Deep moulds

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.

Further information is available in the ebook: Low Temperature Kiln Forming.