That also raises the question in my mind if anyone still uses the open hearth "coal gas" method? Is it correct tom assume that the open hearth method was used by larger concerns doing higher production, and that the small, one man or so shops used hand held torchs? I have heard that Mercian is the last practitioner of the open hearth approach. Anyone know if that is still the case?
Just some observations for the few interested, I looked up "Coal Gas" here is a link:
http://www.zetatalk.com/
looks like its a hydrogen/oxygen mix. This has relatively low heating potential compared to oxy/acetylene which must necessitate the fire brick hearth to retain heat.
One more interesting thing, at least to me is that often, when brazing silver or gold a brick of charcoal can be used to position the piece. This charcoal produces a reducing atmosphere during brazing, reducing the need for flux and or giving a better result. Might the charcoal fire hearth technique accomplished a similar thing?
Dave Bohm
Bohemian