I made 4 similar mugs from the recycled clay and iron filings in a glaze called moonlight. I have learned that a little goes a long way for the iron filings, and next time I think I will use less. Most of the filings came through on the inside of the cup, rusting slightly but covered thinly by glaze. Does any one else have experience with using too much iron filings? Do you feel it causes any safety hazards to the average human?