Friday, 13 July 2018

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 in the kiln is widely popular as it utilises existing equipment, avoiding purchasing additional cold working equipment. This post indicates some elements about fire polishing in the kiln. 


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 in order to give a smooth shiny surface to the glass without extensive cold working with successively finer grits to get a polish. It also can be used to give a variety of textures from a sealed but 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.


Fire polishing range
The temperature range that this occurs between slumping and tack fusing. The normal range is 650C to 750C depending on the glass, the soak time and the speed of advance.  The purpose of this kind of firing is to get the surface of the glass hot enough to form the desired surface without soaking long at higher temperatures, as this is also the devitrification range (700C - 760C).  Normally there would be a minimal or no soak at the top of the temperature range.

When to fire polish 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.

Methods
You can take the fused piece that has been treated to remove the devitrification up at the same rate as for slumping the piece to the tack fuse temperature.  The higher you go, the less soak time is required. Of course, the higher you go, the longer you are in the devitrification zone.  

Some people advocate a quick fire polish.  This is achieved by firing at a relatively slow rate until a low slump temperature is achieved.  Then fire very quickly to the tack fuse temperature with no soak and return to annealing temperature as quickly as possible.

The quick fire polish does achieve a minimum of time in the devitrification zone, but it eliminates all subtely in the surface.  A long soak of up to 90 minutes at a moderate slumping temperature will give a satin appearance to an abraded or sandblasted surface.  A shorter soak will seal sandblasted work without eliminating the texture of the sandblasted image.

In all the cases of fire polishing you need to peek at intervals to determine when the desired surface has been achieved.  This requires careful placement in relation to the place from which you will be able to peek at the surface.  For a fully polished piece, you will see the reflections of the elements.  For more subtle textures, you need to think about what you want to see, peek, close the lid or observation port and think about what you saw.  If it is not yet what you want, peek at another interval in the same way, until you observe the surface you want.

Combining fire polish and slumping 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.

Fire polishing already slumped items 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.

Again, in these more difficult circumstances, you must observe at intervals to ensure you do not over fire and distort your piece.

Schedules
The reason that no indicative schedules are given is that different glasses, and different lay ups require different firing conditions.  These are dealt with elsewhere in the blog.

Alternatives 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.