Fire Polishing of Frit Castings

Image credit: Obsession Glass Studio

 Fire polishing castings is relatively difficult.  Even though people may suggest temperatures for this kind of fire polish for castings from frit:

• ·        They are relevant to particular kilns. 
• ·        They are also dependent on the ramp rate. 
• ·        The presence or absence of a bubble squeeze is important. 
• ·        The size of the casting is relevant.

 The objective is to get a fire polish without distorting the shape of the piece.  The general procedure is to fire slowly to the softening point. This is to ensure the casting is of similar temperature throughout. The softening point for fusing glass is around 540°C/1000°F. You should soak at that point for a time to ensure the glass is all at that boundary between brittle and plastic.

 You may prefer to use a bubble squeeze soak to achieve the same thing.  This has a slightly higher risk of distorting the piece.  If you do use the bubble squeeze, it should be done at the lower end of the bubble squeeze after a slow rise.  The casting will not be subject to much change at 600°C to 620°C/1110°F to 1150°F, if the soak is short.

 The rates to be used are dependent on the size and thickness of the piece.  Larger and thicker pieces need slower rates than thin ones.  Fire at an initial ramp rate for twice the thickness to be sure of heating thoroughly.

 When the softening point is reached, or the slump soak is complete, proceed at a rapid rate to the tack fusing temperature. To get the result you want you will need to observe.  Peek at frequent intervals. Be prepared to advance to the next segment when the gloss appears on the surface.  Your controller manual will tell you how this is done.