Emily Henderson on sun 1 jun 97
Now Mr. Roy and Mr Hansen, Mr. Hewitt, Mr. Hamiliton, Mr. Platt and you
other genii.... Can you help me. I'm stuck out here on the Oregon Coast, 2
hours from Portland and 3 1/4 from Seattle. This is a beautiful place to
be, but not for clay. I have access to a clay which I can drive to and am
trying to fit a glaze to it. (I realize the other way 'round makes more
sense but I can't handle the shipping charges of having clay sent to me.)
So....The stuff I'm using allegedly has a firing range of ^6-10. It is a
very open, very short, fine to throw, less than deluxe to trim buff
stoneware with some minor iron impurities. At cone 10, I have noticed that
high magnesia glazes with low thermal expansions (one at 58.1 X 10^-7, one
at 58.0 x 10^-7 and one at 59.7 x 10^-7) work very over this body with NO
immediate or delayed crazing.
I am trying to develop a ^7-8 base that will not craze over this body. I
have experimented with trying to drop a ^ 10 to ^6 (not too successful) Now
I am trying to take ^6 and move it up. (I need a NON-fluid opaque base
glaze that STAYS-put...)
Here are some things which I have tried and the results:
^9-11 (seems to be a modified Carleton Ball)
Potash feldspar 42.00 5.75% bw of MgO
Dolomite 8.00 6.88% bw of CaO
colemanite 12.00 Est. Thermal Expansion: 58.0 x 10^-7
talc 15.00
ball clay 4.00
silica 20.00
bentonite .30
Nice ^10 very stable, probably ^9-^11
Question? Would a change to soda feldspar drop it to ^8? If so, would
the soda dissolve more easily in the glaze? Would the expansion remain
constant? How much more fluidity could I expect? Do you have any
suggestions to best adjust this glaze and maintain its nice incredibly
stable characteristics?
Vellum
^7-8
Potash feldspar 46.0
Flint 20.0
Talc 15.0
Dolomite 10.0
Colemanite 15.0
zircopax 10.0
epk 5.0
This crazed, but I can't figure out why. I may have the math wrong, I
thought my expansion was 61.1 x 10^-7, MgO at 5.33 CaO at 7.14% and packed
with silicon at 52.13% Maybe I measured wrong. Of course, I could have
confused my containers....scratch head, scratch head....
My experimental blue2
^6-?
Nepheline Syenite 30.60
Dolomite 23.43
Talc 2.85
Whiting 2.85
epk 22.66
flint 12.00
Too low in silica? Not durable enough? Estimated expansion 60.9 x 10 ^-7
This didn't craze but will it be difficult to keep out of solution? If I
had colorants high in metal will this be food safe?
I have also tried wonderful suggestionss by Anne Chambers and Jackie
Lumsden. But, alas, they stewing in my solution of coffee and tea, have
crazed too.
I have the following suggestion from Mr. Hewitt (S2D) which I hope to try
soon but first had to convert his expansion numbers for the ones I'm used to.
Cornwall Stone 517
kaolin 38
dolomite 130
whiting 146
flint 168
ferro frit 3134 100
zircopax 120
tin oxide 90
Estimated expansion 60.1 x 10^-7
^8 gloss pg. 246 from Claywork Form and Idea in Ceramic Design
potash feldspar 30.0
epk 15.0
whiting 14.0
talc 4.0
zinc oxide 6.0
flint 31.0
(I planned to add 10% zircopax)
Ron Roy suggested a recalculated a ^7 glaze Nickel Buckwheat by W.T. Yeh
(Ceramic Monthly 3/95) for Kathleen today. I wondered if it might be a
candidate for me to try too:
Talc 10.5
Whiting 19.0
Custer (G-200) 47.5
epk 7.5
Silica 15.5
(I wondered about 10% zircopax for white and thought I'd like to try it too
I'd like to check to expansion first.)
I'm really dying out here...any help with a glaze...ANY glaze that will fit
my body will be deeply appreciated. I'd love a base clear that I could
opacify for white and is good with colorants...... No pitfires yet this
season and I've even got DRY dung. It just won't stop raining. TIA to you
smart smart people LOL Emily in WET WET Astoria OR
Karl P. Platt on mon 2 jun 97
Emily --
Since you're in that lovliest of cities, despite the rain, let me give
this a whirl. I really do love Astoria and have a very warm spot for
Oysterville. Sometime I'll tell you of my experiences there. For the
moment, however, we have to be concerned with your present dilemma --
misfit clay and glaze.
First, ditch the calculated thermal expansion numbers. The have
essentially no relevance whatsoever. Surprised? Don't be. I'll argue
this point loud and long.
