Firing schedule ∆10 question

[allenc27]

Hi,

I have a test kiln about 20*20*20 CM and a bigger kiln about 75*75*60.

I use the same firing and cooling schedule on both but sometime on certain glaze I keep getting different result on test kiln.

These glaze are ash clay felspar mixed glaze fired in electric kiln both test kiln and the other bigger kiln.

Here some photos, the left test tile is fired by test kiln. I'm guessing firing too hot too fast? Or not enough gap for air? 

 

Please really need help to get this right

Here is my firing schedule.

450° C 3hr @150° C / hr

1050° C 6hr @ 100° C /hr

1200° C 2hr @ 75° C / hr

1280° C 80 mins @ 60° / hr

Cooling schedule

1200° C 30 mins

1050° C 30 mins

800°C 1hr

 

your advices are much appreciated, Many thanks

 

[Pres]

All of this IMHO: Looks like to me, that the top selection is a little thin on application, second  one down  looks like too fast a firing on the left, or a firing that needed a little soak time to let the glaze gas bubbles smooth over, The third one is so different, I would want to know what the glaze chemistry is.. . . Does it contain rutile?

 

 

best,

Pres

[Magnolia Mud Research]

small kilns cool faster than larger ones.

The left tile in the second set appears to have NOT leveled before coming too stiff to level.  Notice that the bottom segment of that tile is smooth; the top section cools faster than the bottom because that segment has less local mass to store thermal energy.  

The tiles from the larger kiln cooled slower.

Neither set of tiles are over fired;  if anything I would raise the firing temperature a bit.  The glaze does not appear to be runny yet.

Based on the test images I would guess that the small kiln tiles may not get as hot as the larger kiln. 

Slow down the cooling in the small kiln or hold longer at or just below the top temperature might help.
 

LT

[Min]

@allenc27, are you using cones to verify what the kiln got to? I've noticed that it seems quite common for potters in some parts of the world to not use cones, they go by kiln temperature alone. Problem with this is thermocouples can read differently and drift. Part of the problem could just be that the smaller kiln is running cool and/or the larger kiln runs hot.

[neilestrick]

Baby kilns and big kilns will not give the same results unless you cool them at the same rate, too. I've got 3 kilns of drastically different volumes, and to get the same results I fire the same schedule going up, at a rate that the big kiln can keep up with, and add a controlled cooling to all 3 kilns from the peak temperature down to 1500F. I cool at 175F/hr, but you can probably go faster than that if you don't wan the cooling rate to affect the look of the kiln. The key is to cool the small kiln as slowly as the big kiln.

[allenc27]

9 hours ago, Pres said:

All of this IMHO: Looks like to me, that the top selection is a little thin on application, second  one down  looks like too fast a firing on the left, or a firing that needed a little soak time to let the glaze gas bubbles smooth over, The third one is so different, I would want to know what the glaze chemistry is.. . . Does it contain rutile?

 

 

best,

Pres

Thank you. Seems soak and slow cool is the issue. 3rd one is 20 wood ash, 60 feldspar 20 local clay, i have not been able to get constant result of grey surface fired on test kiln and large kiln. I think I may need to reduce the amount of feldspar? 

[allenc27]

9 hours ago, Magnolia Mud Research said:

small kilns cool faster than larger ones.

The left tile in the second set appears to have NOT leveled before coming too stiff to level.  Notice that the bottom segment of that tile is smooth; the top section cools faster than the bottom because that segment has less local mass to store thermal energy.  

The tiles from the larger kiln cooled slower.

Neither set of tiles are over fired;  if anything I would raise the firing temperature a bit.  The glaze does not appear to be runny yet.

Based on the test images I would guess that the small kiln tiles may not get as hot as the larger kiln. 

Slow down the cooling in the small kiln or hold longer at or just below the top temperature might help.
 

LT

Thank you very much, will try that

[allenc27]

Thank you everyone really appreciated. Soak time and slower cooling might be it.

 

For 3rd tile I'm thinking ash or feldspar need to be reduced to get that grey color?