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2021 Furnace ('Phoenix') and Lehr at Quarley - Firepit Reconstruction

Projects: 2019 Onwards   Woodshed Build   The Woodpile   IFoG Furnace Removal   Firepit Reconstruction

Furnace Superstructure Build   Phoenix Firing One   Repairs after Firing One   Phoenix Firing Two   Repairs after Firing Two

Reconstructing the Firepit

Hover over the photos as more information is in the captions.

The firepit had deteriorated dramatically due to overheating the Roman tile fragments, and there were large buildups of glass mixed with ash on the surface. We had to strip away all of the old structure except for a small portion at the cold end of the stokehole, and replace it with modern materials more suited for the extreme heat: sillimanite firebricks and castable refractory concrete.

We took the pit down to the chalk, then built it up using a layer of builders sand over hardcore. We set the bricks on this, keeping the slope which was useful in the previous firings.


Excavation

Excavating the firepit revealed a few interesting points. The earthenware tiles had been seriously overheated, cracked and split, and a large spread of glass and ash had fused to them, making it possible to remove most of the remaining spread as one thick sheet. Steve had already removed some of this immediately following the disassembly of the IFoG Furnace, revealing the baked daub in plan and the glass/ash layer in section.

Below this was a thin layer of soil, greyish in the centre and more orange towards the edges of the firepit. This was originally placed there to give a smooth surface on which to lay the tiles.

Under this was a thick layer of hardcore, blackened in the centre by the anaerobic combustion of various pieces of wood, and brown-orange towards the edges. Two fragments of late 19th century bottle glass were found in the blackened area. They had not been altered by heat, leading us to conclude that the temperature had consistently been less than the softening point of the glass - around 560°C.

The chalk itself had been disturbed and scraped horizontal by a digger bucket a few years previously, but it proved a stable surface on which to rebuild the firepit.

Steve sectioning the glass/ash layer Chiselling the glass/ash away from the tiles The glass/ash layer in section
The firepit after the removal of the superstructure The firepit with the furnace superstructure in the background
Section through the glass/ash layer and the tiles, and showing some of the baked daub in front of the section The thin layer of soil under the fired daub. The central grey area is the result of organic matter in the hardcore below which has burnt in anaerobic conditions.
The glass/ash spread from the top The underside of the glass/ash spread, showing the fired daub and the tiles
The cross-section through the glass/ash spread The underside of part of the firepit wall, showing tiles and fired daub
A cross-section through part of the firepit wall, showing tiles and fired daub Below the fired daub
Below the fired daub Section through the thin layer of soil and the hardcore
Section through the thin layer of soil and the hardcore The hardcore three quarters removed
The firepit cleaned down to the top of the truncated natural chalk Two late 19th century glass bottle fragments. These were found in the hardcore immediately below the centre of the firepit. The lack of heat damage shows that the temperature in the hardcore was probably less than the softening point of the glass (about 560°C


The Re-build

To build the firepit walls we used cut-down arch bricks set on a layer of builders sand over a layer of hardcore, all supported by a daub retaining wall. This wall was repositioned to be under the bricks to allow room for the floor of the firepit.

In its turn, the sloping floor was built using hardcore and a layer of sand, on which were set the bricks. The gaps between the floor and the walls were filled with castable refractory concrete rated to withstand 1600°C. The concrete also held the walls in place.

We made the lower stokehole 9" wide, the floor sloping to 5" deep, the inner, parallel-walled section 21" long and the outer, flaring section 9" long, flaring to 18" wide. We lined it with sillimanite bricks, supporting them at the back with sand and Roman tile fragments held in place with daub, and underneath with castable concrete in what were to be the hot areas and ordinary sand/cement mortar in the cool areas.

The upper stokehole is 13.5" wide, and the tunnel is x18" long and 9.5" high from the top of the bricks. The tunnel construction is described on this page.

We made the grate under the tunnel from the ceramic fire bars which had previously been used as the pot support bars in the IFoG furnace, spacing them 1" apart with sillimanite brick offcuts. Three stainless steel bars from the previous grate made the outer grate. We spaced them 1.5" apart and set them into a short daub wall on each side as before. The grate was made so the fire bars would be easily removable, and was 5.5" above the stokehole floor to make raking out ash an easy task.

It took us fourteen days to rebuild the firepit and lower stokehole. It should be a far more durable structure, and we expect it to last for several years and to withstand many repeat firings.

Marking out the new firepit Some of the arch bricks ready for cutting
Mark cutting an arch brick Laying out the bricks
Laying out the bricks Daub retaining wall and hardcore foundation
Laying the bricks on a layer of sand The bricks backed with castable refractory concrete
Bricks and retaining wall Repositioning the retaining wall directly under the bricks
The retaining wall in its final position Ready for the rest of the bricks
Feel the stretch! Adding the refractory concrete backing
Repositioning the retaining wall The repositioned retaining wall
The firepit walls are finished A layer of hardcore as a foundation for the floor of the firepit
The first line of bricks - these are horizontal. More hardcore has been added to create a sloping floor. Planning out the rest of the bricks
Ready to lay the second line of bricks The first two lines laid
Ready for the third line of bricks Three lines now laid
The fourth line at the entrance to the ash hole has been laid The firepit finished with castable refractory concrete. The lower stokehole bricks and the grate temporarily in position.
The firepit finished with castable refractory concrete. The lower stokehole bricks and the grate temporarily in position. Detail showing the two sillimanite brick offcuts acting as stops for the first firebar. These will be concreted in position.
Detail showing the two sillimanite brick offcuts acting as stops for the first firebar. These will be concreted in position. The lower stokehole entrance
Building the support behind the lower stokehole wall Showing the daub wall retaining the castable refractory concrete supporting the bricks
Showing the sand, tiles and mortar behind the lower stokehole walls View of the lower stokehole
The completed lower stokehole The completed lower stokehole
The completed lower stokehole The daub wall supporting the stainless steel firebars

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