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Basic Internet Glaze Course

By Lili Krakowski


This section intends to tie up loose ends.
When you have a frit or feldspar to deal with, you need to divide as usual,
and then multiply out each ingredient, just as you did with dolomite and
clay, above.
What is confusing is when the material does not offer a unit amount.
Colemanite had a formula of  2CaO 3B2O3 5H20 for a molecular weight of 412.
But to use it in calculations one divides it by 2 to get just 1 Ca0 (and 1.5
B203)  and gets the equivalent weight of 206 which one uses.  And talc is
3MgO 4SiO2.  Here we divide by 3 to get that 1 unit of MgO and multiply the
resulting .33 by 4 to get our Silica.  Or .132 SiO2.
When we get to feldspars-here I borrow from Rhodes-if we want .243 K2O to
"satisfy"  the formula, and our spar (he uses Buckingham) supplies .7400,
then again and once more we divide what we need, want, by what is supplied.
We divide the desired .243 by the supplied .7400 and get an equivalent of
.33 by which we multiply all the other ingredients in the spar.  And the
same idea goes for frits.
And now a caveat.
Glaze materials are mined and the mine runs change.  We have discussed this on Clayart many a time.  Furthermore,  in a brilliant article on clay bodies
in Ceramics Monthly December 1985 (Vol 33 #10. p.69-73) Robert Schmitz gives the analyses of diverse clays.  And while the official/generic molecular
weight of clay is 258, these range from 262.60 for EPK, to 622.40 for Red
Art.  The reason:  there are "impurities". such as the iron in the Red Art,
that add to the actual molecular make up and weight.
If this concerned a potter, then the "impurities" also would be calculated
into the formula. Titanium, for instance, in a clay used for glazing, can
have a surprising effect.   But for the everyday need of the every day
potter the system above suffices.  Although it does not prepare one totally
for every surprise, or contingency.
The difficult part of making a glaze from a formula  comes when we need to
make up a new recipe, with different materials.  This would happen if you
moved to a new country where the frits and spars you know are not to be had.
Or when, like Colemanite and Gerstley Borate, a material disappears from the market.
Here you not only need to dance backward and in heels, but you need to do
some clever guessing.
If you are replacing colemanite, you KNOW that there are several sources of
calcium.  But the boron part is not easily found.  So you would try
different frits and figure out how to get closest to your "have to"  Don't
get shook up.  In a good number of cases you will not be able to get the
exact thing.
Here is a recipe from John Conrad:
Colemanite 28
Wollastonite  20
Potash spar 8
Kaolin 44
Figuring the formula is no big deal.
Na2O .016  K2O .038  CaO .882  MgO . 066  Al203 .570   B203  .559  SiO2
As Colemanite no longer is to be had (in the US anyway)  the next step is to
look at the available frits, and see which appears to come closest to what
you want.
Ferro 3124 gives you too much Al2O3 even if you cut out all Feldspar.
Ferro 3134 ends up giving you much too must soda.
Ferro 3195 just will not get the soda adjusted either.
Several other frits also will not "come out" correctly
Because I used GLAZE MASTER -but only as a calculator, NOT its
formula-to-recipe program-- I worked on this for a couple of hours.  Using
only a regular calculator it would have taken a little-not much-longer.
I ended up using two frits, neither one being able to do the job alone.
My new recipe is:
Fusion Frit 34      18.8
Ferro Frit 3195     29.9
Whiting                17.11
Kaolin                   29.9
Flint                        4.3
Which, believe you me, in practice I would round out to 19,30, 17, 30, and
5, the bit of extra silica only doing good.
And the new formula is:
Na2O .199  K2O .004  MgO .008   CaO .789  Al2O3 .424 B2O3 .513
Si02 2.00
Not a perfect match.  But it is as close as I feel I need to come to start
testing. I like the high silica content.  The higher than original alkaline
contents does not worry me.  I like that the Al203/B203 is close. I worry
that 53% of my glaze will not shrink at all.  Do I risk shivering?  (A glaze
calculation program would figure that one out for me)  (I add all this to
indicate in what directions one moves when substituting materials out of
I have never seen the original glaze,  so I am NOT trying for a "look."
And, given an actual situation, I would just find another glaze with
approximately the same balance of ingredients and go from there.
NB.  This is only a BASIC course.  Meant to teach the basic way of doing it.
Glaze calculation is a tool.  Only a tool.
For greater accuracy,  mine analyses (the material fact sheets from
suppliers) give  exact knowledge of what is in each batch of each material.
This info was not available to potters till fairly recently, and may explain
why some old recipes do not work at all.  These fact sheets are extremely
helpful , esp. when you change suppliers, or your supplier changes
I made glazes for decades before such sheets were available, and the generic  ingredient lists worked well enough. And then, in a community studio
situation, it may be impossible for the student to obtain those sheets.
Moreover,  I am of those convinced the most important factor in glaze
results is the body underneath, and its firing. The raw glaze formula is not
affected by this.  Sophisticated lab tests will/ can find out a whole lot
more -but for the average potter the formula is the best tool.
One also has to consider LOI or Loss on Ignition--the loss of weight of a
mineral when heated to where material burns out.  The actual burning out
matters tremendously in the bisque firing-or why many of us insist one
cannot fire too slowly. There has to be plenty of time for gases to escape
before the fire turns the clay hard.  The weight loss can be "very
considerable, up to 50% with dolomite which means the fired glaze will have
only half the amount listed in the recipe." ("Illustrated Dictionary of
Practical Pottery", Robert Fournier)  LOI is an important consideration
when, for instance, when dealing with glaze faults.
LOI is analogous to the waste in cooking.  If you want 1 lb of peas and buy
them frozen, the bought pound= a pound in your salad.  If you buy fresh peas
in their pods  you may need several pounds to get that 1lb of actual peas.
The pods are "waste",  yet they matter greatly when you are writing out your
recipe.  LOI is the same idea.  It tells you what % of the material gets
lost, and allows you to figure how much you actually have in your glaze when
it is fired.
Again:  I potted for decades before this information became readily
available, and relied on the general information --as in Fournier above--to
understand how much of something would be left in the finished glaze.
I urge you to read all you can about the scientific aspect of glaze and
firing, and read it seriously.  This course is only an introduction, meant
to get you started, to enable you to figure out what is in a glaze, and,
when you see a recipe on Clayart or in a book to quickly calculate whether
it is new, or just a variant of glazes you have.
One of the Clayarters on whom I tested this course is planning to buy a
computer glaze calculation program. Probably, once you understand the
basics, you will want to invest in one as well.    These programs, if you
keep the material lists up to date, save considerable time, and provide
greater accuracy.  And as you will learn, glaze calculation is tremendous
A few years ago a "new" system of glaze calculation was introduced.  This
took the % of ingredients as supplied by analysis sheets for materials, and
worked on a parallel system from there.  If you needed 5% of whiting for
your recipe, and the whiting you had contained 93.7 % of CaO  you calculated
from there.  I have neither read not heard about this system of recent, and
can only assume it was found impractical.
Lili Krakowski
Be of good courage



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Originally written for Clay Art Listserv and reprinted here by kind permission of Lili Krakowski.