Kiln Melting Glass over Ceramics

close up of Amber sea glass melted over textured brown ceramic object

I have been experimenting with melting found beach glass and recycled wine bottles over my ceramic sculptures and here are a few things I’ve learned along the way.

Less is more.
It doesn’t take a lot of glass to create transformative effects. Depending on your project, adding small amounts of crushed glass can add little pops of crackled, raised colours over the surface. It requires a bit of testing, as recycled glass can vary in chemical composition.

What temperature does glass melt at?
Glass will generally melt at around 800 degrees Celsius (1472 F) but slumping temperatures – where the glass is only softened but not to a liquid state, can be even lower at 650 C (1200 F).
As a small batch artist, I try to be sustainable with my kiln use which is why I tend to melt my glass at much higher temperatures of up to 1000 C (1830 F). That way I can mix and match my glass firings with other lowfire works. It also means I’m allowing the kiln to get hot enough for my lowfire glazes to activate during the same firing – I will often use glaze to help hold the pieces of glass in place and this can lead to some interesting chemical reactions. If you’re working with stoneware, porcelain or don’t want to melt the glass at such high temperatures, an easy solution would be to first fire pieces with your desired glaze, then refire at the lower temperature with glass only. You could try using Elmers glue to keep the glass in place or simply balance it on the surface.

Will it be food safe?
No. Melted glass will crack and craze heavily as it cools. These cracks are a harbour for bacteria. This is because the clay, glaze and glass all shrink at different rates. This also means there is risk of your pieces cracking, especially if you use clays with very high shrink rates. I have used both earthenware, terracotta and stoneware clays with success. This crackled effect looks great as it pools in the base of a bowl or drips down a vessel but the resulting piece will become a non-functional, decorative-only object. There is also risk of the glass chipping off or some areas remaining a bit sharp. You can fix any sharp areas by very carefully wet sanding and then sealing with a thin coat of clear varnish to help hide any sandpaper scratches.

Do I need to remove the paper labels from glass bottles?
I personally don’t and haven’t seen any issues with my glaze finish. The paper seems to disintegrate and doesn’t leave any residue. If you’re worried about contaminating other works in the kiln or using a shared studio kiln, then the labels can be removed by soaking them in warm water.

Melting glass will make a mess!
I love making a mess and encourage everyone to experiment with different materials in their kiln… however, kiln shelves are expensive! To protect your shelves, I recommend using high-grade silica sand to protect your kiln shelves from gloopy glass drips. It’s cheap and very effective (and can also be used instead of grog in clay bodies).

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