You're looking at a lot of very cool variables:
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- local clay, so there may be iron or something within the clay that is assisting with the results that he's getting
- for example, those brown spots might be iron in the clay
- ash glaze, so once again, the matter that he's getting the ash from (hardwood? softwood?) and impurities might be adding things
- soda firing which could include ranges of temps and additions to get the results that he has
He's got a website - why don't you send him an email and ask?
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You might also like to check out this book about soda firing:https://www.amazon.com.au/Soda-Clay-Fire-Gail-Nichols/dp/1574981676
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gorgeous bowls from her:http://www.ybrm.com.au/lightbox2/nichols.html
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What that what you were after?