The calcining process is used to remove some or all unwanted volatiles from a material (e.g. H2O, CO2, SO2) and/or to convert a material into a more stable, durable or harder state. Varying temperatures are employed to calcine various materials, depending on the decomposition temperature of the volatiles being burned out and the degree of sintering desired in the product. For example, lump kaolins are calcined at high temperatures to form molochite (which is then ground and sized). The powder is calcined at lower temperatures to produce calcined kaolin. Depending on the temperature fired, the material may need to be ground after calcining. However if a material is already powdered and calcined at a low enough temperature that there is no particle bonding, the powder product is ready to use.
What happens when a limestone clay mix is fired to cone 6?
The top bar is a mix of calcium carbonate and a middle temperature stoneware clay (equal parts). On removal from the kiln it appears and behaves like a normal stoneware clay body, hard and strong. However, pour water on it and something incredible happens: in a couple of minutes it disintegrates. With lots of heat.
Badly crawled glaze fired at cone 5 reduction
It was spray applied on the dried bowl (no bisque fire) an was too thick (not to mention under fired). But the main problem was a glaze recipe having too high a clay content. If a glaze has more than about 25% clay, consider a mix of the raw clay and calcined. For example, you can buy calcined kaolin to mix with raw kaolin. Or you can calcine the clay in bowls in your kiln by firing it to about 1200F.
Something is definitely wrong. What is it?
An example of how a glaze that contains too much plastic has been applied too thick. It shrinks and cracks during drying and is guaranteed to crawl. This is raw Alberta Slip. To solve this problem you need to tune a mix of raw and calcine material. Enough raw is needed to suspend the slurry and dry it to a hard surface, but enough calcine is needed to keep the shrinkage low enough that this cracking does not happen. The Alberta Slip website has a page about how to do the calcining.
Calcining Alberta Slip
Calcined Alberta Slip (right) and raw powder (left). These are just 5 inch cast bowls, I fire them to cone 020 and hold it for 30 minutes. Why calcine? Because for glazes having 50% or more Alberta Slip, cracking on drying can occur, especially if it is applied thick (Alberta Slip is a clay, it shrinks). I mix 50:50 raw:calcine for use in recipes. However, Alberta Slip has an LOI of 9%, so I need to use 9% less of the calcine powder (just multiply the amount by 0.91). Suppose, I needed 1000 grams: I would use 500 raw and 500*.91=455.
Alberta Slip as-a-glaze at cone 10R
This is 100% Alberta Slip (outside) on a white stoneware clay fired to cone 10R. The glaze is made using a blend of 60% calcine and 40% raw (as instructed at the albertaslip.com support website). Alberta Slip was originally formulated during the 1980s (using Insight software) as a chemical duplicate of Albany Slip. The inside: A Ravenscrag Slip based silky matte.
Calcining Ravenscrag Slip
This is Ravenscrag Slip, I am going to calcine about 10 pounds of it in this bisque ware vessel to destroy the plasticity. I will fire to 1000F and hold it for 2 hours to make sure the heat penetrates. Why calcine? Because I have found that in some glazes having 70% or more Ravenscrag Slip, cracking on drying can occur if it is applied too thick. I love the working properties of these glazes and want to optimize them to avoid any problems. I am going to mix 75:25 raw:calcine on the next batch of glaze. However, Ravenscrag has an LOI of 9%, so I need to use 9% less of the calcine powder (just multiply the amount by 0.91). Suppose, I needed 1000 grams: I would use 750 raw and 250*.91=227.5.
Fixing a crawling problem with Ravenscrag Tenmoku
Crawling of a cone 10R Ravenscrag iron crystal glaze. The added iron oxide flocculates the slurry raising the water content, increasing the drying shrinkage. To solve this problem you can calcine part of the Ravenscrag Slip, that reduces the shrinkage. Ravenscrag.com has information on how to do this.
Will this crawl when fired? For sure!
This dry glaze is shrinking too much, it is going to crawl during firing. This common issue happens because there is too much plastic clay in the glaze recipe (common with slip glazes). Clay is needed to suspend the other particles (they would quickly settle to the bottom of the bucket without it), but too much causes excessive shrinkage. Fixing this problem is not nearly as difficult as most people think. You can reduce shrinkage by calcining part of the clay or swapping a clay component for another of similar chemistry but lower shrinkage. The best way: Use glaze chemistry to source some Al2O3 (contributed by the clay) from feldspar instead. Of course this involves juggling amounts of other materials in the recipe to maintain the overall chemistry.
Plainsman H450 (left) vs. H550 celadon glazed mugs
The inside glaze is pure Ravenscrag Slip and the outside glaze is a 50:50 mix of Ravenscrag and Alberta Slips. Each of the glazes employs an appropriate mix of calcined and raw clay to achieve a balance of good slurry properties, hardening and minimal drying shrinkage. Ravenscrag needs less calcined since it is less plastic than Alberta Slip.
Two glazes. One crawls, the other does not. Why?
The glaze on the right is crawling at the inside corner. Multiple factors contribute. The angle between the wall and base is sharper. A thicker layer of glaze has collected there (the thicker it is the more power it has to impose a crack as it shrinks during drying). It also shrinks more during drying because it has a higher water content. But the leading cause: Its high raw clay content increases drying shrinkage. Calcining part of the raw clay is an effective way to deal with this. Or doing a little chemistry to source some Al2O3 from other materials than clay.
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