Crystalline Glazes
All of my pieces are hand thrown on the wheel by me using porcelain or fine white stoneware clay.
Crystalline glazed ceramics is time consuming and expensive to produce, due to the time, process and materials used.
There can be many failures when producing crystalline glazed ceramics, but when it all goes well the results are amazing.
The crystalline glaze is mixed using various ingredients, the base of which is usually a Frit, Silica and Zinc oxide. Then colouring oxides are added.
Crystalline glazes are very runny when fired to the top temperature of 1260c - 1280c, so the glaze is applied 4 - 5mm thick at the top of the piece and thinner at the base.
Because of the runny nature the glaze, a glaze catcher is made to exactly fit the base of each piece. This will catch the glaze run off, so the piece does not stick too or ruin the kiln shelf.
The piece is fired in the kiln to 1260c -1280c, then rapidly cooled to between 1020c and 1140c. The crystals form in the glaze between these two temperatures, so is held between these temps for 2 - 8 hours to allow the crystals to grow.
Depending on where the temperature is held, different shape crystals form. Round crystals at the lower end of the temp range, axe heads and spikes at the upper end. Fluctuating the temperature in the crystal growing range can produce halo rings in the crystal.
Once the piece has cooled and been removed from the kiln, the glaze catcher is removed by heating around the joint with a blowtorch.
This cracks the seal and with a slight tap the catcher comes away. This leaves a sharp edge around the base of the piece, which is then removed using a diamond grinding disc.
The piece is then complete.
Some pieces go through a further process of acid etching. This is when the piece is put into an acid bath for 24hrs which leaches some of the colour out of the crystals and makes them stand out more. The piece on the left is a example of this.