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15 Oct 2016 - Experience and one Experiment

The dominant purpose of this day's work was to provide an Experiental Archaeology opportunity for a chapter of the SCA. Never giving up an opportunity, however, there was also a small design experiment going on in the background.

Title: A three pronged approach
Series: Matching Archaeology (ÆZ)

The furnace is based on hearth ÆZ from Ribe - a 30x60cm oval base. For this design the primary air inlet (a standard 2.5 cm I.D. port) was not run all the way through the outer wall. It was instead pushed half way into the wall, and three smaller air ports were put through the inner side of the wall. Since a 2.5cm I.D. inlet has a cross sectional area of 4.9 cm2, the outlets needed a similar combined volume. This called for 7mm radius for the outlet ports.

Specific Question(s):
  1. Will a one-to-three air inlet provide enough air to heat the furnace to the required temperatures to work the glass?
  2. Will the burn pattern on the base, following use, more closely match that seen on the archaeological remains?
  1. Build: Using the cobb mixed build the furnace to shape
  2. Harden: Allow the furnace to air dry
  3. Sinter: Using a wood fire, dry the clay, slowly raising the temperature as the water is driven off to sinter the structure to a ceramic
  4. Preheat: On the day of use - use wood to gently warm the body of the furnace
  5. Fire: open the top, load charcoal, close the top. Seal the top in place using a clay/water mix. Turn on air.
  6. Work: At temperature (c. 800C) make the beads.
  7. Recharge: whent he temperature drops too far to work glass, open the top, refill with charcoal, replace and re-seal the top and start again
  8. Cool: When work is done for the day allow the furnace to cool

The air inlet change was the only difference between this furnace and any number of others we have run over the years.

Exterior length of base50.8 cm
Exterior width of base27.5 cm
Exterior height to shoulder21.5 cm
Exterior height to top of chimney27 cm
Access door height above ground18 cm
Tuyere height above ground9 cm
Access door Length19 cm
Access door width23 cm
Chimney distance from end12.5 cm, 14.5 cm
Chimney diameter7x6, 7.7x7.5 cm
External Tuyere Diameter2.5 cm
Internal Tuyere Diameters1.6 cm

The three prong air inlet system appears to work well. The differing heat in the chimneys can easily be atrributed to different burn rates in the unsized charcoal. The three inlets were not well aligned on the vertical axis. We can likely improve the results through better design of the three-pronged approach.

The burn pattern in the bottom is much closer to the archaeological remains than our previous efforts with air splitters. There are two clear 'white' zones associated with the 'right' and 'center' air inlets. The 'center' one is small and close to the far wall. The 'right' zone spreads across the base from the near to the far side. This matches the physical setup as the 'right' inlet points 'lower'(towards the base) than the 'center' inlet. The 'left' side is a more 'grey' colour but still stands out from the black. It would be an obvious assumption that this presents different temperatures in the left and right sides. During the various burns, however, we noticed that both chimneys had moments when they were too cool, and others when they were both at functional temperatures. It is more likely we are looking at the results of differing ozygen levels during the time that the furnace was sintering. It is likely that we can address this by adjusting the air inlets through a separate experiment series.

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Charcoal Inlet ports End view Central view
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Open to reload Burn Pattern Overview of loading port Colours on the base
Inside air ports

Text © Neil Peterson, 2016
Photographs © Individual artists
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