You could try wedging in some granular ilmenite for the speckles (rather than toxic manganese). Personally, I prefer splattering a bisqued pot with a mix of equal parts RIO, Gerstley (or boron frit), and Neph sy. If the glaze is opaque, splatter over the glaze.

In the clay community, I don't post much. I enjoyed answering technical questions on FB forums. Where does one find something like that?