Grolleg, English Kaolin

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A blended English china clay, combining moderate plasticity, low titania content and relatively high flux content, low shrinkage and blue-white fired color. The low TiO2 content is a key factor for the usefulness of Grolleg. It is excellent for making translucent throwing or casting porcelains. The pottery community uses many Grolleg based porcelains.

Its chemistry is different from a typical North American kaolin, it contains some fluxing oxides (e.g. it has almost 2% K₂O). Thus porcelain bodies employing it require quite less feldspar to vitrify (5-10% at cone 10, for example).

For use in plastic porcelains there is a dilemma that body formulators face: This material has a lower plasticity than American materials like EPK and much lower than #6 Tile and Sapphire kaolins. Thus porcelain bodies employing it require additions of a plasticizer like bentonite (up to 5% to get high plasticity). Since raw bentonites having the necessary plasticity are also exceptionally high in iron, people must balance the advantages of using a white burning and more costly material like Grolleg against the lower plasticity that makes the addition of dirty plasticizers necessary. Theoretically making translucent bodies employing 5% bentonite having 5% iron content should only increase iron in the body by 0.25%. This does not sound like much, but it is. Bodies having 0.1% iron burn much whiter than those having 0.2%. Thus either lower plasticity should be tolerated or white plasticizers (like VeeGum or Bentone) should be used.

Glazes: While the chemistry difference between this and more typical kaolins is certainly worth noting for bodies, it is still likely close enough to the theoretical 1:2 alumina:silica to be substituted in recipes that call for kaolin or china clay. Grolleg may also be preferred because the titania in other kaolins can be detrimental to the development of a color (e.g. titanium can turn celadons to a greenish tint). Transparent glazes employing Grolleg as-the-kaolin used over Grolleg-based porcelains can produce very white and even blue-white color in reduction firing. Another factor making this material an excellent glaze suspender is its inherent stickiness, which is exhibited to a degree much in excess of its actual plasticity. This property also makes for great hardening and durability of the dried glaze layer. New Zealand kaolin has similar properties are can normally be substituted for Grolleg.