OTish: Flue Gas Temperatures

Those here not familiar with gas-fired pottery kilns operating in reduction, will be horrified by what follows, but I can assure them that it's standard practice by studio potters, but you do have to know what you're doing and understand the possible dangers.

I used to run a small gas-fired Laser kiln (model G6 or G7 I think) in a wooden garden shed. One of these

Downdraft, front loading, gas exit slot at the back in the top. The maximum temperature that I usually took it to over an 8-hour firing was 1280°C/Cone 10 although IIRC it would do 1300°C if pushed (I think your pal's idea of Cone 10 being 1200°C is not right. 1200°C is approximately Cone 4 or 5; Cone 10 approximates to 1280°C. Either that or he's firing *extremely* slowly, cones responding to so-called 'heat-work' rather than temperature alone, i.e a time-temperature combination).

The exit gas was not close-coupled to the flue, but there was a stainless steel hood that sat over the exit slot, with a gap of about

3" between the bottom of the hood and the top of the kiln. The hood was available as an extra with the kiln. One of these although mine was bare steel, not enamelled as here, and the gases emerging from my kiln would have been a lot hotter that the 350°C they talk about there! I can't imagine the enameling lasting long under my firing conditions.

I used a double-walled stainless flue, I think made by Selkirk; a single section of one of their larger internal diameter types, probably 8". The flue went up through the wooden shed roof, with conventional fibreglass loft insulation packed into the annular gap between the outside of flue and the roof, and an aluminium plate made to fit snugly around the flue on the outside, sealed with high-temperature silicone sealant to make it watertight and rain-proof. Everything was supported by a framework of Dexion.

Unfortunately, since my wife died and I moved to a smaller house and gave up pottery, I no longer have any pictures of the kiln and its setup.

At temperatures above about 800°C I usually operated my kiln 'in reduction', i.e. with a deficiency of combustion air (I specialised in 'copper-reds' aka 'sang-de-boef' or 'ox-blood' glazes that could only be made under those conditions, and very good they were, though I say it myself!).

Reduction was achieved by limiting the amount of secondary air entering the kiln around the burners in the floor, by reducing the width of the exit slot across the back by sliding a couple of small refractory batts across it. This cased a very slight back-pressure within the kiln and decreased the amount of air intake. In those conditions, the gases exiting through the slot were very rich in carbon monoxide, and at those temperatures it instantly ignited when it came into contact with the air outside the kiln, resulting in a big

12" flame roaring up the chimney. The Selkirk flue got quite warm on the outside, but nothing excessive, and I never feared for the wooden roof or the felt where the flue passed through it. Gas consumption was significant: a 19kg propane cylinder would do about two-and-a-half firings, say twenty hours in all, but hey, it was a hobby!

So, in answer to your query, your pal should use a large diameter double walled SS flue, with a hood above the kiln exit and a gap between to allow air to enter the flue and help keep it cool, and it will be OK.

One caveat: he shouldn't under any circumstances be using his kiln inside his house! Only use it in an outside shed some distance from the house, otherwise he and his family running a serious risk of carbon monoxide poisoning, and/or setting his house on fire. I doubt if his insurance company would pay up under those circumstances.

PS: The overall setup was a bit like this

the shed was slightly larger and the kiln was on a taller stand. The Dexion scaffold supporting the flue was also more substantial than here.

Thanks *very* much Chris - I'll pass it on. The kiln is a good 100m for any houses btw!