Before choosing a ceramic shape to use in draping of glass, you need to consider the characteristics of the two materials. This is one circumstance where CoE is actually useful.
The expansion of the two materials is different. Soda lime glass typically has an expansion rate - in the 0°C to 300°C range - of 81 to 104. Ceramic has an expansion rate - in the 0°C to 400°C range - of 30 to 64. This is important in the final cooling of the project. As the glass expands more than the ceramic on the heat-up, so it also contracts more during the cool. This means that the glass will shrink enough to trap the ceramic or even break if the stress on the glass is too much.
The shape of the ceramic form will have a big effect on the usability of it as a mould. Ceramics with right angles between the flat surface and the sides will not be suitable for draping without modifications or cushioning. The forms suitable for draping need to have a significant draft to work well.
Ceramic forms such as rectangles, cubes, and cylinders do not have any draft in their form.
|A cube shape unsuitable for draping|
|Ceramic cylinders with straight sides|
|Although rounded at the base, the sides are too straight to be a draping mould|
The glass will contract around these forms until they are stuck to the ceramic or break from the force of the contraction around the ceramic.
You can experience this trapping effect in a stack of drinking glasses. Sometimes one glass sticks inside another even though there is a slope (i.e., a draft) on the sides of the glasses. This happens mostly when you put a cold glass inside a warm one. On cooling the warm glass contracts to trap the cooler one. You can separate these by running hot water on the bottom glass, so that it expands and releases the inner, now cool, one.
Effect of Shape
The ceramic contracts at about half the rate the glass contracts (on average), unlike steel which contracts faster than the glass. This means steel contracts away from the glass, while the glass contracts against the ceramic, on the cooling.
Because the glass is in its brittle or solid phase during the last 300°C to 400°C, this contraction tightens the glass against the ceramic, causing stress in the glass, even to the point of breaking.
However, if you choose ceramic forms with significant draft, you can drape over ceramic. This is possible when the slope is great enough and the form is coated with enough separator, to allow the glass to slip upwards as it contracts more than the form. Experience with different draft forms will give you a feel for the degree of slope required.
Compensation for Lack of Draft
You can compensate for the insufficient draft of ceramic forms by increasing the thickness of the separators for the form. The hot glass will conform to the hot ceramic, so there needs to be a means of keeping the glass from compressing the form while cooling. This can most easily be done by wrapping the form that has little or no draft with 3mm ceramic fibre paper. It is possible to get by with as little as 1mm fibre paper, but I like the assurance of the thicker material.
|Kiln posts wrapped in 3mm fibre paper and secured with copper wire|
The fibre paper can be held to the form by thin wire wrapped around the outside of the fibre paper. The advantage of the 3mm fibre paper is that the wire will sink below the surface of the paper. You can tie off the wire with a couple of twists. Cut off the ends and push the twist flat to the fibre paper to keep the glass from catching onto the wire. If you want further assurance, you can put a bit of kiln wash onto the wire.
The choice of ceramic shapes to drape glass over is very important. It needs to have sufficient draft and separator to allow the glass to slip upwards as it contracts more than the ceramic during the cooling. You often can use items with no draft if you wrap fibre paper around the sides of the form.