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Kitty litter, ashes, and water.
Ad in some straw or something els fibrous for better cohesion.
Dirt cheap, and works very well.

You need fire bricks for baking pottery.
Or rather, you don't 'need' them per se, but using firebricks to make an oven for baking pottery is much easier than various other ways.

(hack)saw blades?
I can think of plenty of things, but all off them are going to be more expensive than a piece of O1.
Thin pieces are very cheap to mail, so a piece of O1 or 1084 would be fairly...