Monday, 15 November 2010

Underfired ! - But Why?

In a bid to deal with the number of pots required over the Christmas period
I fired up the second hand kiln I bought on EBay a few months ago,
This was an attempt to fire two kilns one after the other.
But upon opening the Biscuit firing I found it was under fired.
We have had some bad storms and power cuts over the past week.
so I decided to fire the kiln again during the day while I was in the workshop
because all of the moisture had been driven out of the pots
I heated it up at 50 oc per hour up to 200 then moved it to 100 oc per hour
until 400oc, then it went on full power to 885oc.
the heat from 400 - 885 was incredibly quick, no more than 2 hours. 
A classic sign of under fire.
But when I opened it, it was no different.
when I reached temperature I switched over my bentrup controller
to check if it said the same temperature, and it did within 2 0c.
isn't it strange how when you try to push on, something slows you down.
The elements all look OK
and the under fire was even throughout the kiln.

The colours are all wrong and the pots feel as if they have reached about 500 oc
but I'm not sure.
stupid me forgot to put some cones in to check the temperatures,
a natural thing to do when firing a kiln that is new to you.

So any suggestions would be welcome.
I've taken it all out and re fired it in the original kiln and it looks fine.

My thoughts on this sign.
in the time it took them to install this sign,
maybe they could have filled in the pot holes.
or is that just me.!

4 comments:

Hollis Engley said...

Weird. I can't tell by looking at the photos, but does the kiln have a kiln-sitter? Or a computer box that controls the firing? If it does, maybe that component is just badly off in the way it reads the temperature. The interior looks very clean, so it doesn't seem likely the elements are bad. They look fine from this distance. And if the pots fired fine after underfiring in the new kiln, then it's clear there's something wrong with the measurement of temp in the new kiln. And I wouldn't think that uneven electricity would have that effect repeatedly.

Hannah said...

How bizarre. Have you tested the elephants? My kiln fellas are great at suggesting stuff over the phone if that's any help, even when it's not their kiln. You could give them a shot.
I like the potholes sign, I've never ever seen one of those before. Brilliant.

Charles The Potter said...

First thing to do is crank the thing up (empty) until the elements start to glow. If one or more isn't glowing but you can't find a break in the element, then open up the control box and see if one of the connecting wires has burned out. Sometimes this happens if the last person to change an element hasn't tightened the connectors enough. If that looks OK then look for the Relays in either the bottom of the control box, or a separate smaller box. Sometimes they burn out as well. If that is OK then check the fuses on the supply box.

Christine H S said...

My input would be that cones are calibrated to show not only the temperature, but the amount of heat input/work done, which is why in the catalogues they have different melting points for different lengths of firing shedules. I learnt this fact some time ago (the hard way as we do)...that the faster you fire, the higher you have to go, or visa versa. Trust the cones not the pyrometer. So I might be wrong but I think it was the speed of firing, and you were indeed underfired as far as work done was concerned.
Great sign!

Time to start planning for the show season.

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