Paul Lewing on mon 12 feb 01
I got a baffling glaze result out of my kiln this morning, which maybe
someone out there can explain to me.
I've been using a glaze I made up that's 2/3 roadside slip from Montana
and 1/3 gerstley borate. To this I add 5% tin oxide and 5% iron oxide,
and it's always come out a lovely speckled chocolate brown. But it
tends to crawl.
So I calcined some of the slip and made up a batch of the glaze with
half calcined and half uncalcined slip. I didn't bother to adjust for
the loss on ignition of the calcined slip because the proportions don't
seem to be all that critical.
Well, it didn't crawl, but instead of being chocolate colored, it came
out caramel colored. Assuming I weighed everything correctly (perhaps
not a safe assumption), what could possibly be causing this color
difference?
Paul Lewing, Seattle
Ceramic Design Group on tue 13 feb 01
on 2/12/01 1:45 PM, Paul Lewing at pjlewing@WORLDNET.ATT.NET wrote:
> I got a baffling glaze result out of my kiln this morning, which maybe
> someone out there can explain to me.
> I've been using a glaze I made up that's 2/3 roadside slip from Montana
> and 1/3 gerstley borate. To this I add 5% tin oxide and 5% iron oxide,
> and it's always come out a lovely speckled chocolate brown. But it
> tends to crawl.
> So I calcined some of the slip and made up a batch of the glaze with
> half calcined and half uncalcined slip. I didn't bother to adjust for
> the loss on ignition of the calcined slip because the proportions don't
> seem to be all that critical.
> Well, it didn't crawl, but instead of being chocolate colored, it came
> out caramel colored. Assuming I weighed everything correctly (perhaps
> not a safe assumption), what could possibly be causing this color
> difference?
> Paul Lewing, Seattle
Sounds to me that the big variable in your glaze is the "roadside slip".
Perhaps the calcining of this material not only eliminated the chemical
water but also changed this material in terms of its chemistry.Could be that
the iron and other materials that gave the "roadside slip" its deep
chocolate color were altered by calcining.
It would be interesting to have an analysis of your "roadside slip"
material.
Best
Jonathan
Jonathan Kaplan, president
Ceramic Design Group
PO Box 775112
Steamboat Springs CO 80477
voice and fax 970 879-9139
jdkaplan@cmn.net
info@ceramicdesigngroup.net
Plant Location:
1280 13th Street Unit 13
Steamboat Springs CO 80487
(please use this address for all deliveries via UPS, common carrier, Fed Ex,
etc.)
"Custom design and manufacturing for the ceramic arts, giftware and pottery
industries. Molds, models, and tooling for slip casting, jiggering and
hydraulic pressing. Consultation on technical issues such as clay bodies
glazes, and kilns."
Ruth Ballou on tue 13 feb 01
Hi Paul,
The iron sets in Currie's Stoneware Glazes can provide some help here, I
think.The Al:Si ratio has a big impact on the color response of iron. A
change in the ratio will cause a change in the color response as you've
described. If you can't get back the color by adjusting for the difference
between calcined and uncalcined, I think the Ian's recipe approach is just
the ticket here, particularly since you're dealing with a non-commercial
material. His website http://ian.currie.to/ provides all the information,
if you need it.
Ruth
>I got a baffling glaze result out of my kiln this morning, which maybe
>someone out there can explain to me.
>I've been using a glaze I made up that's 2/3 roadside slip from Montana
>and 1/3 gerstley borate. To this I add 5% tin oxide and 5% iron oxide,
>and it's always come out a lovely speckled chocolate brown. But it
>tends to crawl.
>So I calcined some of the slip and made up a batch of the glaze with
>half calcined and half uncalcined slip. I didn't bother to adjust for
>the loss on ignition of the calcined slip because the proportions don't
>seem to be all that critical.
>Well, it didn't crawl, but instead of being chocolate colored, it came
>out caramel colored. Assuming I weighed everything correctly (perhaps
>not a safe assumption), what could possibly be causing this color
>difference?
