The initial experiments in this area were framed by the existing clay 'based plates' found at Ribe Denmark. It should be noted that although there is evidence of glass bead production at a couple of Norse trade / urban sites, there has not been a single complete furnace found. Experimental furnaces we have constructed using these artifact footprints have never been truly effective for actually making beads. There may be some other purpose for the clay slabs (?)
There were a number of continuing problems with the large oval furnaces, as suggested by the artifact bases :
1) Very short effective temperature cycles - in the range of 10 minutes
2) Side ports proved very difficult to work inside - limited space and heat onto hands
3) Top ports proved less than ideal - too large an area for effective control of glass
4) Continuing problems with ash coating surface of beads
5) Large internal volume required considerable charcoal expenditures to operate
6) The top ports were quite effected by any cross winds (operated outside)
Taking my experience with charcoal forge fires, and what we have learned from the much larger charcoal fired iron smelting furnaces, I had suggested this as a possible effective layout:
We tried out a couple of early versions of this system, back in 2009. These were abandoned, mainly because they do not conform to the known artifact 'bases'.
The concept here is that all the exhaust gasses are bottled up and forced out of a top vent hole, which in effect creates a working space much like a more modern torch flame.
Returning to this operating system, Neil constructed a new prototype, based some new observations and suggestions from me, back in mid September. This furnace had been air drying for several weeks. Sunday's workshop saw it fired and operated through several charging cycles:
(images below by Neil Peterson)
|Layout of the Furnace : about 30 cm OD. Annealing pot to left.|
|Lid removed, filling with charcoal. Electric blower used for this test series.|
|Fresh charcoal vents off combustible gasses for about 5 minutes.|
|Stable working flame, working a simple glass bead in the 'stack'|
|End of a working cycle, charcoal has burned away from centre to base.|
1) Effective operating cycle in the range of 75 minutes (!)
2) Higher working temperatures at the upper port
3) Narrow flame created more effective control of the glass itself
4) Ash greatly reduced (mostly absent)
5) Significantly lower charcoal consumption
6) 'Time per bead' rate greatly reduced
This all creates one of the classic problems in experimental archaeology : 'If you can't get the same results, you can't be doing the same thing they did' vs 'That certainly works - but it does not match the available artifact evidence'.
In the actual absence of any complete furnaces (or even upper fragments) from VA sites, my gut feeling is that the few surviving 'bases' may be from some other process entirely. Annealing pans is one possibility.
Our research and experimentation continues...