Tim Willey

Adaptive Construction & Open Firing

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Progressive Open-Firing
Sometimes, particularly with clays that I’m unsure of, a much slower - progressive - open firing is needed. Unlike the firing stack, a progressive open-firing can be taken through the early ‘water-smoking’ stage of firing as leisurely as one likes. The principles are very simple: I start a small bonfire, then let it subside, down to glowing charred-wood, and then I warm the pots gradually with these embers. Below, I have designed and fabricated, what I call a firing grate, simply welded together from mild steel round-bar. This structure allows primary air and fuel combustion to get under the pots and so gives a little more control over reduction/oxidisation. It also enables me to slowly build up the embers, by having the initial fire slightly to the side and then shovelling in the embers gradually, underneath the grate and pots.
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A small bonfire is started near to the grate.
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Embers from the subsided fire are shovelled carefully under the grate and pots. This is a gradual process and might take 2-3 hours to complete - if a pot blows, I slow down! The adjacent fire can be refuelled as necessary.
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The preheat fire is then slowly built up with fresh (bone dry) kindling. Again taking as much time necessary.
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When satisfied that the pots are well over 200c the main fire is built up, first using dry Norfolk-reed bundles or similar material. This provides a good deal of protection from the subsequent application of branch-wood fuel.
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Bone-dry branch-wood is then added to the fire.
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The fire subsides and the pots cool. Here I have removed two pots whilst red hot, for post-firing reduction.
The progressive open firing, whilst not as exciting as the firing stack, does allow me to control that all important water-smoking stage (the removal of mechanically combined water) and it does enable clays, which would otherwise perish, to survive the firing intact.

However, a slow start doesn’t preclude a fast cooling and I’ve found that clays with high levels of quartz sand will put the clay under a different kind of stress as a result of the infamous quartz inversion! This will be covered in the page on tempering.
Below I’ve illustrated a simplified design of the firing grate (Mk2). It’s no more efficient than the round version but it is much easier to fabricate. This particular grate will be used for test firings so it’s quite small (35 x 35cm) with 15cm legs. I start with a welded square made up from 10mm mild steel bar and then weld in 5 rods evenly spaced (6cm centres giving a 5cm gap) and finally weld in the 15cm legs. I guess I could probably extend the area to 45cm square, using the same gauge of round steel bar and retaining the 15cm legs, but it does get very hot (approx.900c) so sagging could be an issue. For this firing I have used waste timber (bone-dry fencing stakes and thin fencing-panel boards).
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A fire is started on the grate using bone-dry timber.
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Embers from the subsided fire are encouraged to fall under the grate. To arrange the embers I use an old garden hoe which has a dampened wooden handle to avoid scorching.
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When the embers have died down till barely alive, the test pieces are placed on the grate. This is a critical time as some clay bodies might spall or explode if too hot.
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The fire is very slowly re-kindled with wood shavings and fine slivers of wood. This stage took about an hour, but it could be slowed down to suit the clay. Also, I’m careful to arrange the embers - spreading them out to quell the fire or building them up to re-ignite. Caution is needed at this critical stage. Again I use the hoe.
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When satisfied that the pots are well over 200c the under-fire is built up, using slightly more substantial slivers of wood.
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Slivers of wood are then slotted into the grate, from the perimeter to the centre.
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Wood is added to the top of the grate.
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Finally, 60cm lengths of fencing board is added to the fire carefully but quickly. The first boards will touch the pots so extra caution is needed to avoid breakages.
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The fire rages and, as seen here, the grate begins to glow orange/red. This is a good sign that the pots have reached temperature (800 - 900c).
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The fire subsides and the pots cool. This is the stage when cooling dunts might occur, so I’m listening out for suspicious sounds!
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The final tests have survived. One tempered with crushed oyster shells and the other with grog.
This very cautious, progressive open-firing, is useful for testing clays of unknown firing-behaviour or clays with modest amounts of tempering (the grogged test -piece, only contains 25% which is quite low for an open firing).

The firing grate has been very effective - it gets heat under the pots, it allows for well regulated pre-heating, and, as any blacksmith knows, the change of the metal’s colour is useful for gauging temperature. Also, it prevents the ware from sinking into a bed of ashes, which at the early stages of firing insulates the pots from the heat above.

The progressive open-firing, I think proves, that if taken slow enough, many clays can be open-fired successfully.

Note: Open-firing is dangerous - I always wear PPE appropriate for hot working.