Calcining is simply firing a ceramic material to create a powder of new physical properties. Often it is done to kill the plasticity or burn away the hydrates, carbonates, sulfates of a clay or refractory material.
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.
To calcine your own ceramic powder simply fire it high enough to destroy the plasticity but not so high as to sinter it (where particles bond to produce agglomerates, or grit, this will affect glaze surface quality). This lower temperature process is commonly called "roasting" (as opposed to calcination, it implies higher temperatures). Typically you only need to fire the uncompacted powder in bisque vessels to 1000F (Cone 022 or red heat, of course testing is needed to be sure it is high enough). For large or heavy-walled vessels fire slower (e.g. 200F per hour) but for smaller ones (especially thin-walled ones holding less e.g. 500-1000g) you can fire much faster (e.g. 500F/hour). Hold for the time necessary for the heat to penetrate (start with 30 minutes). If any black powder remains in the center extend the soak time next firing.
Fireclays and kaolins are the most common calcined materials in ceramics. The calcining process produces a material that still has the desired firing properties but is more suitable to the process in which it will be used. A highly plastic fireclay, for example, could have limited uses because of its high shrinkage, but the calcined form could be mixed with raw to create a more usable material. Likewise, high kaolin glazes are problematic because of the shrinkage and cracking that occurs during drying. Substituting a raw:calcine kaolin mix solves the problem.
The calcining process produces a material having no LOI, if it is being substituted into a glaze this needs to be taken into account. For example, if a kaolin loses 12% weight on firing, then 12% less of the calcine would be used in a glaze recipe.
Calcining can actually produce a less stable form of certain materials, they gradually want to revert to the former carbonated or hydrated state. For a good example of this, mix calcium carbonate with kaolin and make a bar and fire it. Out of the kiln it will appear to be a hard ceramic. But after several days it will absorb CO2 from the air and completely fracture into a powder. Pour water on it and it will immediately fracture and generate an amazing amount of heat.
Calcined clays are not normally used as body ingredients. Potter's need all the plasticity they can get, that means the most possible clay in the recipe and clays of the highest possible plasticity. In industry, where less plasticity can be tolerated, they can simply use kaolins and non-plastic fillers.
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.
This high-Alberta-Slip glaze is shrinking too much on drying. Thus 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, but too much causes the excessive shrinkage. The easiest way to fix this is to use a mix of raw and calcined Alberta Slip (explained at albertaslip.com). The calcined Alberta Slip has no plasticity and thus much less shrinkage (but it still has the same chemistry). Many matte glazes have high kaolin contents and recipes will often contain both raw and calcined kaolin for the same reason.
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.
The glaze on the right is crawling at the inside corner. Why? 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 destroys its affinity for water (which is what makes it plastic), this is an effective way to deal with this. Or doing a little chemistry to source some of the Al2O3 from materials other than clay (e.g. a frit having a higher Al2O3 content).
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 (as it rehydrates). And generates lots of heat as it does so.
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.
An example of how a glaze that contains too much plastic clay 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.
Roasted Alberta Slip (right) and raw powder (left). These are thin-walled 5 inch cast bowls, each holds about one kg. I hold the kiln at 1000F for 30 minutes. Why do this? Because Alberta Slip is a clay, it shrinks on drying. Roasting eliminates that, a 50:50 raw:roast mix works well for most recipes having high percentages of Alberta Slip. And 1000F? Calcining to 1850F sinters some particles together (creating a gritty material) while 1000F produces a smooth, fluffy powder. Technically, Alberta Slip losses 3% of its weight on roasting so I should use 3% less than a recipe calls for. But I often just swap them gram-for-gram.
This is 100% Alberta Slip (outside) on a buff stoneware (left) and iron stoneware (right) fired to cone 10R. The glaze is made using a blend of roast and raw (as instructed at the PlainsmanClays.com product page). Alberta Slip was originally formulated during the 1980s (using Insight software) as a chemical duplicate of Albany Slip. The inside: G2947U transparent. The intensity of the color depends on firing, add a little iron oxide (e.g. 1%) if needed.
This is the Ravenscrag Slip I used to calcine at it 1850F (about 10lbs in a bisque vessel). I am now roasting it to 1000F instead, this produces a smoother powder, less gritty. I hold it for 2 hours at 1000F to make sure the heat penetrates. It is not actually calcining, since not all crystal water is expelled, so we call it "roasting". Why do this? Ravenscrag Slip is a clay, it shrinks. If the percentage is high enough the glaze can crack on drying (especially when applied thickly). The roast does not shrink. The idea is to tune a mix of raw and roast Ravenscrag to achieve a compromise between dry hardness and low shrinkage. Technically, Ravenscrag losses 3% of its weight on roasting so I should use 3% less. But I often swap them gram-for-gram.
|Materials||Calcined Topaz Kaolin|
|Materials||WRA Calcined Alumina|
|Materials||Mulcoa 70 Mullite|
|Materials||Calcined Missouri Fireclay|
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