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Firing Clay according to Color. White Porcelain: being white in color automatically concludes that much purer grades of materials where used in making this body. Grolleg, NZ Kaolin, and Super Standard kaolin have very low rates of impurities and virtually no carbon deposits. Sodium feldspars and standard silica also have little impurities, and contain no carbons. Following the usual bisq and glaze firing schedules should work for this body. Test for absorption and raise peak glaze fire by Â½ cone increments if absorption is more than 1%. Note: due to the composition of these bodies, the COE values are usually significant higher than standard porcelains. White Stoneware: like white porcelain, this type of body usually consists of cleaner clay varieties. However, all fire and ball clay/s have some level of impurities and some level of carbons in them. Part of the attraction of stoneware is the color/s produced by the presence of iron, magnesium, and titanium. The higher the levels of these three elements go: the deeper the toasty color of stoneware becomes. Following standard bisq and glaze firings should work for this body. Note: stoneware has a blend of coarse particle fire clay that must have a relational portion of sub-micron ball clay to minimize porosity. After absorption testing, raise peak temperature by Â½ cone until absorption is below 2%. Grey Porcelain: this body color indicates a utility blend. It is designed to be inexpensive, plastic, and used in the widest range of applications. Grey color indicates carbon, which are usually dealt with in the bisq fire. If pin holes show in the glaze firing; then adjust your bisq fire temperature up by 50-60F with a short 10-20 minute hold to help burn off carbons. Grey Stoneware: can be a particular problem because of bloating. As carbons burn off, they create an impermeable barrier that traps off gassing feldspars that can be small to large in size. As bloating area increases: this also indicates higher levels of carbons. If bloating continues after several bisq or glaze temperature adjustments: it may be wiser to discontinue use. Bodies using higher carbon ball clays are tricky; if accompanying feldspar levels are too high: the problem is nearly impossible to remedy with firing schedules. Brown/Dark Stoneware: brown/dark clay is most always indicative of high iron levels. With iron, comes higher magnesium and titanium levels: these two elements almost always proportionately increase as iron increases. The combination of them creates that warm toasty brown that potters know and love. The problem then becomes, large particle feldspar minerals are always very common in these types of ball clay. Unlike finely ground feldspar in your glaze; these are coarse and hidden in the interior of the clay body where heat takes the longest to reach. Consequently, brown/dark stoneware is the most susceptible to pin holes and blistering because large particle feldspar in the body takes much longer to off-gas. This body is the one that requires a change in glaze firing schedules most often to resolve off-gassing issues. Rarely seen in cone 10 schedules, and often seen in cone 6 schedules. The Remedy given is usually a higher bisq fire which will not work because you are not dealing with carbons; but rather feldspar particles. At bisq temps, there is simply not enough heat to fully resolve feldspar off-gassing issues. At 2190F, normally all feldspar are at the end of their useful cycle; unless they are large and inside a clay body. Some studies suggest it takes heat twice as long to penetrate a clay body as it does the glaze coating the surface. This body then requires a higher peak temp at cone 6: 2230F. It also requires a short to extended hold pending the kiln size. Of course if the goal is not vitrification; then trying to achieve optimum firing results really does not matter. However, some blended fixes may still be required to resolve bloating and pin hole issues. Primarily, higher bisq temps are utilized to burn off excess carbons, and higher glaze temps are used to remedy pin hole and crater issues. Bloating can be fixed or minimized mostly; but often cannot be resolved because of poor formulation of the body. Nerd Large particle feldpsar (20 mesh) inside a stoneware body. 200x
I use Southern Ice porcelain, bisque fired then sanded then fired to cone 9 and sanded again, I don't use glazes. I have started making complex neriage coloured work, using the same clay body. I sometimes develop fine cracks that can successfully be patched and refired to cone 9 but the result is an all over tiny bloating of the surface. I have read a lot about bloating and the possibility it is due to over firing but don't really know what that means. I have twice fired thinner, slip cast work (from the same clay) without developing this problem. Could it mean I need to slow down the firing at one or more stages?
I've had trouble with bloating using Standard 266. I know I'm not the first person to have this issue, but my options for preventing it are limited. I fire my pots in a community studio, so can't change the firing schedule (we bisque to cone 06 and glaze fire to cone 6, both in an electric kiln). Is it reasonable to assume that putting a piece through the bisque again will burn out the remaining organic matter, and therefore prevent bloating? I don't know the exact bisque firing schedule, but it takes around 15-16 hours. Are there other reasons this clay bloats? Other potters in the studio I use have used Standard 266 without bloating, but their pots have thinner walls. The other difference is that I used metallic black glaze on the pots which bloated, while the other potters didn't, if I remember correctly. I'm not sure if this makes a difference in trapping pockets of gass though, especially since I left the outside unglazed, but had bloating on both sides. I should add that I'm not going to be using this clay anymore (mainly because I'll be leaving the US soon), so I'm just looking for a solution that will work for one specific pot, which has already been bisque fired. And yes, I now know the importance of testing a new clay body before using it for a pot you care about.