Mark & Pauline Donaldson-Drzazga on fri 19 nov 99
------------------
Dear Louis,
I have very rarely lost anything in the kiln (30+ years) on cooling. In fact
here in England at some Roof Tile Factories they force the cooling to 900C =
then
let it cool naturally, and then open up at 400C and spray water onto the =
tiles.
In my experience of firing very large and thick, also extremely thin pieces =
that
your major problem is from drying incorrectly. The ware might be damp and =
fired
too fast after 180C, or it might have dried too quickly and trapped moisture
within and cant percolate out in time. The other major crack-maker is lack =
of
compression in the forming of the ware (hence S-cracks as discussed =
earlier),
and can result in other flaws. My kiln cools slowly from 950C down to 350C =
(at
about 20C per hour) when I prise open the door to have a peek to see if all =
is
well, hopefully nothing has blown etc. Having learned years ago to take care=
of
the chemical water ( I fire very slowly from 160C - 360C) and most times the
firing will be OK. This is also when I learned that at 573C at the quartz
inversion that the ware expands about 1=25 and then immediately shrinks ( it=
was
pointed out, in this thread, that a super gizmo thingummyjig disproved the
shrinking back at this stage, just goes to show that we all learn throughout=
our
ceramic careers), and at this time things can go bump in the night.
I must confess that I am rather sceptical about so called dunting, as the =
glass
formed will be on the way up. Some clay fired up to only 1000C ( the clays
maturing temperature) by a Tile Manufacturer, and the cracking was thought =
to be
on the cooling cycle. They tried slowing the cooling with no effect. I =
suggested
a different drying programme, and problem solved.
Different clays have different solutions, and all I can say is if what you =
do
works, then sod chemistry or physics and just do it.
Happy potting Marek http://www.moley.uk.com
Ron Roy on mon 22 nov 99
Dear (is it Mark or Pauline)
Calling my $14,000 Amr (new) dilatometer a "super gizmo thingummyjig"
raises a lot of question in my mind. I am sure you have never seen a
dilatometer chart - as most potters . It is a shame because seeing an
expansion curve - especially one of your glaze superimposed on one of your
clay makes everything so much more understandable.
I am not sure now if you understand the points I was making. Every clay -
raw or designed, even translucent porcelain - has a free quartz expansion
at 573C on the way up and it is exactly reversable at the same temperature
on the way down - and it is on the way down that bisque dunting happens -
all you have to do to stop it is apply the right remedies.
As to opening a kiln at 350C - that may be OK for earthenware bodies - and
maybe even some soft stonewares - but if there is any cristobalite in the
body you better be wearing some glasses and make sure none of that hot
stuff gets close to your skin.
Dunting cracks are different from forming cracks - all you have to do is
see them to tell the difference.
I don't think clay explodes because of the chemically combined water - I
think it happens because the ware is not dried enough before the
temperature gets above boiling.
I have been a potter for longer than you - I started in 1959 - but I know
potters who are even older than us who are still learning - like all of
them.
I do hope I am not being a jerk about all this - I don't want you to think
I enjoy being critical - but I do feel an obligation to get these issues
out in the open where we can all learn from everyones experience.
I just had a peek at your web page - very entertaining - love your roof -
and your work - I will be sure to visit when I get there - if I'm still
welcome.
Thanks for taking the time to share your experiences.
RR
>----------------------------Original message----------------------------
>Dear Louis,
>I have very rarely lost anything in the kiln (30+ years) on cooling. In fact
>here in England at some Roof Tile Factories they force the cooling to 900C then
>let it cool naturally, and then open up at 400C and spray water onto the tiles.
>In my experience of firing very large and thick, also extremely thin
>pieces that
>your major problem is from drying incorrectly. The ware might be damp and fired
>too fast after 180C, or it might have dried too quickly and trapped moisture
>within and cant percolate out in time. The other major crack-maker is lack of
>compression in the forming of the ware (hence S-cracks as discussed earlier),
>and can result in other flaws. My kiln cools slowly from 950C down to 350C (at
>about 20C per hour) when I prise open the door to have a peek to see if all is
>well, hopefully nothing has blown etc. Having learned years ago to take care of
>the chemical water ( I fire very slowly from 160C - 360C) and most times the
>firing will be OK. This is also when I learned that at 573C at the quartz
>inversion that the ware expands about 1% and then immediately shrinks ( it was
>pointed out, in this thread, that a super gizmo thingummyjig disproved the
>shrinking back at this stage, just goes to show that we all learn
>throughout our
>ceramic careers), and at this time things can go bump in the night.
>I must confess that I am rather sceptical about so called dunting, as the glass
>formed will be on the way up. Some clay fired up to only 1000C ( the clays
>maturing temperature) by a Tile Manufacturer, and the cracking was thought
>to be
>on the cooling cycle. They tried slowing the cooling with no effect. I
>suggested
>a different drying programme, and problem solved.
>Different clays have different solutions, and all I can say is if what you do
>works, then sod chemistry or physics and just do it.
>
>Happy potting Marek http://www.moley.uk.com
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
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