Red underglaze, no less. And orange.
Fellow potter Nate Philbrick - lately seduced away from clay by the promise of actual cash money to be made lobstering - used to use a couple of AMACO underglazes to cone 10, with brilliant results. That surprised me, and I made a mental note of it, for possible use at a later date. Nate used them for tiny accents, with no glaze, and on the exterior only.
Hot shades notoriously burn out, and despite the success in the
stoneware kiln, I figured soda vapor would vanish all traces of color.
Expecting exactly nothing, I brushed a little bit on a test piece. And,
hey, WOW, red!! and orange!! Brilliant, almost-true color.
As you can see, I had a hell of a time getting a non-blurry photo of this tiny little piece; but it's enough that you can see the color, and a bit of the sheen, from the soda glass. The orange was shinier than the red but neither was entirely dry.
It bears repeating: exterior only, and I'm gonna guess it would be a less-than-pleasant texture over a broad surface. But for details? Delightful.
The specific underglazes are AMACO 389, Flame Orange, and AMACO 388, Radiant Red, and they were applied after bisquing. (I think that matter but not sure. )