|Monthly Tech-Tip |
Potters and some manufacturers fire ceramic ware twice, once to prepare it for glazing (call bisquit firing) and the second time to melt the glaze onto it.
Generally, bisque firing refers to the practice of prefiring ware without glaze to make it impervious to water, resistant to damage during handling and absorbent for glazing. The porosity of the bisque (generally more than 15%) makes it an ideal medium to absorb water from the glaze suspension and hold it in place, it is typical for the glaze to dry sufficiently for handling in just a few seconds. The temperature of the bisque firing can be adjusted to tune the degree of porosity, and therefore the glazing characteristics. Bisquit should be fired as high as possible. Some companies fire high enough to get some glass formation so the ware has extra strength to resist cracking during fast firing. Bisque also needs to be low enough to provide sufficient absorbency to make glaze application easy. Less porous ware can be successfully glazed by reducing water content of the glaze suspension, heating the ware before immersion in the glaze, additions of binders or flocculants, applying a thinner layer, etc.
Firing bisquit as high is possible is desirable for another reason: Higher temperatures and longer soaks burn off more products of decomposition, this produces more defect-free glaze surfaces in the final firing (since not as many gases need to bubble up through the melting glaze). In many situations, bisque temperature can be increased 1 or 2 cones with little impact on the product process. In most situations the bisque can be soaked at the end to provide more time for gas expulsion. Heavier ware needs to be soaked longer, sometimes alot longer (hours) to fully drive out gases of decomposing organics and carbonates.
'Low' bisque firing is typical for pottery and ceramics while a 'vitrifying bisque' is done for bone china and some types of stoneware.'High' bisque firing is done to mature the body and burn off all the organics. Subsequent firing is usually done to apply a low-fire glaze. Glazing zero-porosity (or very slow porosity vitrified ware) is a challenge, such glazes must have special additives to make them gel and stick to the ware (i.e. calcium chloride, gum) and the production process is able to handle the extended drying time.
At times adjustments to bisque firing are recommended to solve problems like crazing. However crazing is caused by a thermal expansion mismatch between body and glaze, bisque firing is not related to this, it is at best a band-aid-fix for marginal crazing situations, the crazing will appear with time anyway.
Some people wash bisque ware to prevent crawling. This actually does work in some circumstances because the less absorbent bisque makes it less likely to get the glaze on too thick (which causes crawling). But it is better to bisque higher (for less absorbency) or examine the glaze recipe and fix that. Glazes that have too little clay or have not been gelled can build a layer much more quickly. Glazes that have too much clay tend to crack during drying, then crawl during firing (calcine some of the clay to fix this). When the glaze has the right rheology and the bisque is the right density glazing will much better.
Some people go to great efforts to keep dust off bisque ware before glazing. However remember that glaze is a mixture of mineral dust and water so you are removing a small amount of dust and replacing it with alot more.
These mugs are quite thin walled. A glaze has just been applied to the inside. Notice how it has water logged the bisque (you can see the contrast at the base, where the clay is a little thicker and has not changed color yet). Although there may be enough absorbency that a glaze could be applied now, it would still not be a good idea because it would completely waterlog the piece and result in a very long drying time. This is bad, not only because of process logistics, but also because slow drying glazes almost always crack and lift from the bisque (causing crawling).
The buff stoneware mug on the right was bisque fired at cone 02, the one on the left at cone 06. The cone 02 mug was immersed in the clear glaze for 1 second and allowed to dry. The other was glazed on the inside first, allowed to dry, then glazed on the outside with a 1 second dip. Of course, the cone 02 one took longer to dry. In spite of this, the glaze is thicker and more even on the one bisque fired to cone 02. How is the possible? The secret is the thixotropy of the glaze. When that is right, a one second dip will give the same thickness and evenness whether dry or bisque, 06 or 02. Why bisque fire to cone 02? To get a glazed surface free of pinholes on some stoneware clays.
These particles contaminating particles are exposed on the rim of a bisque fired mug. The liqnite ones have burned away but the iron particle is still there (and will produce a speck in the glaze). Remnants of the lignite remain inside the matrix and can pinhole glazes. Since ball clays are air floated (a stream of air takes away the lighter particles and the heavier ones recycle for regrinding) it seems that contamination like this would be impossible. But the equipment requires vigilance for correct operation, especially when there is pressure to maximize production. Ores in Tennessee are higher in coal than those in Kentucky. North American clay body manufacturers who confront ball clay suppliers with this contamination find that ceramic applications have become a very small part of the total ball clay market, complaints are not taken with the same seriousness as in the past.
Example of the oversize particles from a 100 gram wet sieve analysis test of a powdered sample of a porcelain body made from North American refined materials. Although these materials are sold as 200 mesh, that designation does not mean that there are no particles coarser than 200 mesh. Here there are significant numbers of particles on the 100 and even 70 mesh screens. These contain some darker particles that could produce fired specks (if they are iron and not lignite); that goodness in this case they do not. Oversize particle is a fact of life in bodies made from refined materials and used by potters and hobbyists. Industrial manufacturers (e.g. tile, tableware, sanitaryware) commonly process the materials further, slurrying them and screening or ball milling; this is done to guarantee defect-free glazed surfaces.
Example of the lignite particles in a fireclay (Pine Lake) that have been exposed on the rim of a vessel after sponging. This is a coarse clay, but if it were incorporated into a recipe of a stoneware, glaze pinholing would be likey.
These are 10 gram balls of four different common cone 6 clear glazes fired to 1800F (bisque temperature). How dense are they? I measured the porosity (by weighing, soaking, weighing again): G2934 cone 6 matte - 21%. G2926B cone 6 glossy - 0%. G2916F cone 6 glossy - 8%. G1215U cone 6 low expansion glossy - 2%. The implications: G2926B is already sealing the surface at 1800F. If the gases of decomposing organics in the body have not been fully expelled, how are they going to get through it? Pressure will build and as soon as the glaze is fluid enough, they will enter it en masse. Or, they will concentrate at discontinuities and defects in the surface and create pinholes and blisters. Clearly, ware needs to be bisque fired higher than 1800F.
Three mugs. Dry. Bisque fired. Glaze fired. Notice the shrinkage at each stage (these were the same size in the dry state).
These are high temperature stoneware mugs that have been bisque fired, glazed and then bisque fired again to cone 02. This is done so they can be handled without damage (they are being shipped to another location for firing to cone 10R). The glaze is quite durable at this point and would be difficult to damage.
These were fired to cone 06, about 1800F. Of course, there is normally some shrinkage so the bisque piece would be a little smaller. Even though the matrix is very porous and is under developed, the iron in the body is already beginning to impose its color.
These bowls are made from a talc:ball clay mix, they are used for calcining Alberta and Ravenscrag Slips (each holds about one pound of powder). The one on the right was bisque fired to cone 04 (about 1950F). The one on the left was fired to only 1000F (540C, barely red heat), yet it is sintered and is impervious to water (strong enough to use for our calcining operations). That means that there is potential, in many production situations, to bisque a lot lower (and save energy). Primitive cultures made all their ware a very low temperatures. Tin foil melts at 660C (1220F) yet can be used on campfires for cooking (so the temperatures of primitive wares would have been low indeed).
In ceramics, this term refers to the flow and gel properties of a glaze or body suspension (made from water and mineral powders, with possible additives, deflocculants, modifiers).
Manually program your kiln or suffer glaze defects!
To do a drop-and-hold firing you must manually program your kiln controller. It is the secret to surfaces without pinholes and blisters.