Earl Krueger on mon 24 may 04
On Monday, May 24, 2004, at 19:46 US/Pacific, Ron Roy wrote:
> One of the best ways of insuring
> ware does not blow up is to candle a kiln over night - heating the=20
> ware but
> keeping it under 100C accelerates drying. After all the water of=20
> plasticity
> and atmospheric water - then you can go as fast as you like.
Ron,
Are you referring to only bisqued ware or to raw clay as well?
Faced with firing some quite thick objects made by children I
have been thinking on this subject quite a bit. When I fire raw clay
I can detect moisture at two distinct periods using the cold mirror
technique.
The first is when the kiln is just getting warm and is due to the
liquid water in the clay. Then there is a period when the mirror
does not fog. Later, starting at about 400=BAC the mirror again
shows moisture. This would be due to the decomposition of
organic materials and the release of water chemically combined
in the minerals of the clay escaping as the ceramic transformation
takes place.
My question is, how great is the danger of thick pieces (2-5 cm)
blowing up from the release of the chemically combined water
in the 400=BA - 800=BA C range? How slow should I go through this
range?
Thanks...
Earl K...
Bothell, WA, USA
Ron Roy on mon 24 may 04
Just a few words about clay blowing up in addition to what Snail says below.
The way to tell if a piece is as dry as the air around it is to hold it to
your cheek - (the ones on your face) - if it feels cold or cool then water
is still evaporating from it.
There is always some humidity in the air so even if the ware does not feel
cold it does not mean there is not enough water present to make it blow up
if it is fired too quickly.
Much depends on the thickness of the clay - how fast any resident water can
get out (grog helps water get out faster.) One of the best ways of insuring
ware does not blow up is to candle a kiln over night - heating the ware but
keeping it under 100C accelerates drying. After all the water of plasticity
and atmospheric water - then you can go as fast as you like.
You can tell if there is moisture in a kiln by opening top and bottom spys
- turning off any vents or drafts and holding a mirror or some shiny metal
at the top spy. If there is steam coming out it will condense on the metal.
If you need to turn your loads around quickly it would not be difficult to
make a drying cupboard with a few light bulbs in it to heat ware and
accelerate drying.
RR
>Shrinkage, however, has nothing at all to do with
>things blowing up. Only water makes things blow up.
>If it was still damp from applying the engobe,
>or was fired too quickly for its thickness, then
>that is why it blew up. Remember then you apply a
>coating to the work, it is adding more water to the
>piece, and it needs to be allowed to dry again.
>When you put the engobe on the wet piece, you
>probably gave it a decent amount of time to dry
>since you knew it was wet. Perhaps the 'dry' piece
>was put in the kiln too soon after applying the
>engobe, and was not given enough time to re-dry.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Earl Krueger on tue 25 may 04
On Tuesday, May 25, 2004, at 10:28 US/Pacific, Ron Roy wrote:
> It seems that no two authors can agree - at what stage the chemically
> attached water starts to be released. I know it will be a range of
> temperatures - could it be different for different kinds of clay.
>
> Any idea when the chemically combined H2O stopped being released?
Ron, I just got out my kiln logs.
On a bisque firing where I kept detailed notes about
every 15 minutes, I saw moisture on the mirror from:
4:33 - 5:18 pm, 103 - 172=BAC and from
8:30 - 10:00pm, 455 - 540=BAC, dark red heat.
Then I have a note that I saw slight moisture at 10:33,
650=BAC, red-orange heat. I don't think I checked after
this as the note says I put the peep hole plugs in.
These are approximate temperatures (=B1 10%).
You see, I don't have a commercial pyrometer.
What I have is a type K thermocouple, a Fluke
multimeter and conversion tables from the
National Institute of Standards and Technology.
Earl K...
Bothell, WA, USA
Ron Roy on tue 25 may 04
Hi Earl,
I was talking about unfired clay.
I don't think there is any danger from ware blowing up during the
chemically combined water release stage.
