Glazes · Mar 26, 2026

Iron spots, explained

Our clay is iron-bearing stoneware, and the iron is a big part of how the pots look. It is behind the amber edges on fog white and many of the dark spots you'll find on any piece from the studio.

Here is what happens. Near the top of a firing we cut back the air going into the kiln, which is called reduction. With less oxygen available, iron oxide in the clay changes form and starts acting as a flux, melting more readily. Particles of iron near the surface melt and bleed up through the glaze, leaving a dark spot, often with a lighter halo around it.

Iron spots are related to speckles but not quite the same. Speckles come from granules deliberately mixed into our speckled body. Iron spots can show up on either clay, from natural iron in the material, and they vary more: some firings give a scattering, some almost none.

They are sealed into the glaze surface, fully fired and food-safe. They are not rust and won't grow, stain or wear away. A dark spot is also not a flaw in our grading. A pinhole or a bare patch in the glaze is, and those pieces go to the seconds shelf.

If you want to see the effect clearly, look at the rim of a fog white Tide bowl. Where the glaze thins, the iron shows.