Why has my transparent glaze fired ‘milky/cloudy’?

Glossy Transparents:

This is caused by devitrification which is the formation of crystals, particularly of calcium borate, which sometimes occurs in transparent glazes in the first 200°C of the cooling cycle. It especially occurs where the glaze is more thickly applied, and is often seen around handles or indentations where the glaze gathers. Using a cooling ramp to slow down cooling will allow time for crystals to grow, so it is not recommended where this problem is present.

Red clays are particularly prone to this problem because of higher calcium levels: The calcium in the body combines with borax in the glaze to form calcium borate crystals. Lead-containing glazes (low-solubility lead frits) do not have high levels of borax so are less prone to this problem than leadless glazes where borax frit is the primary flux. Firing the clay to a higher biscuit temperature will make the calcium more inert so less likely to react with borax in the glaze.

Milky appearance can also occur if the glaze is a little underfired, and areas of thicker glaze application may not have had sufficient heatwork to become properly mature. Re-firing can sometimes correct this.

Matte Transparents:

A shiny transparent glaze is like a piece of window glass. If you sand-blast the surface of the window glass it will appear less shiny and also more difficult to see through. Therefore, it is very easy to produce a translucent matt glaze by encouraging crystals to grow in the surface of the glaze, but these crystals will impede the transmission of light through the glaze, just like the roughened window glass. Hence the transparency of the glaze will suffer, and the less shiny you make the glaze – the more opaque it will become. It should therefore be described as 'translucent' rather than 'transparent' because it allows diffused light to pass through and not direct light.