Lili Krakowski on mon 30 jun 08
Ann Griffin writes:
"I have been testing a glaze on red and white clays, and have noticed a
difference in the results in the surface texture between red and white
clays even with the same batches of glaze, dipped at the same time.
I'm wondering if the iron in the red glaze would make the glaze
glossier on the red clay. The glazes come out much more matte on the
white clay."
(I am assuming a typo and that Ann means "red clay" not "red glaze" there.
The recipe is as follows
^03 electric w/ox
Gerstley Borate 38
Silica 42
Lithium Carb 10
Neph sy 5
EPK 5
As I have a natural disposition to nag, questions like these are manna
from Heaven.
My main "rule" is: The clay body underneath is the single most important
factor in glaze results!
Well, for pretty much the same reason that a dress that looks fabulous on
Julia Roberts looks like **** on me!
The same reason as the same person sleeping on a bed of rocks gets
different "sleep results" from that person sleeping on a brand new mattress!
It is the INTERACTION of the materials IN the glaze and the materials IN
the body that determine, I would say, half of the glaze result. Actually
this "interface" has been studied too little by potters, but that is for
another day.
If you could do laboratory quality tests on your different clays you
would find that your red clay fuses just a bit more, and that the iron in it
permeates the interface and affects the glaze. That also is why when one
decorates with slip a glaze that is just wonderful on the slip can be awful
on the body, or vice versa.
As interesting--but that is pure guessing--is that the white body probably
contains talc and the magnesium may affect
the glaze.
I am not clear what you plan to do? Add iron to the glaze? I don't think
so.
Lettuce look at the glaze :
Unity Oxide
.46 Li2O
.088 Na2O
.009 K2O
.001 MgO
.442 CaO
1.000 Total
.194 Al2O3
.481 B2O3
.001 Fe2O3
3.161 SiO2
54.9 Exp
Calculations by GlazeMaster™
It is a calcium borate glaze with a good deal of lithium. Its COE is
low, and, by Ron/John's criteria way too low, and likely to lead to
dunting/shivering (NOT good) It really is not a very good glaze to begin
with.
I should be mixing cement to close the holes through which raccoons
invade our house. Much nicer to recalculate your glaze!
48.2 Frit--Ferro 3134
19.6 Silica
11.9 Lithium Carbonate
20.2 Kaolin--EPK
99.9 Total
Unity Oxide
.385 Li2O
.194 Na2O
.002 K2O
.001 MgO
.418 CaO
1.000 Total
.18 Al2O3
.386 B2O3
.002 Fe2O3
2.061 SiO2
.002 TiO2
0 P2O5
11.5 Ratio
70.8 Exp
Calculations by GlazeMaster™
It should do the same, and the extra kaolin should make it easier to apply.
It also fits the COE criteria.
By the way. No idea what the w/ox means....
Back to cement!
Lili Krakowski
Be of good courage
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