These "factors" assume a linearity which simply does not exist --
especially if you go outside the range of composition for which the
values were calculated in the first place. Winklemann & Schott, Englsh &
Turner, Hall, Appen, etc, all worked to obtain this holy grail of an
exact fit between composition and thermal expansion. None of them really
succeeded -- and if they did it was only within the range of
compositions analyzed. I'm not going to go off into the gory details of
this unless provoked, but will repeat that these nmbers are of HIGHLY
limited practical value and bear no scientific value whatever.
This could go on for many pages, so let's return to the matter at hand.
MgO facilitates low expansion in 2 ways. First, it tends to form
crystalline material with (relatively speaking) low thermal expansion,
and it also contributes to glassy material with low thermal expansion.
As well, in the glassy material, it also tends to make the glass more
elastic. That is, it takes more strain to form stress in the glassy
material and the tendency to craze despite misfit is reduces (somewhat).
MgO is a lousy thing to use in colored glazes for reasons beyond the
scope of the present discussion.
One is, of course, a little curious why you'd want to underfire the
body. If it is ideal at Cone 10, firing to cone 6 or 7 leaves a lot of
things incomplete. To the extent that there any alkalies anr/or alkaline
earths remaining in the body which can react with moisture (atmospheric
or otherwise) and swell the body, you'll find that fitting a glaze will
remain highly elusive -- like an 80 Lb Steelhead out there in the
Columbia.
>I have also tried wonderful suggestionss by Anne Chambers and Jackie
>Lumsden. But, alas, they stewing in my solution of coffee and tea, >have craze
>At cone 10, I have noticed....NO immediate or delayed crazing.
Again, why underfire the body? If you're not in a position to modify it
you'll not be able to ensure the long-term integrity of your work if it
is underfired.
Try this. Make a tile, glaze it and fire it to cone 7. If it's not
crazed after the firing, put it in a pressure cooker for a couple hours.
If it comes out crazed there's a good chance that moisture expansion is
a problem. Given your observations to this point and with a result as
described above, you should (must) revert to the higher firing.
I'm not inclined to run all of these glazes though the spreadsheet and
am not sure it'd do much good. Let me ofer this instead. Have a look at
the crazing and if necessary, smear some India ink on the glaze to
highlight the craze lines. The crazing pattern is a function of the
degree to which the glaze is mis-fit. Few broadly spaced cracks suggest
a small degree of misfit and lots of small cracks suggest a large degree
of misfit.
If the glaze is slightly misfit on the _mature_ body you have the
options of:
A. Reducing and complicating the alkali -- less alkali/higher fusibility
B. Increasing Al2O3/SiO2 -- adding clay
C. Adding SiO2
D. A little (1-2 wt%) B2O3 may help -- keeping everything else the same.
E. Complicating the RO a little further -- a smidge of ZnO or SrO -- Not
BaO. This is not owing to any of the leaching hysteria one might read on
Clayart, but simply because BaO contributes to high expansion.
You should have no objection to the higher firing on economic grounds.
The difference in cost relative to the value of the ware is typically
very small. For example, if your cone 10 firing costs $40.00 and the
value of the ware in the kiln is $800.00 if it doesn't craze, it stands
to reason that you don't gain much at all if you lower your firing cost
to $35.00 and get defective ware you can't sell. Is short, the cost of
the firing to maturity is probably <10% of the value of the ware.
Spending less to make ware you can't sell makes no sense at all.
Bejinhos
Karl
Ron Roy on tue 3 jun 97
Karl said:
>First, ditch the calculated thermal expansion numbers. The have
>essentially no relevance whatsoever. Surprised? Don't be. I'll argue
>this point loud and long.
>
>These "factors" assume a linearity which simply does not exist --
>especially if you go outside the range of composition for which the
>values were calculated in the first place. Winklemann & Schott, Englsh &
>Turner, Hall, Appen, etc, all worked to obtain this holy grail of an
>exact fit between composition and thermal expansion. None of them really
>succeeded -- and if they did it was only within the range of
>compositions analyzed. I'm not going to go off into the gory details of
>this unless provoked, but will repeat that these nmbers are of HIGHLY
>limited practical value and bear no scientific value whatever.
I can only agree with the statement that " Winklemann & Schott, Englsh &
>Turner, Hall, Appen, etc, all worked to obtain this holy grail of an
>exact fit between composition and thermal expansion. None of them really
>succeeded." None of them will match the real expansions of glazes.
The preceding statement "The(y) have essentially no relevance whatsoever"
is exactly opposite to my experience over the last 5 years in adjusting
hundreds of glazes to correct fit problems. I use the English and Turner
set and have found them infinitely better than nothing at all and
remarkably accurate, under most conditions, for indicating in which
direction expansion is moving when adjusting glazes.
