Cold working and fire polishing are both methods of making rough glass shiny again.
Cold working involves manually polishing the glass with a succession of grits, from rough to fine. Each finer grit polishes the surface one stage smoother, and then you go on to the next. You can use either diamond polishing pads and an electric polishing machine (lap wheel, grinders, etc) or a polish by hand with a succession of silicon grits and friction on a flat surface.
Polishing with grit:
Fire polishing is heating the finished glass in a kiln just until the surface becomes hot enough to become shiny, but not enough to change the shape of the work. You do have to do some cold working to get the glass somewhat smooth to begin with, remember, the glass won’t change shape, so if you have sharp needle edges, they won’t melt down.
The ideal situation is to combine the two methods in the most efficient way. To do this, use cold working techniques until the glass is optimally smooth and then fire polish. But, what is the optimal amount of polishing? I decided to do a quick test.
First I sawed two identical strips of thick window glass on my tile saw. I’ll be using my trusty bench polisher to do the polishing with diamond pads. If you are unfamiliar with cold working, I intend to polish the edges where the 200 and green line are, then move over one and polish the next bit with 400, etc. This will give me a gradient of polished edges.
After all this cold working, I’m going to Fire Polish ONE of the strips using the slump schedule. This will give me an idea of how much cold working I need to do to get the desired finish.
Here are the polished strips, the two cold worked ones on the left and on the right, the fire polished strip next to a cold worked strip.
For my purposes, cold working to 400 and then fire polishing gives me the perfect finish for the job.