Using a fire horn burns 15 sulfurous ash. I wonder if this was intended because i remember reading that 4 sulfourus ash were required for each blow.
I've long played bards but never dealt with these before so I looked them up. Fire Horns, Slayer and exceptional instruments all came out with Publish 16 which was after Renaissance, so I'm not sure that info from when they were first introduced matters any more than current info on them. Current info says that they take 4 sulfurous ash to use.
These items are currently under review, and are being researched for for proper function and existence.