Coal or wood. I just think it was the fuel source, was not intended to impart a flavor any more then gas or electric would be expected to. Did it? Probably, when compared to the latter 2, but I guess you have to look at the American History of the Pizza. I think Lombardi's in New York was the 1st to do coal and I think that was 1895 or 1905, can't remember. I would venture to guess that a gas fired oven was not even invented at that time, but I maybe wrong.