I once fired some bisque - after I had driven off all the free water - at
999C per hour - by mistake. I discovered the problem when I checked the
kiln about 3/4 an hour after it started and reset to 100C per hour - none
of the pots were damaged.
I have to admit that I don't know if that will be the case with thick ware
but - I certainly don't think 100C per hour would do it.
I wish someone would do some experiments - cubes of clay in a sagger or
something and put this subject to bed once and for all.
It seems that no two authors can agree - at what stage the chemically
attached water starts to be released. I know it will be a range of
temperatures - could it be different for different kinds of clay.
Anyway - thanks for including your observations Earl - good info. Any idea
when the chemically combined H2O stopped being released?
RR
>On Monday, May 24, 2004, at 19:46 US/Pacific, Ron Roy wrote:
>> One of the best ways of insuring
>> ware does not blow up is to candle a kiln over night - heating the
>> ware but
>> keeping it under 100C accelerates drying. After all the water of
>> plasticity
>> and atmospheric water - then you can go as fast as you like.
>
>Ron,
>
>Are you referring to only bisqued ware or to raw clay as well?
>
>Faced with firing some quite thick objects made by children I
>have been thinking on this subject quite a bit. When I fire raw clay
>I can detect moisture at two distinct periods using the cold mirror
>technique.
>
>The first is when the kiln is just getting warm and is due to the
>liquid water in the clay. Then there is a period when the mirror
>does not fog. Later, starting at about 400=BAC the mirror again
>shows moisture. This would be due to the decomposition of
>organic materials and the release of water chemically combined
>in the minerals of the clay escaping as the ceramic transformation
>takes place.
>
>My question is, how great is the danger of thick pieces (2-5 cm)
>blowing up from the release of the chemically combined water
>in the 400=BA - 800=BA C range? How slow should I go through this
>range?
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
=46ax: 613-475-3513=20
John Jensen on wed 26 may 04
When firing my electric kilns in winter I have seen condensed steam
puffing out the peep holes beyond 1300 degrees F. So much visible
condensed steam it looked like there was a fire going on inside. I was
surprised because I had thought (mistakenly) that the clay would stop
putting out water at around red heat.
John Jensen, Mudbug Pottery
mudbug@toadhouse.com , http://www.toadhouse.com
Subject: Re: Vitreous slips and clay blowing up.
> Any idea when the chemically combined H2O stopped being released?
Ron, I just got out my kiln logs.
every 15 minutes, I saw moisture on the mirror from:
4:33 - 5:18 pm, 103 - 172=BAC and from
8:30 - 10:00pm, 455 - 540=BAC, dark red heat.
Then I have a note that I saw slight moisture at 10:33,
650=BAC, red-orange heat. I don't think I checked after
this as the note says I put the peep hole plugs in.
Earl K...
Bothell, WA, USA
Ron Roy on thu 27 may 04
Hi earl,
Thanks for this - excellent information.
RR
>On Tuesday, May 25, 2004, at 10:28 US/Pacific, Ron Roy wrote:
>> It seems that no two authors can agree - at what stage the chemically
>> attached water starts to be released. I know it will be a range of
>> temperatures - could it be different for different kinds of clay.
>>
>> Any idea when the chemically combined H2O stopped being released?
>
>Ron, I just got out my kiln logs.
>
>On a bisque firing where I kept detailed notes about
>every 15 minutes, I saw moisture on the mirror from:
>4:33 - 5:18 pm, 103 - 172=BAC and from
>8:30 - 10:00pm, 455 - 540=BAC, dark red heat.
>Then I have a note that I saw slight moisture at 10:33,
>650=BAC, red-orange heat. I don't think I checked after
>this as the note says I put the peep hole plugs in.
>
>These are approximate temperatures (=B1 10%).
>You see, I don't have a commercial pyrometer.
>What I have is a type K thermocouple, a Fluke
>multimeter and conversion tables from the
>National Institute of Standards and Technology.
>
>Earl K...
>Bothell, WA, USA
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
=46ax: 613-475-3513=20
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