Tuesday, 24 April 2018

Thinfire* and Devitrification

There are reports that Thinfire causes devitrification by rising over the edges of the piece.  There as many saying they have no difficulties with the Thinfire curling.  This indicates there are several factors that may be at work.

If the Thinfire curls over the edge of the glass while firing, it will deposit a fine powder on the edge and perimeter of the piece.  This gives an ideal condition for devitrification to form.

Bullseye recommends placing dams or other kiln furniture on the edges of the paper to resist any tendency for the paper to curl.  Of course, if the paper is put upside down, it is much more likely to rise over the edge.  The smoothest surface should face upwards. Now that Bullseye prints their logo on the bottom, this is unlikely to be a problem.

Cutting the paper to the size of the piece is initially an attractive idea.  However, it does not account for the expansion beyond the initial footprint that glass goes through while heating to the working temperature, and before it contracts to its final size.  The Thinfire must be cut larger than the piece. The amount depends on the thickness of the piece.  6mm larger may be adequate for a 6mm thick piece.

Bullseye does not recommend using Thinfire under multiple small pieces of glass because the paper can shrink and move, disrupting the glass placement on the kiln shelf.  Instead using kiln wash as the separator may be better in these circumstances.

There are other things that can affect the deposit of the separator powder from the Thinfire onto the glass.

Venting – It seems to be good practice to open the peep holes or leave the door/lid slightly ajar during the heat up.  These should be carefully closed once the smell of the binder burning out disappears.  This is usually around 500°C.  The idea here is that the combustion products from the binders are allowed out of the kiln without settling on the glass.  I do not find this necessary, but many do, so it is worthwhile trying it out.  When the smell of the burnout of the binders ceases close the lid slowly and place the bungs gently into the peep holes to avoid disturbing any dust within the kiln.

Opening the kiln or ports - Opening or closing the kiln above ca. 500°C, if done quickly, will create a draft that will distribute the powder around the kiln.  Some of this will land on the surface of the glass. Other parts of the Thinfire will be moved up onto the edges of the piece.  This dust and the pieces of Thinfire will create nucleation points for devitrification.  Always open or close any part of the kiln slowly when there are powders or anything else which can be disturbed by a gentle waft of air.

Over firing - Another element that can bring Thinfire onto your pieces are a too hot a firing.  During high temperature firings, the glass will expand and thin more than usual.  During the cooling phase, the glass will draw back to being 6-7mm thick. This means the glass will have expanded over the Thinfire and drawn some of it back onto the edges as it thickens and retreats.  The solution for this is to reduce the top temperature and possibly lengthen the soak time, but do not do both at the same time.  First see what a lower temperature with a 10-minute soak will do.

Of course, if you are not having problems with Thinfire or Papyros, continue your practice as normal.

*I have used the term “Thinfire” almost exclusively throughout, but remember all these notes apply to Papyros too.