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.
