A mate has asked me about his kiln (for clay pots and things), - obviously, I didn't know, so . . .
It gets to about 1000C, but struggles to reach 1200C. From new it did this quite readily, he says. The slightly odd thing is that it still has the same/consistent energy draw, with no apparent losses such as heat leaks or noises of any kind. So I'm not sure where the energy is going.
Any ideas?
mains down to 187v ? :-)
I would question the claim that it still has the same/consistent energy draw. Assuming the elements are kanthal A1*, as are the elements of most small pottery kilns, they do 'age' over the years. They slowly oxidise and their resistance increases, meaning they draw less power and eventually struggle to get to top temperature. It used to happen to mine in the days when I had a couple of electric kilns.
The solution is to fit new elements. Most potters' suppliers will be able to supply appropriate sets. The only caveat I would make is that the porcelain choc-blocs that connect the elements to the incoming copper wiring will probably be heavily corroded and impossible to unscrew, and will also need replacing. Get a set in advance.
*
What Chris Hogg said... Unless there's something really weird going on, slow heating of kilns (my experience is in fused-glass rather than ceramics, and we 'only' go up to 800c or so) suggests that either the infinite controller on his kiln is faulty, or he (more likely) has at least one set of elements open-circuit.
Should be possible to confirm this by taking a quick look as the kiln heats up - the elements that aren't glowing are open-circuit - either at the wired connections or the element itself. A multimeter check across the individual elements will confirm this.
Typically the kiln has enough power input to heat up, but not as fast or as hot as expected.. How's he measuring the input power?
But why, when its been working? My only involvement with electric Kilns was at school and often it seemed the lagging needed to be replaced, but I'd have thought that would be pretty noticeable!
Thermostat not working properly maybe? IE if it can be turned up would it get to the desired temp faster? Brian
By the time they get above 1000°C, most small pottery kilns are running flat out anyway. The t/stat only serves to cut them off when they get to peak temperature, or a little before that to stop overshoot if they are 'three term' PID types.
A faulty thermostat could cut out power sooner when the kiln is at a lower temperature. If the OP is seeing the exactly the same power taken whilst the kiln is heating up then the elements may be OK but just turned off too soon, at lower temperature, by a faulty thermostat or a self resetting over temperature cut-out.
Yes - it's not a thermostat in the sense of 'keep the water in my hot tank nice & warm' - it's more of a 'we're at temperature, cut off now' device (traditionally operated by a ceramic 'cone' deforming at a preset temperature and cutting the power).
More modern kit might have an electronic controller - so there's some control over the rate of heating..
Glass-work is a bit more complicated - and tends to involve multiple segments with ramp / target temperature / hold time - both on the heating-up and on the annealing/cooling-down stages...
I think that would depend on how the elements were wired with respect to the thermostat. Most kilns have three or more sets of elements, that may be wired in different configurations, for example all in series or all in parallel. In the former case, if the thermostat cuts out early, the kiln will start to cool almost immediately with very little overshoot, and it would be obvious. If the elements were in parallel, each section is often controlled simply by a manual switch, the intention being to control the rise in temperature during the early stages of firing by switching them on in sequence as the temperature rises, with all sections 'ON' to reach top temperature. It's possible that each section could be independently controlled by separate thermostats, and one section could be cutting out early as you suggest, but that's not a usual arrangement for small to medium sized kilns for hobby or small studio potteries, but more appropriate for large commercial operations when zoned temperature control within the kiln may be important.
I have a small kiln and to improve that situation I made wrap-around "pillows" of glass-fibre roof insulation inside "pillow-cases" of the woven glass fibre mat as used in car-body repair. Don't rely on thermostat readings, get a range of proving cones to confirm/calibrate any meter readings
My use of a kiln, to test out versions of things called mathematical tiles.
Whoa! Hold on a minute! I certainly wouldn't put any old glass fibre, whether loft insulation or mat, into a pottery kiln without being very sure it would stand the temperatures I was using without melting. What temperature are you firing to? Your mathematical tiles look like thin bricks, so I imagine you're firing them to temperatures probably around 1000°C, rather than the 1200°C the OP's friend goes to.
Fiberglass varies considerably in composition, depending on method of manufacture (or it did in the days when I was more closely involved), for example whether air-blown (insulation type) or from spinnerets (fibre mat). The OP's mate would have to test small samples very thoroughly before going down that route. And extra internal insulation would take up space in the kiln, which the OP's friend may not want to sacrifice.
If the OP's friend wants to put a pad of insulation under his kiln lid, then I would recommend he use a ceramic fibre specially intended for kiln use, such as Kaowool or alumina fibre.
Ceramic cones 'go down' at different temperatures, depending on their composition and the rate of temperature rise. Potters refer to them as measuring the rather imprecise term 'heat work'. Cones, being made of similar materials to pottery clay and glazes, behave similarly to the pots and glazes being fired, so some potters prefer them to thermocouple measurements. A cone fired up slowly will 'go down' at a lower temperature than the same cone fired fast. For example, cone 7 will go down at ~1200°C when fired up at 15°C/hr, ~1240°C when fired up at 60°C/hr, and ~1260°C when fired up at 150°C/hr, a range of roughly 60°C between slow and fast firing. Not a good method for calibrating a thermocouple.
Presume you're aware by now who supplies McDonalds?
Pillows wrapped around the outside of the kiln. Cones otherwise called witness cones as you can lay them in the area of the clay being fired, not in the wall of the kiln which can be locally hotter or colder than the centre.
Well well, never knew that. Thanks!
Or that.
I wonder what the fire-proof/propogation-supression of the insulation behind
If its getting to 1000C then elemenst must be deemed OK, especially if same power draw. Which leaves 2 options ..... either heat is escaping quicker (ladding or door fit) Or thermostat is not behaving itself (more likely.)
I don't think we ever did get any feedback from the original poster back in May - but a common failure mode on ceramic and glass kilns which have multiple elements is that one (or more) of the elements fails.
The effect is as RJH described - the kiln can heat up initially as normal (though a little more slowly) - but at higher temperatures the reduced heating effect (from having only some of the elements working) prevents the kiln from reaching the desired temperature. I'd imagine this is because the rate of heat loss from the kiln increases as it's internal temperature increases, until it reaches a kind of 'stalemate' situation.
Same effect can be seen on glass and ceramic kilns which have multiple elements, each driven by a separate relay. If one of the relays fails, the kiln will fail to reach the top temperature.
There's probably more than one heating element. So, one might be open circuit. It will have to opened up and a continuity meter used to check they are all OK.
But you'd expect to see a lower power consumption?
In full agreement with these three posts. Has the claim that the energy draw has not changed been properly verified, or was it just a casual and possibly incorrect observation?
Another possibility, perhaps already covered up-thread (I've lost track), is that the insulation is failing in a part of the kiln not immediately visible, and there's a greater heat loss there.
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