First Cast Glass

 Posted by at 4:08 pm  Ceramics
Sep 192012
Love that depth of character and complexity

I’ve wondered for awhile whether my old electric kiln could manage the long slow heat soak required for glass casting. Flat kilns with heating elements in the top are recommended for hot glass work because a) a lot of glass work is flat (plates, windows) and b) heating large flat pieces of glass from the sides isn’t very efficient.

But, if you take your time you can melt glass in any shape of kiln so long as it  can reach the required temperatures and hold at specific temps for long periods of time. Glass slumping, fusing, and casting actually require lower kiln temperatures than firing clay ceramics, but you have to maintain those temps a lot longer than for ceramics.

So one weekend I decided to give glass bottle deconstruction a try. Since this was primarily a test of material and kiln performance, the shape wasn’t particularly important. I grabbed a cylindrical mold I had printed in Hydroperm plaster on my Z406 3D printer. I print these little dishes up by the dozen to use as single-use crucibles for material testing in the kiln. Today that material would be recycled bottle glass.

I grabbed a green wine bottle from the recycling bin, wrapped it in a heavy canvas, and gave it several good whacks with a hammer.  (Hint: start by breaking the neck of the bottle first, then work up to the body).

I filled the mold with glass shards, standing most of the large pieces on edge to pack them in tighter and to reduce the chance of air bubbles getting trapped inside. Less than a third of the bottle’s glass went into this slug.

The kiln firing schedule for glass is very different from firing clay ceramics. Glass needs to be heated and cooled much more slowly than clay to avoid thermal shock. Glass can soak up a lot of heat energy, but it is a very poor conductor of heat. Most of my kiln firings for clay take less than 24 hours to ramp up and cool back down. For this small glass slug, the firing time was almost double that. Glass pieces that are inches thick at the core can require many days of kiln time, mostly for the controlled cooling and annealing soak times.

Molten wine bottle glass in a plaster mold. Temp is about 1500F.

After firing the mold becomes a crumbly powder – still strong enough to hold its shape against the weight of the glass, but a half-inch thick wall will easily crumble when pinched between thumb and forefinger. Demolding the piece is easy – just brush with a whisk broom.

After demolding, there were a few sharps that needed to be ground down. Most were on the top side of the glass, where glass had stuck to the side of the mold and left a thin razor edge shard sticking up from the edge of the disk. A few minutes at the grinding wheel took care of that.

The slug is a little more than half an inch thick. At that thickness, green bottle glass is nearly black except when strongly lit from behind. A thinner casting would show more color with ambient light.

Love that depth of character and complexity

I love the complexity of “stuff” going on inside the slug that’s revealed when held to the light. This would work well for sconces and other back-lit or interior-lit thingamabobs.

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