If glazes are underfired or have an unbalanced formula (i.e. lack silica:alumina) they can be soft enough to 'cutlery mark', that is, scratchor mark by metal instruments.
Glaze Hardness Testing Procedure
Method 1: Using a sharp-pointed concrete nail or dry-wall screw, attempt to scratch the surface of the glaze in question. Also watch for any tendency of the glaze to chip, for small holes to develop as a result of under-the-surface bubbles, and for any black marks that show up. Compare its response with that of a glaze known to be good.
Method 2: Using a standard sharp hack-saw blade attempt to 'saw' an area of the glaze surface. Watch for scratches that develop and for black marking. Compare the glaze's response with that of a glaze know to be hard.
If marks, pits, or scratches are left by either method, attempt to clean the surface. If it cannot be cleaned, reject the glaze for food or drink surfaces
|Tests||Glaze Melt Flow - Runway Test|
Tests conducted on glaze batches used in production (as opposed to tests conducted on the materials used to make those glazes).