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Sunday, July 5, 2009

Another glass bead melt

At a local SCA event this weekend we set up two bead furnaces - the standard oval and one of the newer teapot style. We spent the day Saturday working with new people giving them a chance to try making a bead in the oval, while squeezing in a little time to try a couple of things ourselves.

Quick summary
- two annealing areas - between the chimneys & the crucible in the side. The pot on top reached a temperature of 695C (1283F) while the crucible reached 598C (1108F). Both of these temperatures are well above the 900F we need for annealing. In fact we had a problem with one bead (it picked up a lot of the vermiculite we had in the pot) this might have happened due to the glass getting too hot. This needs to be retested with ash as the insulator rather than the modern vermiculite.

- Much better success rate - 25 beads survived, 13 broke. Given the number of first time bead makers this is pretty good. It will be interesting to see what happens when we can do a run with just experienced bead makers for a day. I'd like to see a higher overall number of beads and a higher success rate.

- Maintaining a useful temperature is HARD in these furnaces. A charcoal load gives time to make a single bead at each side. Tiny changes in the direction of air inflow can make big changes. We need a method of slowing adding charcoal to try to keep a more consistent temperature rather than adding a bunch waiting for it to heat up, the making a bead and repeating. We also need to track the stability of the temperatures in the annealing area rather than just the single point reading I snuck in.

- The teapot furnace had even more difficulties. The fuel load was small enough, and the airflow directed enough that the fuel in the middle of the base burned off but the fuel around the outside wouldn't move in. This implies a cone shaped bottom to help the fuel flow better. In addition the crucible in the spout was not a great fit meaning more heat escaped around it than came out the chimney when it was intended.

- In the oval furnace the smaller chimneys meant working in the chimney was more effective than working in the ports - at least when working with rods. The tesserae require a different approach.

- Darrell had some real luck working with tesserae creating 3 good sized beads that survived. We think we have a solid working method for this, now we just need to test it further.

- I am still troubled by the relationship between the archaeological remains and the remains after we are done. The Ribe pads seem to be flat pieces, finished (not with remains of walls attached to the pad. Yet even when we don't deliberately join the base to the walls we when we break down the furnace we still don't reproduce the archaeological remains. There are clearly some construction details remaining to work out.




Blogger Kathleen said...

What about a third door, on the side, to add charcoal as you go?

July 6, 2009 11:08 AM  
Blogger Neil said...

The crucible on the side opposite the bellows can be lifted out leaving a slot to drop in charcoal.

The problem is to add charcoal (cold) without causing the furnace temperature to crash before the new charcoal starts burning and helps bring the temperature back up.

Each opening allows heat to escape so you also want to limit the openings.

This "fire control" is one variable we are working with pretty much constantly.


July 6, 2009 12:25 PM  
Anonymous chimney liner said...

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October 7, 2009 12:22 PM  

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