How ash glaze works and why it varies

If you've ever bought a ceramic piece with an ash glaze and noticed it looks slightly different from the one in the photograph, that's not a mistake. Ash glazing is one of the oldest ceramic techniques still in regular use, and the variation it produces is built into the chemistry. Understanding a little of how it works makes the differences easier to appreciate.

What ash glaze actually is ¶

Ash glaze is made by mixing wood ash. Sometimes from a single wood type, sometimes blended. With water and a small amount of clay or feldspar to help it adhere to the pot. The ash contains silica, calcium, potassium, and various trace minerals, and the exact proportions depend on what was burned: oak ash behaves differently from rice straw ash, which behaves differently from pine. This is why two ceramicists using 'ash glaze' can produce results that look almost nothing alike.

The glaze is applied to the bisque-fired pot, usually by dipping or brushing, and then fired at high temperature. Typically between 1250 and 1300 degrees Celsius in a UK studio context. At that temperature, the ash melts and flows slightly, which is why ash-glazed pieces often have subtle drips or pooling at the base.

Why the colour changes between firings ¶

The colour of an ash glaze depends on the iron content of the clay body, the atmosphere inside the kiln during firing (oxidation versus reduction), and the mineral composition of the ash itself. A ceramicist who changes their ash source. Switching from one local wood to another, for instance. May find the glaze fires to a noticeably different colour even if everything else stays the same.

This is not a problem to be solved. It's the nature of the material. The grey-green tones common in ash-glazed stoneware come from iron in the clay reacting with calcium in the ash under reduction firing. Warmer, more amber tones suggest a higher iron content or a more oxidising atmosphere. Both are correct. Both are the glaze doing what it does.

What this means when you buy a piece ¶

When a product description says 'glaze varies between pieces', it means exactly that. The piece you receive will be the same form, the same clay, the same glaze recipe. But the specific colour, the degree of pooling, and the surface texture will be unique to that firing. Some people find this frustrating. Most people, once they understand the process, find it the most interesting thing about the object.

If you're buying two bowls and want them to look reasonably similar, it's worth emailing to ask. Helen can usually check the current stock and describe the variation between pieces before you order.

Ash glaze is one of those techniques where the unpredictability is the point. The bowl you receive is the result of a specific combination of materials, temperature, and time that will never be exactly repeated.