Glass fire polishing is a technique used to round and polish the edges and surfaces of a piece of fusing glass that may have been "cold worked" by grinding, drilling or sawing.
It is also used to correct minor defects that can be present between the fusing and slumping stage.
Fire polishing can leave a matte or shiny surface on a fused glass surface that has been re-heated in a glass fusing kiln.
Fused glass size and thickness will determine the firing schedule that is best suited to achieve the outcome.
The thicker the fusing glass the slower the fire polishing heating schedule needs to be. Pieces that are heated too quickly may break into a number of pieces as the heat is increased.
This is known as thermal shock and may happen with a rapid increase in temperature, a slow firing schedule is needed to prevent that from occurring.
A fire polishing schedule can be found at cast glass paperweight. This schedule is designed to cater for a thick piece of glass and to produce a shiny finish.
A matte finish can be achieved by holding the glass at a lower maximum temperature subject to size and thickness.
A two layer fused piece can be fired at a quicker rate to produce a nicely finished surface.
A rate of 300° F (167° C) per hour up to 1100° F (593° C) with a ten minute hold, then lifting to around 1350° F (732° C) with a further 10 minute hold.
Glass kiln temperature should be dropped AFAP to 900° F (482° C) and held for sixty minutes to anneal the glass.
Annealing is critical for fused glass pieces such as dishes, plates,etc. Smaller pieces like fused glass jewelry can be left to cool down in the glass kiln to room temperature without an annealing cycle.
When fire polishing glass it is critical that it is not over fired.
Over firing or holding the piece at high temperature for too long will result in the glass becoming distorted and the edges misshapen.
Temperature should be closely monitored during the upper segment of the fire polishing schedule.
A practical example of cold working and the outcome of fire polishing can be found here at Fire Polishing.
Items that have already been slumped may be difficult to fire polish due to the closeness between the fire polishing and fusing temperatures.
This temperature closeness can result in the slumped item losing its shape. By carefully watching the process and polishing at a lower temperature it is sometimes possible to achieve a result.