My first forge, 12 gal. adobe

Be very careful around the fumes coming off the tub. You could get what my grandfar called "monday Morning Sickness."

burning Zinc can cause "metal fume fever" 

The tub is galvanized aka zinc coated. white smoke coming off of Zinc can cause "metal fume fever"  if it gets hot enough. I would replace the tub with something else if I were you.