Glass casting: part 2, going in

It is a while since I wrote about my first move into glass casting. Having frightened myself working out just how long (for that read expensive) the kiln cycle is for casting glass, I decided to fill the space with a couple of extra castings.

casting glassThe process of making the moulds was exactly the same as with the foot, except of course these moulds didn’t wriggle.

The first is an ammonite. I made one from metal a while ago on the forge and decided that I would like to make a glass one too.

The second casting takes a little more explaining. It is a ‘gold bar’. Using the forge as a heat source to melt lead, I decided it would be fun to cast a mock gold bar using some lead left over from the roof flashings for our extension (that was 10 years ago now). After that I cast one from aluminium too, as I liked the idea of the painted bars looking identical,¬†but providing a surprise when picked up. The glass bar adds another material to the set. I’ll show you all 3 later.

I have used clear glass for the foot and ammonite and a golden tint glass for the gold bar, for obvious reasons. The picture here shows the rough glass ingots in the moulds. To calculate the amount of glass needed you first pour water into the mould, for the foot this was 625ml. then multiply by the specific gravity of glass (2.5), giving 1.5kg of glass.

With the kiln at maximum temperature, the glass should then melt down into the mould, filling it perfectly to the top. The (next) tricky bit comes in calculating the time to anneal the glass after melting. In this case it is 4 days of reducing the heat by 2 degrees an hour. Even then I am told that the stresses can easily cause cracks.

I am a little nervous about this one, partly because of the time it has taken to prepare the moulds, and partly because I have a feeling I know what the electricity bill will be for 5 days of kiln firing. if it works then that is all OK, if it doesn’t…Wish me luck.

Glass casting: part 1, the mould

As mentioned in a couple of earlier posts, I have been playing with glass casting as an extension of fused glass work.

I was inspired by a walk along a wonderful beach in Noordhoek, South Africa. The beach is simply huge, with wide expanses of sand. The wind can be low and harsh, making it feel like your feet and shins are being exfoliated. Anyway, whilst walking along the pristine sand I was hit by the ‘leave only footprints’ saying, and that gave me the idea for my first glass casting.

Much research later and I was ready to have a go.

What I wanted to produce was a glass foot with a small surround, as if I had poured the glass into the footprint in the sand. This makes life a little more difficult, as it meant that I needed to go through 3 stages to get the final kiln mould.

algenate castingStage 1 involves making an impression, in alginate, of a foot. My version of the sand footprint. For this my wife ‘volunteered’. Alginate goes off in a matter of minutes so you need to work quickly. It is mixed from powder and put into a tray. add the required foot, wait 5 minutes and carefully remove. The result is a perfect impression.


DSC04818Stage 2 is to pour wax into the alginate impression to produce a ‘positive’ wax version of the foot. This was then trimmed to replicate what I want the final glass artwork to look like. I managed to make 2 wax impressions before the alginate disintegrated. But from here on it is all one use, so no mistakes allowed.



wax mouldIMG_20160304_154313Stage 3 is to produce the final plaster mould. For this I am grateful to the selfless people who share their experiences on YouTube, particularly Di Tocker, whose videos I have found extremely useful and informative.

Having sourced silica flour and plaster (much cheaper than the ready mixed packs available), I was ready. The wax foot is placed in a tray. The foam is just to reduce the amount of plaster needed. Mix equal amounts of water, silica flour and plaster. Pour over and there we have it. I also added a sheet of wire mesh into the top layer of plaster to add strength. Leave to set for about a day.

plaster mould glass castingWith the mould turned out of the tray, and 2 days in the bottom oven of the aga to melt out the wax and dry the mould, at last I have my final mould ready for the glass.

That’s where I leave this stage. I need to do some more research on kiln programmes, as the annealing stages appear particularly sensitive on large cast items. I must get it right. This is a one use mould, so no second attempts.

Postscript: initial research suggests that I will need to run a 4 day programme, rather than the usual overnight, to prevent stress cracking. This is going to be expensive, so I might try to make a couple of extra small moulds up first to make best use of kiln space and cost.