Pyrax (pyrophyllite) is chemically similar to a clay, it is a hyrated aluminum silicate. But Pyrax has the appearance of a talc. Many properties of Pyrax are quite similar to talcs. Pyrax has a neutral pH and due to the presence of quartz, is highly abrasive. Used in wall tile bodies. It lowers firing temperature; produces low moisture expansion bodies with good craze resistance; increases thermal shock resistance; and greatly increases firing strength of vitreous bodies. It promotes the development of mullite when substituted for an equivalent amount of feldspar or quartz.