Glass Sticking in Cast Iron Moulds

Cast iron bakeware moulds have achieved popularity in decorative glass casting. One problem that seems to be common is that the glass sticks in the mould when cooled.

 

A typical cast iron heart-shaped baking mould

Choosing

 When choosing the mould, try to avoid those with vertical sides. The glass will come out more easily if there is a slope from to top to the bottom of the mould cavity. But it doesn’t stop there.

 The surfaces of cast iron moulds are rough. In casting, the glass conforms exactly to the mould at higher temperatures. On cooling the iron contracts more than the glass, making "sticking" more likely.

Preparation

 Mould preparation should include grinding down the high points to make the mould surface smooth.

 Preparation should also include seasoning of the mould. Clean well with soapy water. Dry. Put a little mineral oil on a paper towel and wipe all the surfaces. Place the mould upside down on short posts. Fire to 300°C/570°F with a 30-minute soak and then turn off. The oil will burn off. You can place fibre paper underneath to catch any excess oil you may have put onto the mould surfaces.

 If you are using kiln wash as your separator, mix it thicker than usual, say 3 parts water to one of powder. This is to give a thick coating of separator on the mould. It may be that you need to heat the mould to avoid runs.

 If you are using boron nitride, it may be possible to add more layers. But this runs the risk of the separator coming off onto the glass.

 Firing

My final suggestion is to use lower temperatures combined with longer soaks.

Releasing

 If the glass still sticks to the mould, turn the mould over. Support the mould with brick or shelf pieces. Tap the back of the mould with a rubber mallet. Not too hard because cast iron is brittle. This most often shifts the glass.

 If the glass is still stuck, put the mould in the kiln upside down on posts a little above the shelf. Fire slowly (say 125°C/225°F) toward 540°C/1000°F. Program the anneal and cool you used previously. Observe frequently to know when the glass has fallen out of the mould. When the glass has separated from the mould, advance to the annealing segment.

 Of course, if the glass has fallen out of the mould by 400°C/750°F, you will not need the anneal soak, although you still will need the controlled cool. So, you can skip the anneal cool and go to the controlled cool down segments.