Lili Krakowski on wed 24 aug 11
Am pursuing this topic/thread because I think it is a good example for =3D
those who want to start mixing their own glazes.
As tedious as it seems, sounds, and to many is, this is how it is, this =3D
is what one does in the glaze room---over and over and over again. And =3D
those of us who love it, can=3DE2=3D80=3D99t get enough. We calculate, mix,=
=3D
test forever. We pursues ideas, we form theories--but there is as yet no =
=3D
"one size fits all" mind set...And most potters have a particular =3D
firing method, particular look they seek--and stay in that "ball park" =3D
all their lives. Our generalizations tend to have little depth. =3D20
Ok, original recipe for a =3DE2=3D80=3D9Cclear=3DE2=3D80=3D9D c. 6 glaze g=
oes like =3D
this:
All ensuing calculations by GlazeMaster=3DE2=3D84=3DA2
25 Silica
15 Kaolin--EPK
25 Gerstley Borate--1999
25 Nepheline Syenite
10 Talc
.214 Na2O
.053 K2O
.311 MgO
.423 CaO
.535 Al2O3
.38 B2O3
.
3.966 SiO2
61.9 Exp
And the glaze=3DE2=3D80=3D99s problem is that COE is below what RonJohn =
=3D
consider safe. i.e.--safe from dunting.
As silica is plentiful here, and silica is one of the two ingredients in =
=3D
this recipe that does not shrink, thereby contributing to low expansion, =
=3D
I simply cut the silica.
And got this (NOT recalculating batch to 100 to make it easier to =3D
compare recipes.)
15 Silica
15 Kaolin--EPK
25 Gerstley Borate--1999
25 Nepheline Syenite
10 Talc
90 Total
=3DE3=3D80=3D80
.214 Na2O
.053 K2O
.311 MgO
.423 CaO
.535 Al2O3
.38 B2O3
.004 Fe2O3
3.282 SiO2
65.2 Exp
The only thing changed is the silica content. The relationship of the =3D
fluxes remains the same. But the COE is now in the safe zone. So I do =3D
not see the flaw Ivor refers to=3DE2=3D80=3DA6.Apologies, Ivor, please =3D
clarify.
Ron went off in a different direction, and I would like--for the sake of =
=3D
clarity--to =3DE2=3D80=3D9Cdefine=3DE2=3D80=3D9D my position.
There are countless glaze recipes out there, and a great many are =3D
re-issues of ancient recipes. A constant example is a glaze that once =3D
used Colemanite, was converted to Gerstley Borate, which in and of =3D
itself has "changed" over the years, thence to a number of variants =3D
using diverse frits.=3D20
Along the way, small changes came into the molecular formula, none =3D
really meaningful. And so, under the banner headline =3D
=3DE2=3D80=3D9CBreathtaking new glaze hits the stands=3DE2=3D80=3D9D we get=
rehashed =3D
well-established recipes.
One of these days someone should make a =3DE2=3D80=3D9Crange chart=3DE2=3D8=
0=3D9D to =3D
show how much each ingredient can be decreased or increased without =3D
meaningful change. How much leeway there really is. In my experience and =
=3D
opinion there is quite a bit--and the major factor in glaze success =3D
remains the body underneath.
In fact I wish there were, I hope will appear, a single, standard, =3D
controlled laboratory-made clay body all manufacturers would sell, and =
=3D
we all would use for tests! I know this is absurd and ridiculous, and I =
=3D
am an idiot, etc--but we all test our glazes on different bodies, these =3D
bodies change whether we like it or not, as clay compositions change =3D
etc. and we all constantly talk about different circumstances that "go =3D
into" glaze tests.
Anyway. I think it important that ,in correcting a glaze fault, one =3D
replace as few materials as possible . The =3DE2=3D80=3D9Cnormal=3DE2=3D80=
=3D9D =3D
potter does not have a glaze lab, endless storage space, nor can she =3D
afford to tie up lots of money. Most of us have very restricted =3D
supplies, restricted access to materials.
Ron recalculated the glaze as follows:
20 Silica
7.5 Kaolin--EPK
25 Gerstley Borate--1999
36 Nepheline Syenite
11.5 Talc
100 Total
=3DE3=3D80=3D80
=3DE3=3D80=3D80
.247 Na2O
.064 K2O
.31 MgO
.379 CaO
=3DE3=3D80=3D80
.456 Al2O3
.329 B2O3
3.384 SiO2
.
67.3 Exp
The COE is higher than what I had, and the calcium has been reduced,
The alkalines have been raised, and the alumina lowered. My guess would =3D
be this glaze would melt earlier than the original, be =3D
glossier=3DE2=3D80=3DA6something that would not really show up till the gla=
ze =3D
was opacified.
There are several other possibilities for this glaze, One could replace =
=3D
the talc by magnesium carbonate, the Gerstley Borate by a frit, and it =3D
always makes me happy to replace clay by Bentonite, if my alumina/silica =
=3D
needs are met otherwise.
Last: Ron says that in his revisions the alumina silica ratio has been =3D
kept the same, which he considers =3DE2=3D80=3D9Cnecessary to get the same =
=3D
look=3DE2=3D80=3D9D This astonishes me as, in my experience, the fluxes =
are =3D
what gives a glaze its look (or why I love high magnesium glazes) not =3D
alumina/silica. Alumina/Silica is a big factor in glaze design; =3D
nevertheless, I have found that the final look of a glaze comes from its =
=3D
fluxes, while melting point and vitrification come from Al/Si. (I once =3D
made up test tiles of ALL the zinc containing glazes I had recipes for, =3D
and ALL the high magnesium ones--and one could tell which was which in =3D
like 90% of the cases.)
It cannot be said enough that we all have ideas about glaze and what =3D
makes and breaks one--but, there are too many variables for any firm =3D
conclusions. It all comes back to testing...Each of us pursues certain =3D
ideas, follows the tests with use on actual pots. I can test X glaze in =
=3D
my studio, and you can test it in yours, and she can test it =3D
elsewhere..and in the end we each is working on a different "final" =3D
glaze because we have materials from different sources, use different =3D
clay bodies, fire in different kilns, and so on.=3D20
=3DE3=3D80=3D80
=3DE3=3D80=3D80
=3DE3=3D80=3D80
Lili Krakowski
Be of good courage
ivor and olive lewis on thu 25 aug 11
Lili Points out the static nature of the R2O components of her glaze...
"The only thing changed is the silica content. The relationship of the
fluxes remains the same. But the COE is now in the safe zone. So I do not
see the flaw Ivor refers to. Apologies, Ivor, please clarify."
Dear Lili, no apology needed.
Using GlazeMaster, I observed the change to individual proportions of
fluxing oxides as the original recipe was reduced one percent of Silica
(Quartz) at a time. From 25% to 15% I found that the fraction of MgO
increased from 0.346 to 0.352 and the CaO decreased from 0.397 to 0.358.
Furthermore, Na2O changes from 0.210 to 0.214 and K2O changes from 0.047 t=
o
0.048.
Recalculating from 90 parts in a batch to 100 parts in a batch after
removing 10% of the quartz has no effect on the fractional proportions give=
n
in the unity formula.
How significant such changes will be on the fired result I do not know but
it would seem the ratios between fluxes does change. To correct this it may
be necessary to add more Calcium oxide, as Whiting.
My concern is that students understand why decreasing the proportion of
silica increases Coefficient of Thermal expansion of a glaze batch.
Best regards,
Ivor
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