Lili Krakowski on tue 8 apr 08
Jim:
One step at a time.
Recipes that fire beautifully at c.8 only need some attitude adjustment
(to use a hateful nonce phrase) to make it at cone 6.
I do not have the most current cone chart, the one I have says the
difference between c.6 and c. 8 is 102 degrees. Not that big a deal at those
temps. Especially when you consider most glazes work fine within a range
of 2 or 3 cones.
You know certain basic things from reading the books. There are fluxes.
Melting agents. They give characteristics to the
look and feel of a glaze, but their role is to melt the silica, or glass
former. Alumina is highly refractory, but is the glue that holds the glaze
on the pot. Your GlazeMaster(tm) tells you it should be between .25 and .50.
Keep that in mind.
Some things to do.
Make up tests in which you increased the amount of one flux. Make up tests
in which you raise each flux in turn. (If 3 fluxes, three sets of tests.)
Example:
Cooper #307 said to be a stiff opaque glaze at c8
Feldspar 60
Whiting 10
Flint 30
without any calculation you simply can up the whiting. How much? Go for it.
Make a straight line blend upping your whiting to 30.
See what gives in between.
When recipes say Feldspar, it is the convention to use potash spar. Ok. But
Neph Sy, which we use as though it WERE a feldspar
while our geologist friends tell us it is not, has a lower melting point
than feldspars. So you might just substitute Neph Sy for spar. (During the
Great Downward Rush of the 80s when so many switched from c.10 to c.6
(whining and cussing all the way, telling us lifelong mid-rangers how
terrible they felt sinking to our [low] level)
the replacement of spar with NephSy was really big.
There are some frits that do not contain boron. Try one of those instead of
your spar.
Ok.
Now to get to your glazes: As you do not give us the recipes, there is no
saying anything specific.
Rhodes gives a very pleasant list as guide to lowering temps.
Which I summarize here.
Raise the more active fluxes. Such as soda and potash.
Add more of the fluxes you have.
Lower silica and alumina (This would be clay content, probably)
Replace less active fluxes--(he includes barium!!) such as magnesium-(and I
add strontium)
by more active fluxes, such as calcium.
Use frits.
To use your calculation software:
The formula for #307 as is looks like this:
Unity Oxide
.138 Na2O
.303 K2O
.003 MgO
.555 CaO
1.000 Total
.477 Al2O3
.003 Fe2O3
5.48 SiO2
Raising the whiting to 20 makes it look like this:
Unity Oxide
.094 Na2O
.207 K2O
.004 MgO
.695 CaO
1.000 Total
.325 Al2O3
.002 Fe2O3
3.736 SiO2
which changes the flux relationships without doing much for Al2O3 and SiO2.
Trying NephSy and not changing other stuff:
Unity Oxide
.382 Na2O
.118 K2O
.008 MgO
.491 CaO
1.000 Total
.553 Al2O3
.001 Fe2O3
4.339 SiO2
does not really impress me....Look at the Al2O3, SiO2!
So some more playing with GlazeMaster(tm)
I come up with:
20 Whiting
55 Nepheline Syenite
25 Flint
Unity Oxide
.261 Na2O
.081 K2O
.008 MgO
.651 CaO
1.000 Total
.377 Al2O3
.001 Fe2O3
2.845 SiO2
A testing possibility.
I gotta tell you. As much as I adore calculation I do not find it saves me
time at the beginning stages. If it saves me huge time
when I have run series of different tests as suggested above, and then have
maybe 5 test tiles that look really good, and I
want to combine them into a single new series.)
Good luck.
Lili Krakowski
Be of good courage
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