karen lovenguth on tue 26 mar 02
Hi everyone!
Thanks for all the help. I have been thinking
all day about all the suggestions and valuable info
you have all sent me. I do have a few questions still
if I may.
Vince when you say that my clay has no grit I was
always under the impression that it did. The fire clay
zacatecas here is widely substitued as grog. I have 30
% in the mix. Do you mean even a larger substance?
Vince you also mentioned sand. What kind of sand is
that? Tommorrow I will call my ceramic supply place to
see what they have. I can find here in town two kinds
of sand, river sand that is fine or mined sand that is
a grey color and used for construction or is it ocean
sand?
Rick thanks also for writing. You had some good
questions that I have also been batting around all
day. Yes rick my vessels are 5 gallon size and some
even larger. It is the size that complicates matters.
My smaller pieces have not cracked as the larger ones
have. Rick suggests larger sized particles (temper) in
the clay. Even grinding up broken fired vessels and
adding that to the clay to open up the body. You also
asked me about the bottoms of the pots.They are all
flat and that got me to thinking about all the pots
that one finds here in Mexico for cooking. They are
all rounded bottomed pots and withstand the fire
daily. Perhaps forms that are rounded on the bottom
can withstand better my pit firing. I have also
noticed but never really made the formal clic in my
head til now that when I lay a pot on its side in the
pit there is less a chance that it cracks. Or if it
does crack ,it does alittle more discreetly.
Rick you also pointed out that lowering my
bisque temperature could help. Is the logic being that
the clay stays more porous and has more room without
cracking to expand and contract?
I have tried local clays and have other types of
problems. I don´t know the technical ceramic word for
it in english but the two local clays that I have
tried had lime in them. After firing thema little
piece of the fired clay would sort of chip off and
them another. My pieces were pock marked all over. I
decided that was not worth it and now I have the
cracking problem. I know one step at a time . I have
lots of other questions. Vince I can´t wait to try
your terra sig recipe that I found recently. I do
though have to find a reliable clay first. If not I am
just stupidly losing pots for a bad clay mix. Have a
nice evening my fellow ceramists and thanks again !
Karen in Ocotitlan Tecacalanco (the name of theland
where I live in Nahuatl)
P.S. One last question that Marck and Slvia Monloch
asked vince about was the mullite? I have a ceramist
friend in Oaxaca whom if my memory serves me correctly
told me that I should add mullite to my pieces and
that would also help me with the cracking. There are
certain types of pottery from oaxaca that have sort of
a shiny substance in the pieces. Is that mullite? It
looks like vermiculitte. I add that to my cactus
mixes. Does anyone know alittle more about that?
PPS Roger did you ever do any of those hydrocloric
acid tests on the local clays? They say that if the
solution bubbles that there is lime. Does anyone know
about that either?It could sure save one alot of time
knowing if their local clays are free from such
impurities. It does sound a bit dangerous though the
acid.
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Andi Fasimpaur on wed 27 mar 02
At 05:56 PM 3/26/02 -0800, you wrote:
>P.S. One last question that Marck and Slvia Monloch
>asked vince about was the mullite? I have a ceramist
>friend in Oaxaca whom if my memory serves me correctly
>told me that I should add mullite to my pieces and
>that would also help me with the cracking. There are
>certain types of pottery from oaxaca that have sort of
>a shiny substance in the pieces. Is that mullite? It
>looks like vermiculitte. I add that to my cactus
>mixes. Does anyone know alittle more about that?
The shiny stuff you're talking about is Mica... (not vermiculite)
depending on the source and the composition, mica will
withstand firings to ^6 without melting. Mica is available from
a variety of sources or you might be able to find a source for
the stone yourself. It is a soft enough mineral that you can
grind it in a mortar and pestle.
Vermiculite is heat expanded mica... I've heard of people
mixing it into clay for temper, but the properties of the mica
are changed by the transformation into vermiculite... The
vermiculite is puffy, with lots of trapped air... In addition,
there has been some concern about asbestos contamination
of vermiculite. More information about that can be found at
http://www.epa.gov/opptintr/asbestos/verm.htm
As a person who is in love with the sparkle of mica, i wish you
great luck... The subtle beauty of it combined with its utility as
a temper make it one of my favorite clay additives.
Best Wishes,
Andi.
Snail Scott on wed 27 mar 02
Kyanite has an excellent effect on thermal shock
when rakuing large pieces. It's available in
very coarse and powdered forms. Mica is also
great for thermal shock - I've seen mica-rich
pieces fired on open flames, unsaggared, in the
wind, with no breakage. Mica is available in
the familiar shiny platelet form, as well as a
powdered form which gives all the thermal-shock
advantages without changing the look of your
work. I've heard good things about pyrophyllite,
too, but have never used it myself.
-Snail
Cindy Strnad on wed 27 mar 02
Dear Karen,
The shiny substance is probably mica. It will increase the thermal shock
resistance of your clay, from what I've read here on clayart. The shape of your
pots does matter. If you make them with a rounded bottom, they should be more
thermal shock resistant.
