Leona Stonebridge Arthen on tue 2 dec 97
Speaking of Cornwall Stone, I used to work with a woman who remembers it
as a white powder. That which I have bought has been a sort of baby blue
and lumpy. The last bag I got was bluer and lumpier yet. I haven't mixed
anything with it yet, but the older stuff had a tendency to thicken glazes
quite a bit and cause crawling problems with the slightest provocation.
Is there anything that can be done to improve the behavior of this Cornwall
Stone? Would calcining it help?
Leona,
In Western Mass. with snow and ice and cold, cold wind.
---
Leona Stonebridge Arthen
leona@javanet.com
Worthington, Massachusetts/USA
David Hendley on wed 3 dec 97
Leona,
The (relative) new book "Out of the Earth, Into the Fire" by Mimi Obstler
goes into great detail differentiating the different
varieties of Cornwall Stone. It lists at least 5 different types.
It is definately worth a look, and worth buying if you are serious about
understanding
ceramic materials.
(I personally have no experience with Cornwall Stone & can't help you.)
Best wishes,
David Hendley
At 10:21 AM 12/2/97 EST, you wrote:
>----------------------------Original message----------------------------
>Speaking of Cornwall Stone, I used to work with a woman who remembers it
>as a white powder. That which I have bought has been a sort of baby blue
>and lumpy. The last bag I got was bluer and lumpier yet. I haven't mixed
>anything with it yet, but the older stuff had a tendency to thicken glazes
>quite a bit and cause crawling problems with the slightest provocation.
>
>Is there anything that can be done to improve the behavior of this Cornwall
>Stone? Would calcining it help?
>
>Leona,
>In Western Mass. with snow and ice and cold, cold wind.
>
>
>---
>Leona Stonebridge Arthen
>leona@javanet.com
>Worthington, Massachusetts/USA
>
>
David Hendley
Maydelle, Texas
See David Hendley's Pottery Page at
http://www.sosis.com/hendley/david/
Bill Amsterlaw on thu 4 dec 97
Hi Leona:
There is a good discussion on Cornish Stone in Hamer's _The Potter's
Dictionary_ (source of my information below).
It is a highly variable material that contains feldspar, quartz, kaolinite,
mica, and fluorspar.
The fluorspar is responsible for the purple coloration. Samples containing
more flourspar also contain more feldspar and melt at a lower temperature.
Because fluorine gas is harmful, suppliers now process some by flotation
to remove the fluorspar and ferreous micas. The processed product is
called "D.F. stone".
The thickening tendency you observed is caused by the clay content which
can vary from 5-12%. When it is used in a recipe that already has a high
clay content, pre-firing cracking may occur because the total amount of
clay in the mixture is too great.
Remedy (my suggestion) for the cracking/crawling problem.... If the recipe
calls for bentonite, drop the bentonite. If there is no bentonite in the
recipe, sub some calcined kaolin for some of the clay; run tests to see how
much.
You wrote:
>>
Speaking of Cornwall Stone, I used to work with a woman who remembers it
as a white powder. That which I have bought has been a sort of baby blue
and lumpy. The last bag I got was bluer and lumpier yet. I haven't mixed
anything with it yet, but the older stuff had a tendency to thicken glazes
quite a bit and cause crawling problems with the slightest provocation.
Is there anything that can be done to improve the behavior of this Cornwall
Stone? Would calcining it help?
<<
- Bill Amsterlaw (wamster@slic.com)
Plattsburgh, NY
David Hewitt on thu 4 dec 97
This ( a pale blue) is the normal colour to-day for Cornish Stone in the
UK. I presume that you have the same material. For health reasons it has
to supplied with a degree of moisture, as is also the case with quartz.
Mine is always lumpy. I would advise that you do not worry about this
and just use it as it is.
David
In message , Leona Stonebridge Arthen writes
>----------------------------Original message----------------------------
>Speaking of Cornwall Stone, I used to work with a woman who remembers it
>as a white powder. That which I have bought has been a sort of baby blue
>and lumpy. The last bag I got was bluer and lumpier yet. I haven't mixed
>anything with it yet, but the older stuff had a tendency to thicken glazes
>quite a bit and cause crawling problems with the slightest provocation.
>
>Is there anything that can be done to improve the behavior of this Cornwall
>Stone? Would calcining it help?
>
>Leona,
>In Western Mass. with snow and ice and cold, cold wind.
>
>
>---
>Leona Stonebridge Arthen
>leona@javanet.com
>Worthington, Massachusetts/USA
>
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
URL http://digitalfire.com/education/people/hewitt.htm
Fay & Ralph Loewenthal on fri 5 dec 97
Leona I have used James Chappell's recipe to make up my
own Cornish Stone since Ferro raised the price about 5
times. I was pleasantly surprised that the glazes I used it in
were better than with the bought C S. Here is the recipe from
his book "The potter's complete book of clay and glazes".
67 parts Feldspar - I use Potash
22 parts Silica
11 parts Kaolin
Since I am in South Africa I have different ingredients to you,
so you would have to test. Hope this helps Ralph in PE SA
Bob Hanlin on sun 7 dec 97
At 10:21 AM 12/2/97 EST, you wrote:
>----------------------------Original message----------------------------
>That which I have bought has been a sort of baby blue
>and lumpy. The last bag I got was bluer and lumpier yet.
Leona:
I just bought a bag of Cornwall from Trinity in Dallas and was surprised
that it was GREEN and lumpy.
