Anyway, you take mustard seed, soak it for two hours, then grind it up, pat it into a lump, put a coal on it, and dump some water with natron over it, dump that, and mix with vinegar.
I took mustard seed and soaked it for two hours, couldn't find my modern mortar and pestle, ran it through the coffee grinder, which sort of worked, didn't feel like dealing with coals at ten pm at night, poured a bit of water with baking soda and salt over the stuff and rinsed it, and mixed in some vinegar.
HOLY SHIT THIS STUFF IS SHARP.
So I have an iteration of Roman mustard.
The coal and water and natron thing is supposed to leech out the sharpness of the mustard. And it;s very, very coarse, because the coffee grinder was not feeling the soggy mustard seed thing. I am going to get myself a molcajete in the next couple weeks and try again. This particular recipe is for common, every day mustard, and Columella has further instructions on how to make banqueting mustard (mixing in ground almonds and pine nuts, I believe).