I have been working on series of glazes to test expansion of clay bodies
using a dilatometer (measures the actual expansion of clays and glazes) to
compare the calculated expansion against the real expansion. The numbers
are never the same - it does not matter which set of factors I use or which
glaze calculation software I use - which confirms what Karl said about the
numbers not matching. However, in every case so far, when the calculated
expansions says it's going down or up the real expansion confirms this.
The calculation of bodies however is another story altogether. There is no
way to assess the amounts of free silica in a body and because quartz and
cristobalite have such a profound effect on expansion/contraction any
attempt to calculate will give misleading figures.
We can argue forever about this - I won't - all we need is an experiment -
of your design? - and we can all calculate the expansion and I will furnish
the real expansions. Don't give me any matte glazes because they are truly
unpredictable - at least at this point in time. I should add, I prefer to
work with glazes which at least approach the definition of durable glass.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/ronroy.htm
David Hewitt on tue 3 jun 97
Emily,
If your experimental blue2 does not craze on your clay body then I would
expect that the the glaze S2D that I suggested should also not craze as
its coefficient of expansion is slightly less than your blue2. Perhaps
it is worth a try.
In message , Emily Henderson writes
>My experimental blue2
>^6-?
>Nepheline Syenite 30.60
>Dolomite 23.43
>Talc 2.85
>Whiting 2.85
>epk 22.66
>flint 12.00
>
>Too low in silica? Not durable enough? Estimated expansion 60.9 x 10 ^-7
>This didn't craze but will it be difficult to keep out of solution? If I
>had colorants high in metal will this be food safe?
>
>I have the following suggestion from Mr. Hewitt (S2D) which I hope to try
>soon but first had to convert his expansion numbers for the ones I'm used to.
>
>Cornwall Stone 517
>kaolin 38
>dolomite 130
>whiting 146
>flint 168
>ferro frit 3134 100
>zircopax 120
>tin oxide 90
>
>Estimated expansion 60.1 x 10^-7
>
This glaze has % mol Parts
Basic 23.39%
Ampho 7.42%
Acidic 69.19%
It is perhaps interesting to note that Ron Roy's suggestion below with
10% Zircopax added would have the following very similar % mol Parts
Basic 22.74%
Ampho 7.10%
Acidic 70.16%
the coef. of expansion, however, is higher on my figures.
>Ron Roy suggested a recalculated a ^7 glaze Nickel Buckwheat by W.T. Yeh
>(Ceramic Monthly 3/95) for Kathleen today. I wondered if it might be a
>candidate for me to try too:
>
>Talc 10.5
>Whiting 19.0
>Custer (G-200) 47.5
>epk 7.5
>Silica 15.5
>
>(I wondered about 10% zircopax for white and thought I'd like to try it too
>I'd like to check to expansion first.)
>
David Hewitt
Tony Hansen on tue 3 jun 97
Karl P. Platt wrote:
> First, ditch the calculated thermal expansion numbers. The have
> essentially no relevance whatsoever. Surprised? Don't be. I'll argue
> this point loud and long.
Sounds like predicting expansion is on about the same level as
everything
else in ceramics. It's a highly imprecise science. I find calculated
expansion less than reliable when comparing glazes of different oxide
systems, but highly useful in predicting the direction and magnitude
of expansion change when making adjustments to a recipe.
Still, I've got dozens
of measured frit expansions to compare with calculated numbers and
the consistency is quite striking. I'd never want to throw this all out
and resort to guessing on the material level.
--
=================================================================
Tony Hansen, IMC thansen@mlc.awinc.com
David Hewitt on wed 4 jun 97
In message , "Karl P. Platt" writes
>
>First, ditch the calculated thermal expansion numbers. The have
>essentially no relevance whatsoever. Surprised? Don't be. I'll argue
>this point loud and long.
>
>These "factors" assume a linearity which simply does not exist --
>especially if you go outside the range of composition for which the
>values were calculated in the first place. Winklemann & Schott, Englsh &
>Turner, Hall, Appen, etc, all worked to obtain this holy grail of an
>exact fit between composition and thermal expansion. None of them really
>succeeded -- and if they did it was only within the range of
>compositions analyzed. I'm not going to go off into the gory details of
>this unless provoked, but will repeat that these nmbers are of HIGHLY
>limited practical value and bear no scientific value whatever.
>
At the risk of starting a long debate I do believe that you have
dismissed the work of these eminent ceramists rather unfairly. I agree
that this is far from an exact science and seeing the variation in the
coefficients produced by each ceramist shows that one cannot take any
resultant calculation of expansion as an absolute definitive figure.
Relative figures for different recipes using one ceramists figures is,
however,IMHO, of very practical value. I do not dismiss your rule of
thumb guidance, but I personally would always also do a glaze expansion
calculation as well to see in which direction and by how much the
figures had changed with each change in recipe. It is by far the best
indicator that we have before actually testing a recipe.
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
URL http://www.ceramicsoftware.com/education/people/hewitt.htm
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