I called Trinity and they did the best shrugging of shoulders on the phone
I've ever experienced.
It's just how they get the stuff. Mine was a bit damp, also. Dried it out
on some platters while
bisque firing.
I'm glad to hear that the lumpy and colored stuff is a universal thing. I
used Cornwall Stone years
ago and it was white and powder. I guess that they're sluffing the
pulverizing to us.
Bob Hanlin
bhanlin@ionet.net
Oklahoma City, OK
ClayCoyote on tue 9 dec 97
When mixing Cornwall Stone in a glaze, we've found that if you put the
cornwall into nearly the total needed water, it will disolve well. The add
the other ingredients. I know Karl recommends putting in the clay ingredients
first to help suspend everything else, but this alternative method seems to
work best with lumpy CS. The other alternative is to put in clay ingred, then
cornwall and other stuff, and then soak over night before screening.
Don't know if this presaok of the CS changes anything, but it doesn't seem to.
PS Does the W.G. we see on the bag of CS we get mean Wet Ground?
Tom Wirt
Clay Coyote Pottery
Hutchinson, MN
clay[ot@hutchtel.net
Stephen Mills on tue 9 dec 97
When I first started potting here in the UK in the 60s Cornish stone
came in two versions HP & MP, these stood for Hard Purple & Mild Purple.
in fact the HP was a pale bluish green and the MP a pale greenish blue.
We can only get the latter now. Incidentally all raw material powders in
the UK have to be supplied containing 5% moisture for safety reasons,
important when you consider how dangerous inhaling silica dust is.
Steve
Bath
UK
In message , Bob Hanlin writes
>----------------------------Original message----------------------------
>At 10:21 AM 12/2/97 EST, you wrote:
>>----------------------------Original message----------------------------
>>That which I have bought has been a sort of baby blue
>>and lumpy. The last bag I got was bluer and lumpier yet.
>
>Leona:
>I just bought a bag of Cornwall from Trinity in Dallas and was surprised
>that it was GREEN and lumpy.
>
>I called Trinity and they did the best shrugging of shoulders on the phone
>I've ever experienced.
>It's just how they get the stuff. Mine was a bit damp, also. Dried it out
>on some platters while
>bisque firing.
>
>I'm glad to hear that the lumpy and colored stuff is a universal thing. I
>used Cornwall Stone years
>ago and it was white and powder. I guess that they're sluffing the
>pulverizing to us.
>
>
>Bob Hanlin
>bhanlin@ionet.net
>Oklahoma City, OK
>
--
Steve Mills
Bath
UK
home tel: (44) (0)1225 311699
work tel: (44) (0)1225 337046
pedresel@3-cities.com on fri 12 dec 97
So do you correct for the moisture when weighing out ingredients?
-- Evan in Eastern Washington State whose hand only seems to crack where he
holds his pen -- must be in the wrong line of work
At 07:03 AM 12-9-97 EST, you wrote:
>----------------------------Original message----------------------------
>When I first started potting here in the UK in the 60s Cornish stone
>came in two versions HP & MP, these stood for Hard Purple & Mild Purple.
>in fact the HP was a pale bluish green and the MP a pale greenish blue.
>We can only get the latter now. Incidentally all raw material powders in
>the UK have to be supplied containing 5% moisture for safety reasons,
>important when you consider how dangerous inhaling silica dust is.
>Steve
>Bath
>UK
>
Stephen Mills on sun 14 dec 97
No, most powders available to us come that way so I personally
have never bothered to adjust, besides I am by nature lazy. My
glazes didn't vary batch to batch
Steve
in Bath
UK
What's a pen for? :-)
In message , pedresel@3-cities.com writes
>----------------------------Original message----------------------------
>So do you correct for the moisture when weighing out ingredients?
>
>-- Evan in Eastern Washington State whose hand only seems to crack
>where he
>holds his pen -- must be in the wrong line of work
>
>At 07:03 AM 12-9-97 EST, you wrote:
>>----------------------------Original message----------------------------
>>When I first started potting here in the UK in the 60s Cornish stone
>>came in two versions HP & MP, these stood for Hard Purple & Mild Purple.
>>in fact the HP was a pale bluish green and the MP a pale greenish blue.
>>We can only get the latter now. Incidentally all raw material powders in
>>the UK have to be supplied containing 5% moisture for safety reasons,
>>important when you consider how dangerous inhaling silica dust is.
>>Steve
>>Bath
>>UK
>>
>
--
Steve Mills
Bath
UK
home tel: (44) (0)1225 311699
work tel: (44) (0)1225 337046
Ron Roy on mon 15 dec 97
To dry or not to dry - this depends on how much of the material the glaze
calls for and how sensitive the glaze is to small changes - the material
will dry gradually over time so the amount you weigh out will be more as
time goes by. Why not leave the bag in a warm spot and let it dry.
The technique for finding out if there is a need would be to weigh out a
quantity while wet, dry it and weigh again - then do a test of the glaze
using the different amounts.
>----------------------------Original message----------------------------
>No, most powders available to us come that way so I personally
>have never bothered to adjust, besides I am by nature lazy. My
>glazes didn't vary batch to batch
>Steve
>in Bath
>UK
>What's a pen for? :-)
>
>In message , pedresel@3-cities.com writes
>>----------------------------Original message----------------------------
>>So do you correct for the moisture when weighing out ingredients?
Ron Roy
93 Pegasus Trail
Scarborough,Canada
M1G 3N8
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm
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