Firing AFAP - Kiln Forming Myths 19

Firing as fast as possible harms your kiln, or at least will wear out the kiln elements.

I believe this comes from the days when ceramic kilns were commonly used.  Certainly this is still the mantra among ceramicists today.

A number of people fire their kilns as fast as they can, especially small ones, all the time.  Refractory fibre kilns are not affected at all by rapid changes in temperature. 

There might possibly be some small damage to the light weight refractory brick used in glass kilns in that the frequent expansion and contraction may cause crumbs to fall from the brick.  But this will happen anyway as the brick expands and contracts the same amount every time it is fired.  There is no definitive information on whether rapid increases in temperature have any greater effect on brick than slower increases. 

Any rapid change in temperature is unlikely to affect the kiln elements.  Attempting to bend the elements while cold is very likely to break them, as a compound is formed on the surface which makes them brittle when cold.  But this is very different from rapid changes in temperature.  As an analogy, the elements in electric fires are made of the same material and are always heated as fast as possible from cold.  They have a long life, so there should be no difference in effect on kiln elements, which are generally thicker and less exposed to drafts and rapid temperature changes once hot.

It could be said that firing as fast as possible would reduce the stress on the relays in the controller, as they will be closed for the whole of the temperature rise, with no opening and closing.  Thus, the number of firings will be increased without equally increasing the number of cycles the relays have to perform.

However rapid rises in temperature affect the kiln is secondary to how it affects the glass.  Except for small pieces, extremely rapid rises in temperature increase the likelihood of the glass breaking.  This is the more important consideration when thinking about afap firings.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Rapid Rates of Advance to Avoid Devitrification - Kiln Forming Myths 18

Firing as fast as possible, or at least, very fast above annealing point will avoid devitrification.

Of course, this is true in one sense. Moving quickly through the devitrification range will reduce the time the glass has to crystallise – the action we call devitrification.

It will not on its own prevent devitrification.  Nowadays fusing compatible glass is formulated to resist devitrification during the firing.  However, devitrification still occurs during prolonged soaks at high temperatures, and slow rises or falls in the temperature range of 720°C to 760°C.  So you should always be trying to fire quickly through this range, whether up or down.

The contaminants that can form nucleation points for crystal growth can be oils from fingers, or cutters, residue from glass cleaner or refractory fibre papers, or even dust. 

This means the first line of defence against devitrification is cleaning.  Cleanliness is next to perfect results in kiln forming.  Use glass cleaners without additives.  In the UK, Bhole produce excellent glass cleaners.  In the USA, Spartan glass cleaner is recommended by Bullseye.  These may be better than clean water if your water supply contains a lot of minerals or additives for health purposes.

If you feel the need to make your own cleaning fluid do not use denatured alcohols such as rubbing alcohol.  They contain additives which may leave residues.  Use something like isopropyl alcohol and distilled water.

The drying of the glass should be accomplished with a thorough buffing to squeaky clean with plain paper towels or lint free cloths that have been washed without softeners in the washing.

The burn off of organic binders in fibre papers can produce enough residue to affect your glass, so it is best to keep your kiln vented until the burnout has completed – around 400°C.

To prevent dust settling on your pieces, clean and place into the kiln immediately.  If this is not possible, make sure the surface is well covered until placing in the kiln.

Hand Finishing to a Flat Edge.

Hand finishing an edge does not require expensive electrical tools, although they do make the process quicker.  This is a note on how to get good-looking edges without expensive equipment. Only a few materials are required.

• ·         A thick sheet of float glass for the grinding plate
• ·         Aluminium oxide or silicon carbide grit in approximately 80, 180, 400, and 600 grits to act as the abrasive.
• ·         Wet and dry sandpaper of approximately 1200 and 2400 grits
• ·         Paint pens (white and gold work well)
• ·         Paper towels for drying
• ·         Water for rinsing
• ·         Large bucket or basin to collect the rinsing water

The thick flat float glass acts as the grinding plate.  It is flat and smooth, making the grinding and polishing flat. 

