Comment

Comment by Tom Anderson on May 14, 2017 at 3:43am

This is the easiest and best resource for the various temperatures potters need to be concerned with.

http://www.quartzpage.de/gen_mod.html

Tom

Comment by Tom Anderson on May 13, 2017 at 8:06pm

Norm: absolutely agree- was just giving Joseph a quick explanation. Would only apply to stoneware in some cases, and never to porcelain because of the flux molarity differences. Mr. Hill is just exercising an abundance of caution.
Tom

Comment by Joseph Fireborn on May 13, 2017 at 4:52pm

Makes perfect sense. Glad I questioned this. I have saved several hours in the last week no longer using this part. Although my entire schedule is a lot different than his. 

Comment by Norm Stuart on May 13, 2017 at 4:50pm

With a "Cone Fire" digital firing kiln it's easy to add multiple-hour 200 F pre-heat to any firing.

"Kiln Sitter" controllers don't allow you to add a pre-heat to a firing, so people owning those sort of kilns adapted with a slower ramp to 500 F.

Comment by Joseph Fireborn on May 13, 2017 at 4:39pm

Norm,

This makes sense. So I preheat and soak for a good 3-4 hours during the day before my glaze firing at night. I am certain my stuff is dry by then. Thus I can continue to use my 300F/hour to 500F, which makes me save a good 1 hour or more from the 100F/hour to 500F. I never understood that, but that makes sense. He is trying to make sure other people don't explode their ware if they dont preheat sufficiently. 

Comment by Norm Stuart on May 13, 2017 at 4:25pm

Tom & Joseph -  A several hour pre-heat at 200 F is very effective at thoroughly drying greenware. Some have kilns which cannot easily add this pre-heat so they try other techniques which are not as effective.

On Digitalfire Tony Hansen shows this example of a different firing which quickly heated to 220, above the boiling temperature of water, and held at 220 F for 60 minutes. The temperature then rose again and a bowl with a thick base which was not thoroughly dry exploded. - https://digitalfire.com/4sight/education/firing_what_happens_to_cer...

I call this "case hardening", I'm not sure anyone else does.  By ramping quickly to 220 the exterior of the clay dries and shrinks quickly, mechanically trapping the evaporative water deeper in the ware. So you can attempt to solve the lack of a 200 F pre-heat with a very slow temperature ramp between 220 and 500 F - the range in which mechanically trapped water escapes.

If ware is especially damp it's important to lift the kiln lid during the pre-heat to allow moisture to escape. A tall stack of damp greenware plates or test tiles will see the plates or tiles on the bottom of the stack fire will fire fine, but there will be an increased level of cracking at the top of the pile, because the top ware is soaking in a steam cloud from the ware lower in the stack so doesn't dry completely. 

.

Quartz / cristobalite expansions at this temperature range and inversions as the fired ware cools would not benefit from a slow firing from 220 to 500 F. These 0.8% expansions are affecting tiny particles in the clay with no real effect on the greenware. Cristobalite doesn't form and remain in that state until well above Cone 10, when the silica and clay was formed. I've heard there is additional cristobalite formation above Cone 2, but I have no proof that's the case. Rapid cooling between 500 and 300 F can accentuate any cristabolite shrink problem as these crystals are more structurally incorporated after firing to where the clay is densified.  But cristobalite definitely doesn't form between 220 and 500 F.

Comment by Joseph Fireborn on May 13, 2017 at 2:19pm

Tom,

So do all the other profiles just not care about it. Because every glaze and bisque profile goes faster than 100F. I dont understand I guess. I won't worry about it anymore anyways. Everything came out fine so I am just going to remove the 1+ hour spent there.

Thanks for your answer though.

Joseph

Comment by Tom Anderson on May 13, 2017 at 2:16pm

Joseph:

I will leave the technical chemistry out of my reply- it puts people asleep. At 270C crystobalite polymorphs can become actual crystals.. Short answer- to stop crystobalite formation.

Tom

Comment by Joseph Fireborn on May 11, 2017 at 11:20am

John,

I am not really sure either. I am not certain that between 220 and 500F the clay is shrinking that much, is it? Also since the glazes are sprayed I assume they are already pretty dry. The inside being poured could be a reason for this. I am not sure either. I was going to email him about it, but last night I ran a firing with a different ramp of 300F/hour from 220F to 500F. Cutting about 1.5-2 hours from the total firing time. I noticed no differences from before. Nothing exploded, but I know my pots are dry because I preheat them 3 hours during the day then let them sit in the warm kiln until I fire that evening. 

Thanks for the response though. 

Comment by John Lowes on May 10, 2017 at 6:44pm

An educated guess on that ramp relates to SH's single firing. Since his glazes need to dry on the green clay, going slower is beneficial so the glaze can shrink with the clay as they are headed to bisque temps and beyond.

John Lowes
Wynhill Pottery LLC
http://wynhillpottery.weebly.com

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