The Pitting Enamels technique can be tricky, affected by amount of metal in the kiln and firing length. As the project says, too long a firing will result in the bubbles flattening out. Mary Stafford feels you have to experiment with the amount of metal and length of firing for each grouping - amount of metal includes gauge of metal, dimensions of metal, type/size of trivet and firing screen (use a self-standing trivet for less metal). If you experiment with similar materials and figure out your optimal firing timing, then you can reliably fire a finished piece. She also watches it in the kiln and when it “boils” its ready to be removed-immediately!!! Waiting even seconds can cause the voids to decrease to unusable results! Another tip is to make sure your kiln is heat saturated - that is, has been up to temperature for awhile, didn't just reach temperature. This allows the kiln to stay hotter when the door is opened. She also puts the potassium nitrate on sifted enamel, not previously fired.
A few of us have used Spectracide Stump Remover (can be gotten at Amazon), but other brands are also being experimented with. This is still an experimental technique and any findings you come up with I’d appreciate being told so I can post them here.