Occasionally
fused pieces come out of the kiln with one side thicker than the
other. There are several things that need to be done for the present
piece and for the future.
Level
First
check how level your kiln is. The best for this is to begin with a
check of the bed of the kiln. Check the level in four directions –
left-right, front-back and the diagonals. If it is practical, wedge
up the legs of the kiln to make the bed of the kiln as level as
practical.
Then
check how level your shelf is. Put in your shelf supports and then
place the shelf on them. Again check with a spirit level the four
directions. Place pieces of fibre paper under or on top of the
supports to level the shelf. It is only after these checks have been
made that you can consider firing your piece to help it return to an
even thickness. As part of your kiln maintenance you should check the level of your shelf at least monthly, if not every time you prepare to fire.
Variation in Thickness
Now
that you know the shelf is level, you need to consider what the
variation in thickness across the piece may be. The firing schedule
needs to be more conservative than just for the thickest part. As
the thinner parts will heat through more quickly than the thickest
parts, you need to fire less quickly than you normally would for the
thickest area. A rule of thumb – not always correct of course –
is to add the difference of the thick and thin areas to the thicker
and fire for that calculated thicknesses. This will make the firing
schedule slower and so allow the thicker part to be the same
temperature as the thinner. For example, a piece 6 mm at one side
and 10 mm the other would have a difference of 4 mm. Add this 4 mm
to the thicker 10 mm and then fire for 14 mm.
Temperature and Soak
You
also need to consider the top temperature to use and the length of
soak required. Glass flows relatively slowly at kiln forming
temperatures. The conservative approach – one that allows further
work if necessary – is to use the previous fusing temperature and
extend the soak by at least twice the length of time on the previous
firing, even perhaps to a couple of hours.
Bubbles
One
thing that will happen is that the bubbles that previously were near
the surface will rise and burst giving pin holes on this extended
soak. So you should consider cleaning the bottom and putting the top
face down on a separator between the shelf and the glass. This will reverse
the direction of flow for the bubbles. Few if any will break through
the new top and there should be no pin holes when flipped.
Further Firings
When
the piece is cool, check it for the even-ness of the piece all
around. If it is not even enough, you will need to consider
re-firing again. If you decide to do so, you should go no faster
than the rate of advance as previously – probably even slower - but
consider raising the temperature or extending the soak. Remember
that achieving the heat work required at the lowest temperature is
the guide line for kiln forming. So an extended soak should be
preferred over a higher temperature, unless there are strong
indications that a higher temperature is required.
Fire Polishing
Of
course, you will now need to throughly clean the face down side and
re-fire to fire polish the original top. The rate of advance should
be the same or slower than the firing to even the thickness. Once
you have achieved about 600C, a soak of about 30 minutes will ensure
that the glass is thoroughly heated through. Then you can advance at
a quick rate to the fire polish temperature with a soak of no more
than a minute. This allows the surface to change without giving the
rest of the glass time to begin to move. Of course, a thorough annealing is required.
This
procedure for re-firing can be used when re-firing pieces for any reason. You only
need eliminate the considerations on the uneven thicknesses.
