Re-firing issues

[Libba Adams]

Hello,  I recently fired a ^6 load that didn't reach temperature on the bottom shelf, evidenced by witness cones 5, 6 and 7 not bending at all.  The top 2 shelves were perfect, with ^ 6 tip touching, ^5 melted, and ^ 7 bending.  The kiln elements (new) all seem to be working properly, doing a visual test in darkened conditions.  The firing schedule I used was an adapted E3 schedule from John Britt's book on mid-range glazes, and had a 10 minute soak at peak temperature, with a slow cool from 1900-1400F.  Ambient temperature was in the low 30's.  I'm thinking of re-firing, and leaving the bottom shelf empty.  My question is whether I need to use the same slow cooling schedule as before, or whether it would make a difference to use a faster schedule if all I'm trying to achieve is fully melting the glaze at the rims of pots.  Attaching a pic showing the turquoise pots from the bottom shelf.  The liner glaze, also on the rim, is the version of John's blue in the original edition of the book.  This glaze has been corrected to use Frit 3134 instead of Gerstley Borate, and I like how it works as a liner in the original form.  

 

[Min]

55 minutes ago, Libba Adams said:

My question is whether I need to use the same slow cooling schedule as before, or whether it would make a difference to use a faster schedule if all I'm trying to achieve is fully melting the glaze at the rims of pots.

If any of the glazes needs a slow cool to get the desired look of the glaze then no, I would not use a faster schedule. If all the glazes are fine and look the same with the kiln just doing a free fall then yes it would be okay. You might find refiring them causes the glazes to be a bit overfired if going to cone 6 as they have already had some heatwork. If in doubt I would include one pot in the next firing and see what happens.

Bigger issue would be why the bottom shelf is underfiring.  Is this a kiln with a single thermocouple in the middle of the kiln? If so then going forward pack less mass in the bottom of the kiln, ie taller pots on the bottom, more density (short pots and more shelves) in the middle of the kiln. If it's a 3 zone kiln then I'ld be looking at doing a tc offset for the bottom thermocouple.

[Libba Adams]

Thanks very much.  There is just one thermocouple.  Thanks, too, for the suggestion about the heatwork and overfiring.  

 

[Mark C.]

I put my refires (gas kilns) in cooler spots and go very slow at  1st as they are tight and can crack if heated to fast