Advice for setting a hold at top temp for ^6 firing

[Caroline E]

Hello all, 

I am experiencing some small blistering on glaze ware that were on the top shelf. I put 3 cones bottom, medium, and top shelves in the kiln. It turns out that my top shelf was firing at a slightly lower temperature, and the pots on the top shelf were the only ones with blistering. I was recommended to put a hold at the top temp of my Skutt kiln to try to level out the glaze. One person told me to put a hold at top temp for 10-12 minutes and another said 1-2 minutes. Which hold time is better? My initial question was that the longer I hold at top temp, I may run a chance of overheating? I am using Opulence Glazes from Mid-South ceramics. 

 

Thanks for any advice,

Caroline

[Babs]

Hi, is it possible to post a photo of blistered pots and cones? That way folk can better understand the extent of underfiring and length of soak required. A hold of a minute or two won't account for much imo.

 

[Pres]

 How many thermocouples in your kiln?  Can you do a thermocouple offset?

As Babs says, 1-2 is not much, maybe try starting with a 5 minute to see what happens.

 

best,

Pres

[neilestrick]

A 15-20 minute hold equals 1 cone hotter, so yet, the hold will increase heat work on the rest of the kiln, too. I would first try loading the middle of the kiln tighter and loading the top looser. Put low mass pieces up top, like wide bowls.  If that doesn't work, I would try firing a cone lower and holding for 15-20 minutes to achieve the next cone.

[shawnhar]

13 hours ago, neilestrick said:

A 15-20 minute hold equals 1 cone hotter, so yet, the hold will increase heat work on the rest of the kiln, too. I would first try loading the middle of the kiln tighter and loading the top looser. Put low mass pieces up top, like wide bowls.  If that doesn't work, I would try firing a cone lower and holding for 15-20 minutes to achieve the next cone.

This is what worked for me. I use 12 to 17 minutes based on how dense the load is. I also use an offset like Pres mentioned.

[Caroline E]

Hi All, 

Like always thank you so much for the generous knowledge you share. I have attached a picture of my cones from the firing. I cannot get a good picture of the blistering on the pot because they are really small, but they are a few scattered around and they are sharp. 

To answer your questions, I have one thermocouple. I wasn't aware of the term "offsetting the thermocouple", but earlier this year, I did program my skutt kiln to fire a little cooler with the help of Skutt customer service. At the time, my kiln was firing a cone too hot and the blistering on the pots were worse than they are now. I thought that blistering on the glaze was a result of the kiln being too hot, now I have learned that blistering can happen when the kiln is too cool. Can someone please explain what temperature blistering happens? Please also  let me know what you think about my cones. 

I am starting to think it may be a good option for me to try a hold at top temp instead of messing with offsetting again. 

thanks,

Caroline

[dhPotter]

@Caroline Ennis

My schedule has a ramp from 500*F  to 2050*F at 400*F/hr. Then ramp at 108*F to 2185*F. A hold of 15 minutes at this temp. Then free fall drop to 2085*F and hold for 30 minutes. This has stopped the blistering(pinholing). Your blistering(pinholing) having sharp edges means the blister occurred after you hit top temp and the kiln was cooling and there wasn't enough heatwork to heal over the blister.

[neilestrick]

Those cones are very close in terms of evenness. I wouldn't expect them to be any more even than that. If you're going for a full cone 6 then a 5-10 minute hold ought to do it.

[graybeard]

Just a thought.

When I had problem with pin holes in my glaze it was caused by out gassing because my bisque fire was not complete (not hot enough?) When I raised the bisque temp that took care of it. Like I said, just a thought.

Stay safe, Graybeard