A blended English china clay, combining moderate plasticity, low titania content and relatively high flux content, low shrinkage and white fired color. 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 slightly less feldspar to vitrify. - Digital Fire ceramic materials database
A glaze or clay body will almost always specify Grolleg if Grolleg is necessary. If it just says china clay or kaolin, other kaolins are a more cost-effective option.
55# bag; white bag, blue text Imerys Quality China Clay