|Monthly Tech-Tip |
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 most commonly used to remove some or all unwanted volatiles from a material (e.g. H2O, CO2, SO2 - thus eliminating the LOI) and/or to convert a material into a more stable, durable or harder state. However, in ceramics it is also useful to destroy the plasticity of a clay for use in glazes (non-plastic clays reduce drying shrinkage). Varying temperatures are employed to calcine materials, depending on the decomposition temperature of the volatiles being burned out or the degree of sintering (and thus physical characteristics) needed.
The cement industry is by far the largest consumer of calcined clay. And the largest calcined powder producers. They heat-treat the kaolin in rotary kilns at 1450C (mixed with limestone and iron ore to form clinkers that are ground with gypsum to get the final product).
To produce molochite, lump kaolins are calcined at high temperatures (but lower than that of the cement process), they are then ground and sized. The powder is calcined at lower temperatures to produce calcined kaolin (which also is ground to a powder). However when already-processed powders are calcined at temperatures below their sintering point (where particle bonding occurs), the powder product is ready-to-use.
Potters typically calcine clays to enable using them in higher percentages in glazes, slips and engobes (the electrolytics of clay particles are destroyed by this process resulting in less shrinkage while drying). Typical calcine temperatures are not necessary to accomplish this, only about 1000F is needed (cone 022 or red heat). At this temperature the process is commonly called "roasting". Typically the loose powder can be fired in bisque vessels (any pottery clay can easily withstand this temperature) to 1000F (Cone 022 or red heat). For large or heavy-walled roasting vessels, fire slower (e.g. 200F per hour). For small amounts, 500F/hr should be fine. Hold at temperature 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.
The calcining and roasting processes produce 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 needed in the 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 considerable heat as it disintegrates into a powder.
Calcined clays are not normally used as ingredients for traditional bodies, especially because of cost. But for refractories and high-tech products the use of calcined materials is common.
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).
From Henan Hongxing Mining Machinery Co. in China. This unit is capable of 7 tons/per/hour and can fit in a standard shipping container.
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.
This is 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. To make sure the heat penetrates for this size vessel I hold it for 2 hours at 1000F. If your container is smaller you could do less time, if any black material remains inside, then do a longer soak on the next firing. 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||Calcined Missouri Fireclay|
|Materials||Mulcoa 70 Mullite|
In hobby ceramics and pottery it is common to layer glazes for visual effects. Using brush-on glazes it is easy. But how to do it with dipping glazes? Or apply brush-ons on to dipped base coats?
Traditional Japanese high feldspar glazes having cream to orange color flashing or blushing. Potters today seek to emulate the Shino appearance using a wide range of recipes.
In ceramics, glazes are suspensions. They consist of water and undissolved powders kept in suspension by clay particles. You have much more control over the properties than you might think.
A ceramic glaze fault that occurs during firing of the ware, islands of glaze form as it crawls, leaving bare patches of body.
The calcination of kaolin
Calcination of Alumina
Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.
|Temperatures||Dehydroxylation in clays (480C-600C)|