Rice straw ash (not to be confused with rice hull ash - hull ash has a different chemical composition) is a major component of Japanese ash glazes.
I think rice straw ash is an incredibly good solution to several problems. Rice, like many other plants in the grass family, is high in silica content. Since rice is a common crop in Japan, the rice is a waste byproduct and is therefore readily available for use in pottery. Rice straw ash is also a great glaze ingredient because the silica particles are small and will melt well in the glaze. In contrast, European glazes use the mineral flint as the silica source, which requires grinding. To summarize, Japanese potters have easy access to a very cheap and high quality source of silica.
Compared to using flint as a source of silica, rice straw ash is markedly less pure. Its main contaminants are K2O, Na2O, CaO, and MgO fluxes; iron; and phosphorus. Plant ash is also much more variable - the chemistry will vary from crop to crop. Whether these differences are deficiencies or advantages is up to you.
Due to its phosphorus, iron, and high silica content, rice straw ash is a great ingredient for making Phase Separation glazes.
Rice straw ash is black. If a raw glaze is black or gray in the bucket, it may be a rice ash glaze.
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