>Paul Lewing, Seattle
>
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Cindy Strnad on tue 13 feb 01
I wouldn't think the calcining of the roadside slip would make a difference,
but maybe it would. Is it possible you may have gathered this batch of
"wild" clay from a slightly different location? Even deeper in the bank? You
know how it is, trying to tame wild things.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
earthenv@gwtc.net
http://www.earthenvesselssd.com
Lili Krakowski on tue 13 feb 01
The delight of Clayart is meeting new problems--those one has been spared
over the years. My instant guess is that the crawls (little lumps) by
creating a bead --i.e. the glaze is infinitely thicker than spread
out--gives a different visual effect/optical illusion/whatever. Before I
got out my Little Scientist Kit (bought from Ignorance Is Bliss
Educational Toy Co.) I'd make a deep but small pinch pot, and when it is (if
you bisque) or at the stage at which you normally glaze)
spoon some glaze (dry is fine) into it so that when it is glaze fired
you have the approximate thickness of those beads. Proceed from there
Good luck
Lili Krakowski
michael wendt on tue 13 feb 01
Paul,
Great illustration of the importance of the materials lost due to firing.
Try the same experiment again, but weigh out the half you calcine before you
calcine it and please report to us what you find. My guess is you have too
much slip for the Gertsley Borate after calcining. The same experiment could
be done using a line blend with more and more slip or less GB until the
color change is the same.
Thanks for sharing,
Regards,
Michael Wendt
Edouard Bastarache on tue 13 feb 01
Hello Paul,
in 33 years of dabbling in pottery I have always been told that
to duplicate a glaze it is better to use the same raw materials
because of possible changes in them, mainly the "impurities".
Calcining may have modified them.
Later,
Edouard Bastarache
Irr=E9ductible Qu=E9becois
Sorel-Tracy
Dans / In "La Belle Province"
edouardb@sorel-tracy.qc.ca
http://www.sorel-tracy.qc.ca/~edouardb/
Erik Buitenhuis on tue 13 feb 01
Well, the only experience I have with crystal water is with
chemicals that dissolve in water, for which the only difference is
that you need more product if it contains crystal water (1 mol of
FeCl3*6H20 weighs 270 gram, while 1 mol of FeCl3 weighs 162
gram), so I don't really understand why removing the crystal water
would make a difference if you are then going to proceed by adding
water. Apparently, it does make a difference.
Anyway, other chemical changes can occur. For instance
carbonates can get converted to oxydes: CaCO3 -> CaO + CO2 at
about 1000 =B0C.
Cheers,
Erik Buitenhuis.
John Hesselberth on tue 13 feb 01
Paul Lewing wrote:
>Well, it didn't crawl, but instead of being chocolate colored, it came
>out caramel colored. Assuming I weighed everything correctly (perhaps
>not a safe assumption), what could possibly be causing this color
>difference?
>Paul Lewing, Seattle
Hi Paul,
Here is another guess. Color depends on physics as well as chemistry. I
would guess that your calcining changed the effective particle size of
your clay AND you are not getting complete melting to melt that new
particle back out. Hence you have a different physical structure and,
therefore, a different color.
Regards, John
"The life so short, the craft so long to learn." Hippocrates, 5th cent.
B.C.
Ron Roy on wed 14 feb 01
I think Michael is exactly right here. If you don't take LOI into
consideration you are simply going to get more alumina and silica than you
bargained for.
Weigh out 1000 grams of the ditch slip and calcine it - then weigh it again
to see how much was "lost." It will be around 10% - when you use that
calcined clay in your recipe as a sub for the uncalcined - simply use 10%
less - or what ever the LOI actually is. The glaze will then look the same
- Wana bet?
RR
>Paul,
>Great illustration of the importance of the materials lost due to firing.
>Try the same experiment again, but weigh out the half you calcine before you
>calcine it and please report to us what you find. My guess is you have too
>much slip for the Gertsley Borate after calcining. The same experiment could
>be done using a line blend with more and more slip or less GB until the
>color change is the same.
>Thanks for sharing,
>Regards,
>Michael Wendt
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
Paul Lewing on mon 19 mar 01
Remember this thread from a few weeks back? I had made a test of a slip
glaze I've used many times before, but had calcined half the slip to
prevent crawling, and the result had come out much lighter than the raw
version. I didn't understand how that was possible. Several people
told me I may have changed the Si:Al ratio and that I had to compensate
for Loss on Ignition.
Well, I tested it again. I calcined some more, calculated the LOI,
carefully made a batch using all raw slip and one using all calcined
slip, compensting for LOI. And just for fun, I made one with all
calcined slip without compensating for LOI.
The result: all three looked exactly alike. Conclusion: Lewing
screwed up something when he measured the first time. So come to my
discussion group at NCECA on using glaze calculation software. I'll be
moderating, but maybe there'll be some REAL glaze experts in the
audience. I hope so. I'll need them.
Paul Lewing, Seattle
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