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.

Related Pictures

Alberta Slip GA6-A glaze slow-cooled goes matte

This is GA6-A Alberta Slip base glaze (80 Alberta Slip:20 Frit 3134) fired using the C6DHSC firing schedule (on Plainsman M390 iron red clay). If this is cooled rapidly or 1% tin oxide inhibitor is added, it fires to a glossy clear amber glass with no crystals. This base is needed to make GA6-C rutile blue, otherwise we recommend the GA6-B base (which uses frit 3195 instead), it has a lower thermal expansion and guarantees gloss regardless of cooling speed. If, on the other hand, you want to enhance this effect, just add a little more frit (to gloss it more) and more iron oxide (that should enable matting even at faster cooling speeds.

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.

The matteness of this glaze depends on the cooling rate

This is the G2934Y matte cone 6 recipe with a red stain (Mason 6021). The one on the left was fired using the C6DHSC slow-cool schedule. The one on the right was fired using the drop-and-soak PLC6DS schedule. The only difference in the two schedules is what happens after 2100F on the way down (the slow-cool drops at 150F/hr and the other free-falls). For this glaze, the fast cool is much better, producing a silky pleasant surface rather than a dry matte.

Ravenscrag floating blue color affected by cooling rate

Ravenscrag Slip really shines in its ability to produce a good floating blue glaze at cone 6, this is the GR6-M recipe. The speed of cooling in the kiln affects the fired appearance. The mug on the left was cooled faster, using our drop-and-soak PLC6DS firing schedule. The other one was slow-cooled using the C6DHSC schedule. The latter schedule is preferable for these because the G3914A black inherits a much smoother surface. Of course, the slow-cooled lighter blue could be darkened by adding a little cobalt.

The appearance of this commercial glaze varies with cooling rate

This is Amaco PC-20 Rutile Blue. These mugs are the same clay, Plainsman M390. Both were brush-on glazed the same way. The one on the left was fired to cone 6 using the C6DHSC (drop-and-hold, slow-cool) schedule. The one on the right with the PLC6DS (drop-and-hold and free-fall) schedule. The label on the jar just says to fire to cone 6. But this is a rutile blue, and behaves like one. Since most people would fire fully loaded kilns slow cooling is a natural consequence of this and the color should be blue. That being said, variations in density of pack will still produce variations in the way this turns out. If you fire in a small kiln, the way we do, then it is necessary to program the cool cycle of the firing to be sure it does not drop too fast.

Same glaze fires hyper-matte and very glossy at cone 10. How?

Actually, this is the same mug. It was refired the first time at cone 10 reduction to see if it would gloss up a little more. It did not. Then it was refired again in cone 10 oxidation - yielding the high gloss on the right. The glaze is G2571E, a satin matte base employing Mason 6600 black stain. A combination of the stain, atmosphere and cooling rate increased the matteness of the base recipe in the gas kiln and increased its gloss in the electric kiln.

Links

Glossary	Crystallization Ceramic glazes form crystals on cooling if the chemistry is right and the rate of cool is slow enough to permit molecular movement to the preferred orientation.
Glossary	Glaze Chemistry Glaze chemistry is the study of how the oxide chemistry of glazes relate to the way they fire. It accounts for color, surface, hardness, texture, melting temperature, thermal expansion, etc.
Glossary	Melt Fluidity Ceramic glazes melt and flow according to their chemistry, particle size and mineralogy. Observing and measuring the nature and amount of flow is important in understanding them.
Oxides	B2O3 - Boric Oxide
Oxides	Fe2O3 - Iron Oxide, Ferric Oxide

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