Potash, if my chemistry serves correct whould have been obtainable and is used as a source of potassium in explosives. Potassium does have a naturally high burning temperature.
If the druids did indeed have advanced knowledge of chemistry, would they be able to perform vitrification as a "sign" from the fire God.
Camulodunum - Almondbury - vitrified fort - means (I think) castle of the fire god.