Ash Glazes

In the heat of a ceramic kiln, many materials burn or melt. Plant ash is one of the most traditional glaze ingredients and is widely used by East Asian potters. Classic glazes such as celadon, chun and tenmoku come to mind. In fact, it is difficult to name an East Asian glaze that does not use ash, or did not use it in its original formulation.
The composition of plant ash can vary with the plant species, its age and even whether it is healthy or diseased. Monocotyledons, such as grasses and rice, are richer in silicon, a refractory and glass-forming element. Dicotyledons contain calcium and magnesium, two fluxing elements.

Sake set by Michael Coffee. Unwashed ash glaze. Reduction firing to cone 10.

To prepare ash, the plant material must first be burned. However, only about 1% of the plant mass becomes ash, as much of it is consumed in the fire and releases heat. The non-combustible fraction, including basic minerals, remains as ash. These minerals result from the plant's absorption of the inorganic elements it needs to live. Ash is, so to speak, an extract of the essential elements supplied by the soil.
To use ash as a glaze, it is desirable for the kiln to reach at least cone 10, approximately 1,300°C. Rice ash, which is richer in silicon, requires even higher temperatures. Reduction firing is also particularly suited to ash glazes.

Preparation

Preparation is not complicated, but it takes work. You can burn the wood in a barbecue, if you wish. First scrape and thoroughly clean the inside so that rust does not contaminate the ash with iron. It is also best to burn only one type of wood: this gives you a better chance of obtaining similar results when you repeat the recipe. Burn it slowly to ensure complete combustion. This can take two to three days.

Burning the wood

Once the material has burned completely, separate the ash from charcoal and other impurities using an 80-mesh sieve. Caution: ash contains highly caustic substances that can enter the respiratory system and cause serious irritation. Wear a mask and gloves during this process!
After sieving, you can either wash the ash or use it as it is in a glaze. Washing removes soluble components that can make the glaze run excessively. Put the ash in a bucket, fill it with water and stir thoroughly. The next day, pour off the water and any impurities floating on the surface. The ash should have settled at the bottom. Repeat four or five times, until the water you pour off is clear. Then dry the ash on a plaster slab and store it for use in glazes.

Composition

The next step is to test how the ash you have prepared behaves after firing.
Here are some suggested test recipes from Brian Sutherland's book, Glazes from Natural Sources:

Test 1:

Test 2:

Note: weigh the clay when completely dry.

Conclusion:

Using ash offers potters a way to connect with some of the most basic natural materials. It requires very little processing and gives potters greater control over ceramic production. The variability of the material also offers a wide range of effects to explore.

Sources:

• Forrest, Miranda. The New Ceramics: Natural Glazes, Collecting and Making. University of Pennsylvania.
• Sutherland, Brian. Glazes from Natural Sources. University of Pennsylvania Press.