You’ve probably noticed, I love to experiment. When I first started out in ceramics, I spent hours researching glazes, clays, firing, glaze fit, everything. I love finding out about new techniques as much as I love putting them in to practise.
One of my early discoveries was that you could add glass in to your clay designs with beautiful effect. Glass is made from the same basic ingredients as ceramic glaze… silica, sodium and calcium. The main difference with a glaze is it contains alumina which will stop the glaze from running during firing. When fully melted, glass is extremely runny, so this has to be accounted for by creating wells in your design to contain the glass.
This technique works best at stoneware temperatures. At earthenware temperatures, the glass doesn’t become liquid enough and will create drops of glass rather than a pooled effect.
My favourite thing about this technique is the crackling that appears as the clay and glass cools. The refraction of light on the deep crackles makes your finished pieces glow like jewels.
Clay expands and contracts through the heating and cooling cycle. You may of heard of COE (CTE) when it comes to using glass, or the coefficient of thermal expansion. This is a measure of the rate of expanding and contracting during firing. The same principle is used for clay, you have to match your glaze to your ceramic body so that they work together. Without matching the two, you will get crazing or crackles through your work. With glass this can be used to your advantage.
I have recently rediscovered this technique while trying to make beads that look like rock pools. I’ve had an idea coming to me for a while of how to get the glass in to a bead rather than a cabochon, and have come up with nuggets!
First I made some beads in clay with a little well in the centre. They are then bisque fired.
I cut glass rods from my lamp working supplies and pile the glass in to the well. This takes a bit of experimenting to know how much to put in so that it doesn’t run over the edges and spoil your bead.
The beads are laid out on ceramic fibre blanket to make sure they are level. This is crucial… if they’re not, you will end up with a hot mess in the bottom of the kiln! They are then fired up to full temperature as you would using normal glaze.
And here are some of them finished…