In addition to the BD stamp, I use a number on the bottom of each piece. The first digit indicates the pounds of clay I used (not what the finished product weighs), and the second is a code for the type of clay. After the bisque firing (and before the glaze firing) the different clays can look very similar, so this is a way to know what clay I am glazing. Clays change quite a bit at 2300 degrees in the glaze firing. Some of the numbers: 1 - B-Mix; 2 - B-Mix with Grog; 3 - Recycled; 4 - Soldate 60; 6 - Black Mountain; 8 - Porcelain (usually Coleman). So, for example, this "53" started as 5 pounds of recycled clay or a "46" would be 4 pounds of Black Mountain.