Fuel Use
To fire the furnace to an average working temperature of 1074°C, we used 1708 kilos (1.708 tonnes) of wood over ten days, at an average rate of 14.05 kilos per hour, or 170.8 kilos for each average 12.13 hour firing day. Although slightly higher than Firing Five (13.83 kg/hr), this was to be expected as we were running the furnace at an average temperature of 34°C higher.

For most days, as in Firings Four and Five, we raised the temperature to 1000°C using Chestnut, then switched to another wood (either Opepe or Sycamore), reasoning that as Chestnut was high in sulphur, not using it for blowing would be sensible. Unfortunately, we still had attack on the surface of the glass using these single species burns.

Although we used a variety of small, medium and large pots in this firing, we again allowed room for the flames and waste gases to flow through the furnace, lessening burnback.

The firing on the final day (Day Eleven) has been treated as a separate firing, and is recorded on this page.

Furnace Atmosphere
As we had not changed the methods of firing from Firing Five, and had not set up or trimmed the furnace differently, we would expect the furnace atmosphere to be similar to that firing.

After the first four days, the colour of the ash changed to a dark grey - probably due to a greater proportion of charcoal in the ash. This possibly indicates a more reducing atmosphere, but does not appear to be related to the wood species or the furnace temperatures.

Furnace Condition
The section on the Removal and Resiting of this furnace contains details and photos of its condition after this firing. They show that the furnace was in dire need of replacement!

Pot Filling
We tried a new approach to filling the pots in the morning: that of packing pre-made glass ‘cakes’, 3” to 4” in diameter and 7mm to 8mm thick into the pots when cold and heating them along with the furnace in the morning. The idea was to lessen the amount of small bubbles caused by using small fragments of cullet that trapped too much air. Instead, these were to trap large air bubbles which would quickly dissipate.

This approach worked, but we found that the cakes which were annealed tended to split and cracked on the reheat - in one case the explosions completely destroying a pot, whereas those which were not annealed tended to split much less. See Pots, Collars and Breakages for photos of this destruction.

Glass Vessel Surfaces
The hot glass surfaces were still attacked by the furnace atmosphere, even though we tried various ways of combatting the effect. These included running the furnace very hot - at 1100°C, raking out the ash every morning, burning single species of wood and reheating as little as possible whilst maintaining a calm furnace.

None of these trials worked completely, so we have concluded, reluctantly, that the furnace design is causing the problem: allowing sulphur and possibly too much flame to contact the glass surface during the reheats.

It is worth noting that we encountered very little of this problem at Stourbridge in 2019, and the only design feature we have introduced is the grate in the stoke hole tunnel. Again, we will address this problem in the rebuild, making it easy to cut down the air introduced into the tunnel, as well as introducing shelves to divide the furnace into a firing chamber and a melting / reheating chamber and sheltering the glass as much as possible from the flames.

The lehr caused no problems, and annealed the glass well. We tried one experiment: putting one goblet under a bell jar, so protecting it from the atmosphere, and left the next goblet exposed to the atmosphere. There was no difference: both had glass attack from the furnace. So it was not the lehr causing the problem.