Calcined Kaolin

  • Friday, 17 February 2023
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Calcined Kaolin

Calcined kaolin is an ultra-fine clay that is heated to high temperatures in a kiln to increase whiteness and hardness, altering the size and shape of the particles. The calcination process also changes the chemical composition of the kaolin.

Imerys kaolins are used to improve the mechanical properties, porosity and appearance of a range of construction products such as cements and concretes, plasters and mortars. They are also employed as aluminizing agents to enhance the performance of fiberglass and as multi-functional fillers in adhesives, sealants, plastics and rubber.

In paints, calcined kaolin can partially replace the use of titanium dioxide to reduce cost and improve paint properties. It also works as an anti-settling agent to prevent agglomeration, and can improve the abrasion resistance and aging properties of a paint formulation.

It can be applied as a fine-particle slurry or a dry powder in a water based paint system to offer additional properties such as gloss and sheen control, enamel holdout and color enhancement. It can also be used in clear coat varnishes to provide better burnish and scuff resistance.

Ceramic Glazes

Raw kaolin is an excellent second or primary source of Al2O3 for ceramic glazes. It is readily meltable and supplies the necessary Al2O3 and SiO2 to all glaze recipes, while being an excellent insulator. It is an essential component of a slurry because it suspends the slurry to help keep it from moving during drying and firing, and because it helps to harden the dried glaze layer.

However, raw kaolin shrinks rapidly when it is fired and can cause crawling or crazing. This is particularly common in matte glazes with over 20% clay content.

This shrinkage can be easily controlled by adding a small amount of calcined kaolin to tune the drying shrinkage while maintaining fired properties. The calcined kaolin can be added to a glaze slurry before the glaze is pressed into the mold or during a step of the production cycle.


Like glazes, engobes have higher clay percentages than slips and it is more important to control their drying shrinkage. They are commonly used in low fire stoneware, vitreous terra cotta and soft body refractory castables and furniture, thermal insulation bodies, low expansion bodies, permeable ceramic compositions and investment casting (see molochite).

It is important to remember that a calcined kaolin will not change the color or texture of a glaze. It will not affect the fire-resistance of a glaze, but it can be a useful additive in a slip or engobe recipe to tune the drying shrinkage and plasticity of these materials.

The calcined kaolin can also be added to a glaze slurry to increase its abrasion resistance and stability. The kaolin is also useful in providing an anchor for the glaze to the ceramic body.

Calcined kaolin can be characterized by a number of infrared spectral features in the SWIR, MWIR and LWIR ranges that are relevant to the different steps involved in its calcination. These features include the kaolinite crystallinity (Kx) index and the depth of the water absorption feature (1900D). The characterization of the calcined kaolin reaction sequence using SWIR spectroscopy can significantly reduce the turnaround time required for quality control, and provide the basis for an on-line analysis system.

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