If you have a lot of glass to take off to get a straight edge use 60 or 80 grit.  If there is not much to take off, start with 120 or 180 grit.  There is no need to make deep scratches on the edge that will take time to eliminate, if a finer grit will do the job.

Put 80 (or finer) grit aluminium oxide or silicon carbide on the glass grinding plate and make slurry with water. 

Slurry mixed and the circular motions of grinding can  be seen
photo: hisglassworks

Move the edge firmly in circular or figure of eight motion over the grinding plate until flat. If the slurry becomes pasty, add more water as you do not want a thick grinding mix. Maintain the same angle of the glass piece to the grinding plate at all times so you have only one plane of glass to take to a polish.

When the edge is flat, clean and dry the glass, and especially the ground surface to remove all traces of the coarser grit.  Set the piece aside to dry.

While the piece is drying, clean off the grinding plate. Scrape off the slurry into a pot set aside for that grit for further use, or into newspaper or other temporary container and then into a waste bin, not the drains.  It is a heavy material and will block drains. Rinse the plate off in a basin of water to ensure there are no coarse grains on the glass.  The residue will settle to the bottom and you can decant the water off once it clears. These grits are not very expensive so repeated use is not essential, just economical.

The next step is to paint the now dry glass edge with a white paint pen.  This will allow you to see when you are ready for the next step, by the disappearance of the paint from the scratches.  Of course, if you are grinding a white or other pale glass, a gold paint pen will be better to see those scratches.

While the paint is drying, make a slurry of the next finest grit. Then begin grinding. The first element in each grinding stage is to give an arris to the edge of the glass.  This prevents chipping the sharp edges.

 

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When the white paint is gone from the edge, you can progress to the next grit.

At each stage of grinding you can reduce the grit size by half (double the number). This is the generally accepted reduction of grit size to make the removal of the scratches of the previous grit least time consuming. You can reduce the grit size by more than half if you want. Most often reducing grit by large amounts means more time is spent at each stage.  Experience will show you how much you can reduce the grit sizes beyond the accepted intervals.

Stopping at 600 or 800 grit will enable an edge to be fire polished with ease and minimum heat.

At each stage you need to clean the glass and grinding plate as for the first change of the grit size. This repeated cleaning usually means that the artist either has separate grinding plates for each grit, or the grinding is saved up until there are a few pieces that need the same treatment.

A piece of wet and dry sandpaper fixed to a glass plate

After 800 grit, you may wish to progress to wet and dry sandpaper for the finer polishing, using 1200, 2400 and, if you want. 6000 grit. Fix the paper to a glass plate.  Often, simply folding two edges under the glass will be enough.  Add water and proceed as for loose grit.  Hand finishing to this level will eliminate the need for fire polishing. 

Of course, for smaller areas, you may wish to use diamond hand pads.  The need to use water and rinse between grits still applies.  The diamond hand pads are usually most suitable for short straight edges. The longer ones need the kind of treatment outlined above.

Borax Characteristics

Borax is a glass making flux used to reduce the melting temperature of glass. 

It is almost colourless - grey, white, or yellowish; seldom bluish or greenish; and colourless in transmitted light.

The chemical composition of Borax is:  Na₂(B₄O₅)(OH)₄ · 8H₂O

It has a hardness rating of 2 – 2.5, about half that of glass at approximately 5.5.

The melting point is 878°C. At this temperature borax dissolves numerous metal oxides. In spite of this high melting temperature, it acts as a flux reducing the softening point at the surface of the glass at kiln forming temperatures.

The specific gravity of borax is approximately 1.7, considerably ligther than glass at ca.2.5.

Borax is sparingly soluble in cold water, although readily soluble in boiling water. It is insoluble in ethanol.

Description of devitrification
Temperature range

Devitrification spray