Dale,
The whole "open-hearth" mystery is really interesting. Maybe you could contact Bill Betton at Mercian and interview him about their methods. The Mercian I recently ordered is pinned at the bottom bracket so I assume they are indeed still using some take on the open-hearth method. His e-mail address is:
billbetton@merciancycles.com
Jim Cole Memphis, TN
<< It's not an "open-hearth" method being described at all. It's a air/coal-gas torch that's being used to heat the frame and it's sitting on a hearth full of "heat retaining materials" Firebricks. >>
I understand that that IS what an open hearth method is. They build this brick semi-surround (hearth) and then have a static, held-in-place torch that has a big soft flame heating the brick and anything in close proximity...
The frame section (head tube, bottom bracket, etc.) is pre-assembled using pins to hold the parts together, and then placed in the flame path on the hearth and brazed. The entire frame area is heated up uniformly rather than in smaller areas as with hand held torch brazing.
The result of this method is a frame that has not been held in a jig to tack braze, and which often therefore needs more cold setting to get aligned afterwards. This is why the hearth brazing is considered a more crude method than current practices.
OR a technique requiring more skill to get a top quality result than modern methods...
Dale Brown
Greensboro, NC