[Clayart] Preheat follow up question

[Kenneth Chase]

Thanks Robert:
My pieces came out fine after a slow schedule. . I noted from 650 degrees to somewhere around 1600 degrees there was still
dampness on the mirror. I really don’t know what conclusions to draw from a my observations.
My kiln is programmed with a set pre heat temp so I’m not sure I can change it or if it’s necessary to do so.
I live in the desert so maybe my concerns aren’t warranted but I still would like to understand.
Appreciate your reply
Best,
Ken
> On Feb 24, 2017, at 6:27 PM, Robert Harris <robertgharris at gmail.com> wrote:
> 
> Hi Ken,
> 
> A couple of things. Preheat should be at 180F at minimum (this is generally
> what is recommended or programmed into most kilns). 150F isn't going to
> show you much.
> 
> BUT, you have to remember that during the process of bisquing water that is
> chemically bonded to clay is released. This occurs at 660F and up. Unless
> you are going really fast (350F or maybe more per hour), this water does
> not induce breakage.
> 
> If your mirror looks dry at 200F you would probably be OK. Personally I
> preheat at 230F as this is not sufficiently high to build up the head of
> steam required to crack a pot, but dries it a lot faster and more
> thoroughly than a 180F pre-heat.
> 
> I have thrown, trimmed and bisqued on the same day (I'm in dry CO which
> helps!), and the pots are pretty moist when I put them in the kiln. For
> this sort of craziness, I preheat at 180 for 1.5 hrs, and at 230 for 2.5hrs
> with no problem given relatively thin functional pots).
> 
> Under normal circumstances a 1.5 hr preheat at 230 is sufficient for the
> majority of slightly damp (i.e. just off bone dry) pots.
> 
> Robert
> 
> On Fri, Feb 24, 2017 at 9:25 AM, Ken Chase <kchase235 at gmail.com> wrote:
> 
>> Hello:
>> I set a preheat for my electric kiln at 5:30 AM.
>> About 7:30 when temp was about 150 degrees
>> I held a mirror up to both port holes and there was no sign of fogging so
>> I assumed the pots
>> Were dry and reprogrammed for a slow schedule bisque 04 fire.
>> After taking my dog for a walk, out of curiosity,
>> I held the mirror up to the top port hole and saw
>> Fog on the surface. I'm confused but believe the
>> Mirror shouldn't be used until the temp is much higher. Am I right? If so,
>> what temp should I check?
>> I think my load is safe because the heat rise takes several hours.
>> Thanks much
>> Ken
>> 
>> Sent from my iPad
>> 
>> 
> 
> 
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