Ceramic Drape Moulds

Characteristics of materials

One of elements you need to consider in selecting a mould for draping is the characteristics of the ceramic material in relation to the glass being draped.  

Ceramic

Ceramic materials have what are called inversions. These are  points at which the ceramic has a quick change in expansion both on the heat up and cool down.  The two major ones are cristobalite inversion temperature at around 225°C and the quartz inversion at about 570°C.  The Crystobalite inversion is a sudden change of about 2.5% and the quartz is a sudden change of 1%.  These are very sudden and dramatic changes in comparison to the average of around 0.1% over the temperature range of 570°C to 800°C.  The crystobalite inversion does not occur until ca. 225°C.  This means that the whole structure of the ceramic is contracting less than the glass – exhibiting a CoE of ca. 66 rather 90 to 96.

Ceramic drape mould from Creative Glass Guild

Glass

We are used to saying glass expands and contracts at a standard rate, depending on the glass this may be a CoE of 83 to one of 104.  This is not the case.  The coefficient is an average calculated between 20°C and 300°C.  If you change the temperature range, the coefficient will also change.  And if you look at the range 570°C to 580°C you find the CoE is around 500.  This means that as the glass cools into the annealing range, it is contracting about 7 times faster than the ceramic. 

This dramatic difference in contraction means that the glass is attempting to crush the ceramic by enclosing it tightly.  Sometimes it does it so strongly that the strength of the glass is exceeded, and it breaks.

Possibilities

It is possible to drape over ceramic in certain conditions.

Influence of draft

The term “draft” indicates the slope of the sides of the form.  The steeper the sides, the more likely the glass is to trap the ceramic mould.  To be useful, the draft of the mould needs to be sufficient for the glass to slide upwards on the mould as it cools. This means the mould needs smooth sides and be well covered with a separator.

Compensations

You can compensate for steep drafts by wrapping the ceramic form in 3mm refractory fibre paper.  You can bind this with high temperature wire to ensure it stays throughout the firing. The fibre paper can be compressed and so provides a cushion between the rapidly contracting glass and the slowly contracting ceramic.

These need a circle of 3mm fibre paper over the open top of the kiln posts that have no draft at all before use. Of course, they need to have a circular piece of fibre paper over the hole in the post.

The use of ceramic forms to drape over requires care about the draft of the ceramic or addition of a cushion to avoid the greater contraction of the glass than the ceramic grabbing the mould so tightly it cannot be removed.