You're welcome, it is from Newcastle College ceramic department. Not sure who came up with it. Do you have some pictures of the tests? 6% seems like quite a bit and will be changing the glaze chemistry as you can see. Could be if you are colouring the glaze with that much stain each one could need a slightly different recipe to have the right surface.
Have you tried using it to colour slip or paint on as underglaze and using a transparent over that? It could help the problem but maybe lose what you are looking for in colour.
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