Monthly Tech-Tip | No tracking! No ads! |
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.
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.
Buy me a coffee and we can talk