Good luck,
Cindy
=====================================
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
cindy@earthen-vessels-pottery.com
http://www.earthen-vessels-pottery.com
CM Critique discussion forum
http://www.earthen-vessels-pottery.com/toforum.html
vince pitelka on wed 27 mar 02
> Vince when you say that my clay has no grit I was
> always under the impression that it did. The fire clay
> zacatecas here is widely substitued as grog. I have 30
> % in the mix. Do you mean even a larger substance?
> Vince you also mentioned sand. What kind of sand is
> that? Tommorrow I will call my ceramic supply place to
> see what they have. I can find here in town two kinds
> of sand, river sand that is fine or mined sand that is
> a grey color and used for construction or is it ocean
> sand?
Karen -
I am not familiar with your fireclay. Unless it has particles of hard grit
(like sand or grog) in significant quantities, then it will not serve in
place of grog. I have never heard of a fireclay being used in place of
grog, but I am only familiar with our domestic materials in the US.
Even if the fireclay can be used in place of grog, I still do not think
there is enough grit. If it is in fact mostly clay content (and why else
would they call it fireclay?), then there cannot be that much grit in it.
Claybodies used in bonfiring often have more than 50% tempering materials
(sand, grog, crushed seashells, volcanic ash, etc).
The sand we use in claybodies in the US usually comes from a silica
supplier, which means that it is probably crushed, meaning angular with
sharp edges. Beach sand and river sand are much more rounded, and will not
give as much support. I really do not know what the difference is in the
firing. I would think that the mined sand might be better, as long as it is
silica sand with minimal impurities.
Kyanite is an aluminum silicate mineral, and is a good low-expansion source
of silica and alumina, so it increases thermal shock resistance. You can
use milled kyanite in place of some of your clay (also an aluminum
silicate), and/or granular kyanite as a grog.
Good luck -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
Craig Clark on wed 27 mar 02
Cindy, another "rule of thumb" is that the wider (more open) and flatter the
piece the more subject to thermal shock. Closed forms tend to fair better
than open forms.
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
----- Original Message -----
From: "Cindy Strnad"
To:
Sent: Wednesday, March 27, 2002 11:28 AM
Subject: Re: re coiled pots and pit fire breakage
> Dear Karen,
>
> The shiny substance is probably mica. It will increase the thermal shock
> resistance of your clay, from what I've read here on clayart. The shape of
your
> pots does matter. If you make them with a rounded bottom, they should be
more
> thermal shock resistant.
>
> Good luck,
>
> Cindy
> =====================================
> Cindy Strnad
> Earthen Vessels Pottery
> RR 1, Box 51
> Custer, SD 57730
> USA
> cindy@earthen-vessels-pottery.com
> http://www.earthen-vessels-pottery.com
> CM Critique discussion forum
> http://www.earthen-vessels-pottery.com/toforum.html
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
Craig Clark on thu 28 mar 02
Snail, I'm going to be mixing up my old clay body again to use for
larger forms in raku. The bottles will be as large as 48 inches, shapes from
Amphora to standard jar, platters to 40 inches and spheres to 36 inches.
I may substitute the kyanite for the grog that I used. I had a
proportion of 20% grog before. It worked well on pieces that were smaller
(bottles 18 to 24 inches. Platters at 22 inches). The problem was that with
that high a percentage of grog the body is extremely abrasive on the ole
hide and when I trim there is the scaring problem as well (all those smaller
pock marks from the grog being pulled out of the body. I normally use a
sieved slip of the body over the trimmed area to correct the pock marking)
Do you know wether or not the use of Kyanite will eleminate any of this?
Will it impart a greater degree of thermal shock resistance than the grog so
that I may be able to use less of it in the body? I was using a 28 mesh AP
Green grog.
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
----- Original Message -----
From: "Snail Scott"
To:
Sent: Wednesday, March 27, 2002 10:30 AM
Subject: Re: re coiled pots and pit fire breakage
> Kyanite has an excellent effect on thermal shock
> when rakuing large pieces. It's available in
> very coarse and powdered forms. Mica is also
> great for thermal shock - I've seen mica-rich
> pieces fired on open flames, unsaggared, in the
> wind, with no breakage. Mica is available in
> the familiar shiny platelet form, as well as a
> powdered form which gives all the thermal-shock
> advantages without changing the look of your
> work. I've heard good things about pyrophyllite,
> too, but have never used it myself.
>
> -Snail
>
>
____________________________________________________________________________
__
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
Snail Scott on thu 28 mar 02
At 06:16 AM 3/28/02 -0600, you wrote:
>Snail...
>I may substitute the kyanite for the grog that I used...
>...that high a percentage of grog...is extremely abrasive...
>Do you know wether or not the use of Kyanite will eleminate any of this?
>Will it impart a greater degree of thermal shock resistance...
Kyanite is available as a fine powder as well
as in the granular form, so it should throw just
fine. It will do much more for thermal shock than
grog can, too. I'd probably keep the percentage
(20%) about the same for now, since your work is
so very large. If you prefer a slightly groggy
body, maybe try 10% granular kyanite and 10%
powdered kyanite, or 5% and 15%, or some such.
(I haven't really done many experiments with the
powdered or milled form, so perhaps other folks
can recommend their favorite percentages for
very large raku work.)
-Snail
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