Schedules for Specific Mould Shapes

A suggested temperature for a slump can only be a starting point and not all moulds will have slumping or draping temperatures given. The main factors to be considered are mould shape, span and depth, and glass thickness and profile.

Fortunately, Bullseye have a mould tips document on suggested slumping temperatures (in Celsius and Fahrenheit) for a variety of moulds.

The mould shapes are of course are those that Bullseye sell. If the shape is not one of theirs, a similar shape can be chosen and the suggested temperature and soak time can be used as a starting point.

Study of this tip sheet can give you a lot of information on various aspects of time and temperature for variations:

• Angularity – Moulds with sharper angles require longer times and sometimes higher temperatures.

• Span – The smaller the span of the mould, the longer the time required to slump into the same shape.

• Depth – It is not surprising that greater depth requires longer soaks

• Ramp rate – The rate up can affect the temperature and time of the slump.

What the Bullseye table does not give is information on the effect of the thickness of the piece. The Mould Tips document assumes a 6mm/0.25” flat fused blank. Thicker pieces will need a combination of slower ramp rates and longer soaks to fully form than given in this document. They will also need longer anneal soaks and slower cools.

It does not provide information on the effects of the profile either. Tack fused pieces need slower ramp rates and longer annealing than full fused blanks. The additional rates and times can be worked out by applying the following multipliers and firing for the resulting thickness:

• Flat fuse – the actual thickness

• Contour fuse – 1.5 times the thickest part

• Rounded tack fuse – 2 times the thickest part

• Sharp tack fuse – 2.5 times the thickest part

The key to getting the right temperature and soak length for a successful slump or drape is to observe the progress of the firing with quick peeks from the start of the top temperature. Slumping temperatures are effected by all the factors above, so that no one temperature can be given for even one mould.

My ebook Low Temperature Kilnforming gives more information on the factors in writing schedules for slumping. Available here and here.