Matte Glaze Base for Cone 6
Recipe
Notes
A cone 6 boron-fluxed MgO matte developed at Plainsman Clays by Tony Hansen (a link below will take you to its page there). This page contains technical and mixing information about the recipe, their page, under code MG6CDM, contains mixing and usage information. This recipe has the best suspension and application properties when it is thixotropic (that involves mixing it thinner than normal and gelling it using Epsom salts). Target a specific gravity of 1.43-1.44 (about 90 water to 100 powder, by weight; that means that in 1900g of slurry there is 900 water and 1000 powder). Then about add 1g of Epsom salts per 1000g powder to increase thixotropy. This should make it creamy and it should gel after a few seconds on standing still (add more Epsom salts if needed, but be careful, it is easy to overdo it). Screen through 80 mesh (there are tiny agglomerates that will not break down without screening). Important note: The degree of matteness is dependent on the cooling rate of the firing. In our circumstances, fast cooling (e.g. free-fall in a lightly-loaded or smaller kiln) produces the desired silky matte surface and slow cooling (e.g. a heavily loaded kiln) produces a matter and drier surface. Do test firings to determine if your cooling rate will accommodate this or whether you need to blend in some glossy G2926B to shine it up a little (for example, try 75% matte and 25% glossy, the mixing can be done by simply pouring together, volumetrically, the two slurries). One more thing: Certain colors will matte this more than others, so specific adjustments might be needed. Again: Be sure to control production firings so their rate-of-cooling matches that of the test firings. The degree of matteness we prefer it does not cutlery mark and has good (but not too much) melt flow. This glaze has plenty of SiO2 and Al2O3 and good melt fluidity, these contribute to durability and resistance to leaching. The calcined kaolin is needed (if you use all raw kaolin the glaze will shrink and crack during drying and crawl during firing). If you do not have calcined kaolin make your own by bisque-firing a container of kaolin powder. If you do not have EP Kaolin, just substitute another. If it settles convert some of the calcined kaolin to raw. If it does not dry hard enough or does not suspend well, use more raw kaolin and less calcined.* Although this is a matte glaze it flows well (it is well melted). If fired ware has pinholes the solution lies elsewhere. Bisque ware as high as possible. If you gel the slurry a little, or preheat the ware, bisque can be even hotter. Try applying a thinner glaze layer and use whatever technique necessary to get an even and quality laydown. If still needed, consider using the double-soak firing schedule. With certain stains and opacifiers this base can present issues with glaze crawling. It is not completely clear why this is, test in your circumstances before making large batches or glazing high volumes are ware. Plainsmanclays.com makes this recipe as a premixed powder. *If you adjust raw:calcined kaolin proportions more than 5%: Raw kaolin has 12% weight loss on firing, more is needed to supply the same amount of SiO2 and Al2O3 to the fired glaze. For example, if you drop the calcined kaolin by 5 you need to increase the raw kaolin by 5.6 to maintain the same overall chemistry (5 + (5 x 12% / 100 = 5.6). If your kaolin is not too plastic you might be able to use all raw kaolin (18.3 + 13.9 + (13.9 x 12% / 100) = 34.
Description
A base MgO matte glaze recipe fires to a hard utilitarian surface and has very good working properties. Blend in the glossy if it is too matte.