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Grog, also known as chamotte, is a granular material obtained from grinding calcined (fired) clay at high temperature (between 1400 and 1600ºC). In ceramics, it is used to regulate the plasticity of the clay and facilitate drying and firing properties, avoiding tensions and, consequently, cracks and breakage caused by drying and firing shrinkage. It also improves the permeability of the piece, favoring the evacuation of water during drying and gases during firing (mainly water vapor and CO2).
It is supplied in different granulometric curves depending on the use. The most common curves are from 0 to 0.2 mm (impalpable), from 0 to 0.5 mm (fine), from 0 to 2 mm (medium), and other coarser granulometries (up to 3 mm) for the manufacture of special refractories.
Different granulometries of chamotte: impalpable (0-0.2 mm), fine (0-0.5 mm), medium (0-1.5 mm) and coarse (1-3 mm).
It is important that the chamotte be as refractory as possible, because it must have an inert behavior that does not react during firing with the rest of the clay in the ceramic body. Thus, chamottes usually have high alumina content, which can reach 40% or even higher.
In artistic ceramics, chamotte is also used to give texture, obtaining a rough appearance that gives character to the piece, especially in sculpture. In the manufacture of refractories, chamotte favors resistance to thermal shock, since the coarse grain prevents the propagation of microcracks that are produced by thermal stress.
Effect of chamotte on the texture of ceramic clays (SIO-2® PRN black clays fired at 1200ºC).
Strictly speaking, if chamotte is grinded calcined clay it means that chamotte is grinded ceramics, so any ceramic material that is grinded is chamotte; be it a brick or a piece of sanitary porcelain. This is true, however, in practice, no one makes a ceramic product to later grind it to make chamotte. It only occurs in the case of pieces that have come out of the kiln defective. So it is a by-product and it may have some specific uses, but with supplies that are usually limited.
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