This section intends to tie up loose
When you have a frit or feldspar to deal with, you need to divide as
and then multiply out each ingredient, just as you did with dolomite and
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
But to use it in calculations one divides it by 2 to get just 1 Ca0 (and
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
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
"satisfy" the formula, and our spar (he uses Buckingham) supplies
then again and once more we divide what we need, want, by what is
We divide the desired .243 by the supplied .7400 and get an equivalent
.33 by which we multiply all the other ingredients in the spar.
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
weight of clay is 258, these range from 262.60 for EPK, to 622.40 for
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
into the formula. Titanium, for instance, in a clay used for glazing,
have a surprising effect. But for the everyday need of the
potter the system above suffices. Although it does not prepare one
for every surprise, or contingency.
The difficult part of making a glaze from a formula comes when we
make up a new recipe, with different materials. This would happen
moved to a new country where the frits and spars you know are not to be
Or when, like Colemanite and Gerstley Borate, a material disappears from
Here you not only need to dance backward and in heels, but you need to
some clever guessing.
If you are replacing colemanite, you KNOW that there are several sources
calcium. But the boron part is not easily found. So you
different frits and figure out how to get closest to your "have to"
get shook up. In a good number of cases you will not be able to
Here is a recipe from John Conrad:
Potash spar 8
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
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.
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
Which, believe you me, in practice I would round out to 19,30, 17, 30,
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
Not a perfect match. But it is as close as I feel I need to come
testing. I like the high silica content. The higher than original
contents does not worry me. I like that the Al203/B203 is close. I
that 53% of my glaze will not shrink at all. Do I risk shivering?
calculation program would figure that one out for me) (I add all
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
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
This info was not available to potters till fairly recently, and may
why some old recipes do not work at all. These fact sheets are
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
situation, it may be impossible for the student to obtain those sheets.
Moreover, I am of those convinced the most important factor in
results is the body underneath, and its firing. The raw glaze formula is
affected by this. Sophisticated lab tests will/ can find out a
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
mineral when heated to where material burns out. The actual
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
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
only half the amount listed in the recipe." ("Illustrated Dictionary of
Practical Pottery", Robert Fournier) LOI is an important
when, for instance, when dealing with glaze faults.
LOI is analogous to the waste in cooking. If you want 1 lb of peas
them frozen, the bought pound= a pound in your salad. If you buy
in their pods you may need several pounds to get that 1lb of
The pods are "waste", yet they matter greatly when you are writing
recipe. LOI is the same idea. It tells you what % of the
lost, and allows you to figure how much you actually have in your glaze
it is fired.
Again: I potted for decades before this information became readily
available, and relied on the general information --as in Fournier
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
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
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
A few years ago a "new" system of glaze calculation was introduced.
took the % of ingredients as supplied by analysis sheets for materials,
worked on a parallel system from there. If you needed 5% of
your recipe, and the whiting you had contained 93.7 % of CaO you
from there. I have neither read not heard about this system of
can only assume it was found impractical.
Be of good courage