Monday, 13 February 2012

Fire Polishing

Polishing of glass can be done in the flame, in the kiln, by acids or by grinding with successively fine abrasives depending on the nature of the piece and the equipment available.

Fire polishing
Fire polishing is the technique most often available to kiln formers. This is the process of heating the glass to less than a full fuse to achieve a smoother texture on the glass. It is often used after sandblasting or hand sanding a piece to give a smooth shiny surface to the glass. It also can be used to give a variety of textures from an almost unchanged sandblasted surface, through a satin-like finish to a very subtle difference between full polish and slightly textured surfaces in the same piece.

The temperature range that this occurs within is that of slumping to tack fusing. The normal range is 650C to 750C depending on the glass, the soak time and the speed of advance. Normally there would be a minimal or no soak at the top temperature. The purpose of this firing is to get the surface of the glass hot enough to form the desired surface without soaking long there, as this is also the devitrification range

As this temperature range is above the slumping temperature, fire polishing should be done after fusing and before slumping. As this will be the last operation before forming, you also should do any work to shape the edges and deal with any other imperfections, before fire polishing. After doing any grinding or other work on the edges or surface of the piece, thoroughly wash and polish the piece dry.

It is sometimes possible to fire polish and slump at the same time, but this is a risky technique often leading to changes in shape or an uprising of the glass at the bottom of the mould. It is possible to fire polish glass as low as 630 with a long soak – 60 minutes or more. If you are determined to fire polish and slump at the same time, it's essential that you watch the piece very carefully to prevent over-firing.

Similarly, re-firing already slumped items to a fire polish rarely succeeds. Distortion of the piece is more likely than achieving a fire polish on an already slumped item.

Alternatives to fire polishing include acid polishing, which can present a health hazard, and is normally an industrial process. The other common method of polishing is to cold work the piece. This often requires specialized equipment, but can be done by hand if you have the time.