All Glossary

Calcination

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.

Key phrases linking here: calcination, roasting, calcined, calcine, roasted - Learn more

Details

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 change) 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 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. Powdered calcined kaolin is burned at lower temperatures.

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". The loose powder can be fired in bisque vessels made of pottery clay (any clay can easily withstand this temperature. 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 considered. 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.

Calcined clays are not normally used as ingredients for traditional clay bodies (because of cost and plasticity loss). But for refractories and high-tech product manufacture the use of calcined materials is common.

Related Information

Roast or calcine your Ravenscrag Slip (or other clays) for much better results

Calcined or roasted clays are indispensable in making many types of glazes, they reduce drying shrinkage (and thus cracking and crawling) compared to those made using raw clay. In a glaze, you can fine-tune a mix of raw and roast clay to achieve a compromise between dry hardness and low shrinkage. This is Ravenscrag Slip, we roast it to 1000F (roasting is adequate to destroy plasticity and produces a smoother powder than calcining at higher temperatures). To make sure the heat penetrates for this size vessel I hold it for 2 hours at 1000F. Calcined koalin is getting harder to find, this same process can be used to make your own from a raw kaolin powder. One thing is worth noting: Weight lost on firing actually means that less of the roasted powder is needed to yield the same amount of material to the glaze melt, it can be anywhere from 5-12% less.

Roasting Alberta and Ravenscrag Slips at 1000F: Essential for good glazes

Roasted Alberta Slip (right) and raw powder (left). These are thin-walled 5 inch cast bowls, each holds about 1 kg. I hold the kiln at 1000F for 30 minutes. Why do this? Because Alberta Slip is a clay, it shrinks on drying (if used raw the GA6-B and similar recipes will crack as they dry and then crawl during firing). Roasting eliminates that. Calcining to 1850F sinters some particles together (creating a gritty material) while roasting to 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.

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 (as it rehydrates). And generates lots of heat as it does so.

Fixing a crawling problem with Ravenscrag Tenmoku

Crawling of a cone 10R Ravenscrag Slip 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.

Will this crawl when fired? For sure!

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 roasted Alberta Slip. The roasted 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.

Two glazes. One crawls, the other does not. Why?

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).

A rotary drier suitable for drying and calcining ceramic powders and granulates

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.

A rotary drier made by Feeco International

They have exceptional support and are very open about their engineering and designs.

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Badly crawled glaze fired at cone 5 reduction

Glaze cracking during drying? Wash it off and then do this.

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