Under Firing?

[Pam S]

Sorry for the long post but I thought the more detail the better. I purchased a used Jupiter J18 kiln (230 amps, 5520 watts, 240 volts). It has three control knobs with the settings low, 1, 2, 3 4=med, 5, 6, 7, 8, 9, 10 and hi. The kiln sitter is a model P. The previous owner said he had changed the elements 8 months before I purchased it. His work was all low fire. I’m firing cone 6 buncombe white clay. No problem with the bisque firing at cone 04. I’m having trouble with my glaze firings. Several of my own mixed glazes are fine but I’m having trouble with two. I’m also having trouble with the Coyote glazes I’ve purchased. My white silk (a very waxy, soft matte) is turning out shiny with a slight crackle finish and my ketchup red is a muddy brown. The Coyote glaze colors don’t seem to develop either. For instance the eggplant is a greenish color and you can see just a hint of the purple at the edges of the glaze line. The red gold is muddy brown as well. I’m thinking they may be under fired. I’ve done multiple test firings to cone six. I’ve fired using the following schedule:

 

B – bottom, M – middle, T – top

 

Hour 0, B1, M 1, T 1 lid propped open, both peep hole open

 

Hour 2, B 2, M 2, T 2

 

Hour 4, B 3, M 3, T 2

 

Hour 5, B 6, M 6, T 3 close lid, plug bottom peep

 

Hour 7, B 10, M 10 T 10 close bottom peep

 

 

 

The sitter shuts off at about hour 10 and a half and the witness cones look fine. I’ve also tried it using a soak at medium for two hours, then at low for an hour. Any input would be much appreciated. I’m getting very frustrated and my work is sitting on shelves.

 

 

 

Thanks!

[Guest JBaymore]

I’ve done multiple test firings to cone six.

 

Are you using any witness cones siting on the shelves with the ware , or believeing the kiln sitter as to the "cone 6"?

 

best,

 

..................john

[Pam S]

I have used witness cones to make sure we're reaching temp. ^5 is slumped completely, ^6 slumped properly and ^ seven slightly.

[Guest JBaymore]

I have used witness cones to make sure we're reaching temp. ^5 is slumped completely, ^6 slumped properly and ^ seven slightly.

 

 

If that is true, then you are not "underfired". The cones measure the heat work accurately. So it is something else.

 

The satin-matt that is now shiny points toward the possibility of faster cooling than your prior kiln. As does the

muddy colorations of the glazes that contain iron. These usually require percipitation of oxidized micro crystaline phase seperations on the surface of the glaze to get the red color. The iron in solutuion will tend to remain "muddy".

 

So I guess the next question is what are you comparing these results you are now getting TO? Was it your own kiln....and if so, did it have better insulation or a larger thermal mass?

 

Or was the comparisons you asre making to a firing over which you do not know the particulars of the cooling cycle?

 

The cooling cycle almost has more to do with glaze development then the up cycle.

 

 

Off on another potential tack on this........

 

The guy who did the low fire work that you bought the kiln from..... was he using lead based glazes, and how long did he fire that kiln......and how often?

 

best,

 

...................john

[Pam S]

The kiln I was previously firing in was at a city recreation center. They closed the studio due to the economy =0(. The kilns were full size Skutts, and very old. I never did the firings there so I have no comparison other than the results. I have spoken with the instuctor from that studio who said that she fired using a similar schedule to mine and only used the kiln sitter as cone representation. No witness cones. My kiln is small maybe 4 cubic feet so the thermal mass is quite different. The guy I bought the L&L kiln from did functional pottery, mainly mugs and dinner ware, no lead.

[Guest JBaymore]

The kiln I was previously firing in was at a city recreation center. They closed the studio due to the economy =0(. The kilns were full size Skutts, and very old. I never did the firings there so I have no comparison other than the results. I have spoken with the instuctor from that studio who said that she fired using a similar schedule to mine and only used the kiln sitter as cone representation. No witness cones. My kiln is small maybe 4 cubic feet so the thermal mass is quite different. The guy I bought the L&L kiln from did functional pottery, mainly mugs and dinner ware, no lead.

 

 

Ah... bigger kiln .....more thermal mass... better ratio of mass to surface area...... slower cooling curve. Very likely the culprit here.

 

Also without witness cones... it is possible that the end point of the ware actually was not the "cone 6" that likely was in the sitter. And the up-cycle may have been slower too,.... since the kilns were very old.

 

Might also be a bit of a combination of slower up and slower down. But first choice is the cooling for me.

 

Try firing down instead of simply shutting the kiln off at the end of the firing. Much easier with a computerized controller....... but still doable with the switches. So instead of turing the kiln "off", try turining all the banks to low or even maybe medium for an hour to two hours or so and see what that does.

 

You'll have to contend with the sitter wanting to shut it completely off though. You may have to use a higher end point cone in there to compensate so that the slow cool does not trip off the sitter before you are "done" with firing down. Then use witness cones on the shelves thru the spyport to decide when to shift from the up-cycle to the cooling cycle.

 

Plot out the cooling curves on your firings (record the pyrometer readings) so that you can understand what is happening. It would be good to have one for the "not fired down" kiln for the comparison.... but hate to make more bad ware for you.

 

 

best,

 

..................john

[Pam S]

Thanks for the input. Luckily I've only ruined one batch of work. It's all been test tiles since.

[Deb Evans]

for older manual kiln shutoffs.

easy fix if you want to continue cool down firing after sitter has shut off - just put the metal piece back up , jam a used cone tip into gap and push buttom in.