I didn't notice the reply but --- I'm at the forge now and it still won't mix.
Right way: drag salt over coal, correct? Has to be the way really because, otherwise, one gets a big, red "X". The only thing that MAY have been different is that the ingredients were not in my hero's inventory but, instead, in that of the DK. When that didn't work, I put them on the ground (had a handy save) and tried it. I'll move them to the hero's inventory to see if that makes a difference but, my hopes are not high.
EDIT: Well, I'll be! It worked! It has to be in the hero's inventory and no where else. Darn! No cool explosion and lava flow through the citadel, though.