Wednesday, 17 February 2016

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.