What is the relation between the diameter and depth of a mould
and the diameter of the blank to be slumped into it? Is there an equation? For example, a mould of
6.5 inches diameter and 3.5 inches deep.
The above example is a deep
mould. When compared to the diameter it is more than half the
diameter deep. It is a difficult style of mould for many kiln formers.
To explain, some of
the differences between deep and shallow moulds needs to be noted. Deep and shallow are descriptions for the
relationship of the span (or space from side to side) to the depth (the
distance the glass must slump into the mould).
 |
| A 150mm square bowl that is 50mm deep |
Shallow moulds are easier as there
are no steep curves to form into. Gentle compound curves generally provide no
greater challenges. Shallow moulds with angular corners or abrupt changes of
curve to the bottom are moderately more difficult than simple curves.
Deep moulds are difficult. An
example of this is the successful deep vessels of Karl Harron where it is
necessary to form the glass through three successively deeper moulds. His vessels are often deeper than the
diameter, making them among the most difficult kilnformed vessels to achieve except
roll-ups, which are a combination of kilnformed and blown.
The difficulties
with deep moulds are multiple.
·
The glass
must stretch more than in a shallow one where only a change in shape is
required.
·
As the
depth increases, the upper rim is heated more than the centre, being closer to
the heat source.
·
The edge
develops needle points and stretch marks.
·
The
blank becomes smaller in diameter than the rim of the mould.
·
As it
slips down into the mould the softened rim of the glass catches on the mould
and produces stretch marks.
·
If the
upper rim is significantly hotter than the centre, needle points are left on
the edge where it catches the mould and stretches to very thin points.
Because of the depth, you will
find the finished glass diameter will be smaller than the rim of the mould.
 |
| An example of an apparently shallow bowl until you take account of the size of the inner part, making this a deep bowl. |
Over hanging blanks
Do not be tempted to make the
blank much bigger than the mould. The most you should risk is 12mm larger diameter
than the mould. This means only 6mm is over the edge of the mould.
In deep moulds where I am
doing only one slump, I find better success in making the glass slightly
smaller than the mould diameter, so the glass does not hang up on the edges.
 |
| A blank larger than this mould is likely to hang up on the narrow rim of the mould, and the blank should be slightly smaller than the interior of the rim. |
Multiple Moulds
Because deep moulds are
difficult to do in one firing, it is best is to start with a larger blank in a shallower
mould and transfer the formed piece to successively deeper moulds.
Some experimentation will be
needed to determine the starting size, of course. A guide would be to measure the length of
inside curve of the deep mould from one edge along to the bottom and up to the
other side with a flexible measuring tape.
This will give the approximate diameter needed for the blank.
I start off with a blank of about
25mm less than the measured diameter. This
is to allow for the stretching that is going to occur even at low temperatures.
Cut and fuse a clear blank for the test. Find a shallow ball mould of that
diameter or larger and slump into it. A second mould may be needed that is
deeper and of the size of the now slumped piece with a smaller than original
span. By this second slumping, the piece
may fit into the deep mould for the final slumping.
Firing for deep moulds
To fire deep moulds successfully,
you need to go slowly (maybe only 75C per hour for 6 mm) up to a low slumping
temperature (maybe 630C for 60 to 90 minutes).
This slow rate of advance allows the heat to be distributed evenly
throughout the piece. A low temperature
avoids over-heating the rim of the piece during the slumping. The long soak allows the glass to gradually
conform to the shape of the mould without excessive marking.
It is important to monitor the firing of deep moulds. It is common for the glass to slump unevenly. Peeking and being prepared to reach into the kiln to shift the glass or even tilt the mould so the lowest part is elevated to receive more heat is important to succeed in slumping into deep moulds.
Deep moulds require
a lot of effort to achieve successful results. You must give special attention
to temperatures, rates of firing, soaks and consider the use of multiple slumps
– each deeper than the previous.
