Thanks guys!

Snuffy has what I came up with. The fact that there was too much iron seems so small on the one hand -- only 100 extra grams in a 10,000G batch .... but since it's percentages, BOY does that alter it, eh?

....and Cluin, you're right -- too much pottery$$$$ to risk. What I do is mix the batch and fire one piece to make sure it's right.

Thanks again!