Fuel Use
This was heavy for the first few days until we cut down the air supply. We are now approaching 12 kg/hr using Opepe, though it will take some more adjustments to the air inlets and hot gas outlets before we regularly achieve this!

To fire the furnace to an average working temperature of 1024°C, we used 1610 kilos (1.61 tonnes) of wood over ten days, at an average rate of 14.37 kilos per hour, or 161.0 kilos for each average 11.24 hour firing day.

With the stoke hole inlet unblocked, we burnt 18.43 kg/hr of Opepe over one day, at an average temperature of 1014°C. With the lower stoke hole inlet blocked, we burnt an average of 12.54 kg/hr of Opepe over four days, at an average temperature of 1029°C . With the lower stoke hole inlet blocked and the upper stoke hole inlet partially blocked, we burnt 12.51 kg/hr of Opepe over three days, at an average temperature of 1027°C. This shows the value of regulating the air supply.

Over the ten-day firing, we mainly kept the temperatures below 1050°C - this was substantially exceeded only once in order to help to refine the coloured glass cullet that we had in at the time.

To fire the furnace up to 1000°C, we used mixed wood species, and for glassblowing, we used Sycamore for the first three days, replacing this with Opepe for the rest of the firing.

Furnace Design and Firing
It proved to be an easy furnace to fire, and the inclusion of the ceramic fibre blanket in the walls, reducing the rate of heat loss made temperature control very simple. The insulation also kept the furnace from cooling rapidly, resulting in early morning temperatures around 200°C. This was, unfortunately, not hot-enough to stop the pots from cracking, but it did mean that the furnace climbed to 1000°C in a couple of hours or less.

During the firing, new vertical and horizontal cracks appeared in the inner wall: a few smaller ones on and near the ceiling, and larger ones at and below the shelf level. They appeared to exploit weak points associated with the holes for the shelf supports and were caused by the wall not being able to move inwards as it shrank.

The cracks on the outer wall also grew and multiplied, but none appeared serious enough to threaten the integrity of the furnace.

Over the ten days, the firing chamber floor became peppered with solidified lumps of glass which dripped from the shelves and combined with ash whilst molten, and which will be difficult to remove.

For clearer photos of the damage to the interior of the furnace, see the section on the Repairs after Firing Two.

The slot between the shelves allowed all of the waste gases into the melting chamber with no problems, and we experienced very little - if any at all - burnback.

The shelves themselves were successful, neither cracking nor moving, although two of the small brackets for the support bars broke away. The shelves suffered from glass spillage, and the ceramic fibre cloth offered very little protection.

The four top holes were useful, but did spend much of their time closed or partly-closed to preserve heat for the gathering holes. The hotter area tended to be at the rear of the furnace.

The stoke hole and tunnel did not need to be lengthened, with Steve concentrating the burn at the very end of the tunnel. Excess air was reduced by stopping up the lower inlet (below the grate). This lessened the attack on the glass and lowered the rate of fuel consumption. Stopping the upper air inlet and stoking hole also lowered the excess air and perhaps lessened the glass attack even more. More work needs to be done on controlling the amount of air and reducing the amount of wood burnt - something we address in Firing Three.

Glass, Glass Attack and the Lehr:
A lot of glass was made (about 275 vessels in total, all of which survived). The bloom was less than that when using the previous furnace, and lessened towards the end of the firing, mainly due to the changes we made in the furnace and lehr:

• Blocking the lower stoke hole inlet on the furnace. This cut down the bloom.
• Partially blocking the upper furnace air inlet and stoke hole. This appeared to have a slight positive effect.
• Lowering the lehr temperature to 420°C throughout the day for one day. This only caused us to lose some vessels as the glass was below its strain point.
• Isolating some vessels from the atmosphere of the lehr by putting them inside an aluminium ‘box’. This had no effect (due, unfortunately, to there being a large gap between the lid and the body! See the lehr photos below.)
• Only firing the lehr with Opepe for one day. This made no difference, so we went back to using mixed wood.
• We also experimented with not completely closing the lehr stoke hole overnight as one possibility was that the atmosphere became too reducing. This did not make a difference, so we went back to closing it off completely.

The glass all went into the lehr in what appeared to be a fire-polished state, but many had a slght bloom on them the next morning. Those which had not been reheated in the furnace (mould-blown beakers) had no bloom or surface attack. Those which had minimal reheating had only the slightest mistiness, if any. The goblets we made each day all had a slight bloom, but this decreased during the firing - connected to the blocking-off of the air inlets. We allowed one goblet to cool down out of the lehr. Although shiny when finished, one could feel a slight resistance when rubbing the outer wall of the goblet when it had cooled, and there was a slight bloom. It seems that the time in the lehr does not add to the bloom effect.

It does appear that the glass attack occurs in the furnace, with the excess air reacting with the sulphur to produce sulphuric acid, which attacks the glass. This attack may be developed in the lehr, but does appear to be present even on unannealed vessels purposely left to cool down naturally. So, the less wood burned and the less air in the melting chamber, and the less the length of time spent reheating the vessel, the less the glass attack.

We had no problems in firing the lehr, and as discussed above, the lehr does not appear to contribute towards the bloom on the surfaces of the glass vessels.

Following the success of closing down the air intakes on the furnace, in Firing Three we will try narrowing the lehr stoke hole to let in less air. (This will also cut down heat loss by radiation.)