugs use the same cone 6 oxidation high-iron (9%), high-boron, fluid melt glaze. Iron silicate crystals have completely invaded the surface of the one on the left, turning the gloss surface into a yellowy matte. Why? Multiple factors. This glaze does not contain enough iron to guarantee crystallization on cooling. When cooled quickly it fires the ultragloss near-black on the right. As cooling is slowed at some point the iron will begin to precipitate as small scattered golden crystals (sometimes called Teadust or Sparkles). As cooling slows further the number and size of these increases. Their maximum saturation is achieved on the discovery, usually by accident, of the likely narrow temperature range they form at (normally hundreds of degrees below the firing cone). Potters seek this type of glaze but industry avoids it because of difficulties with consistency.

A gunmetal glaze I have wanted for decades!

I finally have a perfect functional, durable gunmetal black! It has an incredible silky surface. It does not cutlery mark. It does not craze on anything. It is easy to clean. This is G2934Y with 6% Mason 6600 black stain fired using the PLC6DS schedule. I had to tune it a bit, adding about 15% glossy G2926B, because it was a little too matte on initial firings. But now it is perfect. These are heavy mugs made using the M340 casting recipe (and the casting-jiggering process). The speckled mug was made by casting a thin layer of the speckled version of the slip first, then filling the mold with the regular slip. I used a 40-minute cast to get walls nice and thick!

Links

Glossary	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.
Glossary	Firing Schedule Designing a good kiln firing schedule for your ware is a very important, and often overlooked factor for obtained successful firings.
Articles	What Determines a Glaze's Firing Temperature? The oxides contributed by glaze materials determine its chemistry. The chemistry is the main factor determining melting behaviour. But the particle sizes, shapes and mineralogies also come in to play.
Recipes	G2934Y - Cone 6 Magnesia Matte Low LOI Version The same chemistry as the widely used G2934 but the MgO is sourced from a frit and talc instead of dolomite. It has a finer surface, less cutlery marking and staining.

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