Lili Krakowski on tue 9 may 06
ABOUT RECIPES: PART III
GLAZE FAMILIES
Let me explain the concept. Glazes (like cooking recipes) belong to
families. As in cooking, the families can be very large. Stews are a good
example. There are meat stews, fish stews, vegetable stews. We can think =
of
them as analogous to our different firing temperatures. We can think of m=
eat
stews (X firing temp) as subdivided among beef, veal, lamb, chicken. As w=
e
can think of firing electrically, oxidation in a fuel burner, reduction i=
n a
fuel burner, Raku. And there is the soup family, and the vegetable one, a=
nd
so on. As there are feldspathic glazes, dolomitic ones, those high in zin=
c,
or boron, glazes with ash and so on. There are many overlaps which each
potter sorts out for herself.
The following are three dolomitic, or high magnesium glazes:
DE BOOS I
Nepheline Syenite 35.3
OM#4 Ball Clay 29.4
Dolomite 29.4
Flint 5.9
(originated with Janet De Boos, an Australian potter, and adapted by me t=
o
c.6)
ALISA CLAUSEN WHITE (adapted from a T. Martens glaze)
Nepheline Syenite 49
Dolomite 25
Wollastonite 6
Ball Clay 20
COOPER #163 Mottled White Glaze
Feldspar 40
Whiting 5
Dolomite 40
Ball Clay 10
Flint 5
(from =93The Potter=92s Book of Glaze Recipes=94 by Emmanuel Cooper) N.B.=
: it is
customary, when a recipe only specifies =93feldspar=94, to use a potash o=
ne.
These recipes essentially contain the same ingredients. You can tell that
with the naked eye.
What you cannot tell is the amount of each.
Cutting to the chase: We have here three magnesium glazes, also called as
dolomitic glazes. These happen to be my personal favorites, because of th=
eir
characteristic soft sheen. Often referred to as =93satin=94 finishes, the=
se
glazes have the soft glow of peanut butter, or cold bacon grease, or suet=
.
(A possible downside of magnesium is its effect on cobalt--which it turns
purple).
At this point you can test. Or you can resort to calculation and find
out which contains the most magnesium.
The analyses (Thanks to GlazeMaster=99 )
DE BOOS CLAUSEN WHITE=20
COOPER 163
Na2O ___________.141 ______________.183_________________ .037
K2O _____________.051______________ .061________________ .08
MgO _____________.381 ______________.311________________ .377
CaO _____________.427_______________ .445________________ .506
AL2O3 ___________.400 ______________.392________________ .176
SiO2 ____________1.779 _____________1.703 ________________ 1.157
My guess is they are in the same ballpark. That tests would show the
characteristics of magnesium, with very small differences. NO earthshakin=
g
differences here. DeBoos has the most alumina, Cooper least--I would expe=
ct
the latter to be more mature than the former=85nothing to fall off your c=
hair
for! And these need work in the coefficient of expansion,silica area.
But that is for further tests.
These recipes, and the chart that "examines" them brings me to the point
I have been leading up to. That the =93rush to
recipe=94 so often evinced on ClayArt, and fostered by magazines that inc=
lude
a few recipes in each issue, along with workshop =93handouts=94, are, in =
a
sense, futile. They create a false impression. Not that the recipes are n=
ot
wonderful, but that expectation or claim of uniqueness, any hope that THI=
S
recipe is a breakthrough, and will make you a better potter are wrong. (T=
he
only breakthrough recipe would be one that incorporates an ingredient =93=
we=94
have not used before.)
With glazes one has to learn all about the ingredients, what they do, wha=
t
one can expect. How they interact (zinc enhances cobalt blues, magnesium
turns cobalt purple etc.) If one fires reduction, or uses an electric kil=
n
whose digital controls allow much fine tuning of the rates of heating and
cooling, then one also must go beyond the recipes to the firing technique=
s.
There is no point in begging for more recipes, testing them quickly, curs=
ing
the source when the glaze does not turn out as hoped for.
Why did it not turn out right? For a good number of reasons. It was not u=
sed
on the body the originator used. It was fired a trifle higher or lower,
quicker or slower. The feldspar, whiting, dolomite, clay in the glaze wer=
e
not identical to those the originator had. You applied the glaze more
thickly, or thinner.
I end the Internet Glaze Course with:
ADVICE
LEARN YOUR INGREDIENTS. Read Rhodes, Fraser, Parmelee, Cooper, all the bo=
oks
you can get your hands on. Read articles, study up on the materials.
Compare the fact sheets the suppliers give you. Buy small quantities of
different spars and frits and test them against each other=85Experiment w=
ith
replacing whiting with Wollastonite, Dolomite with talc or magnesium plus
whiting, or magnesium and Wollastonite.
Take any recipe you have that interests you and experiment with its limit=
s.
Decide what characteristics you like, and test in that direction.
As in the above Cooper recipe: I would be curious to see if I could repla=
ce
the Feldspar with Neph. Sy, which might allow me to drop the whiting, and
raise the dolomite.
I do not know. One of these days I will try for a recipe with a huge amou=
nt
of magnesium. It might be totally frightful. It might end totally gloriou=
s!
Who knows? But BY KNOWING MY INGREDIENTS, I would be aware of what is
taking place, and able, after a bunch of tests to maneuver the glaze to
what I want.
Another point, generally ignored when glaze is discussed, is the =93inter=
face=94
between glaze and body. We speak readily of the effect of the body on the
glaze=85but the existence and influence of their interaction generally is
relegated to discussions of glaze defects.
As ClayArt member, glass scientist Carol Knighten writes:
=93The layer between the clay body and the glaze is neither clay nor glaz=
e,
rather an intergrowth of the two. With stoneware and porcelain bodies, wh=
ere
the body has vitrified, the interfacial layer can be understood as a
transformation region between the two and can approximate in depth the gl=
aze
layer, with no sharp demarcation between clay and glaze. Like a thin laye=
r
of water on mud, water on top, hard earth underneath, but where did the
water end and the solid earth begin? The layers blend into each other and=
=20
are clay
under interface, under glaze. If the glaze layer is sufficiently thin, th=
ere
may be no pure glaze layer, rather only the interface.=94
This =93interface=94 all by itself affects the look and quality of your f=
inal
glaze. And more recipes will not help you with it!
It is a pointless waste of time and money to collect recipes unless and
until one understands what makes them what they are, and how to adjust th=
em
to what one desires--and wants to do it!
Learn your ingredients, test several recipes in each family that appeals =
to
you, fine tune these to your taste, and focus on application, and firing.
To speed up the process I refer you back to glaze calculation=85which is =
not
an end in itself, but an invaluable tool towards the understanding of gla=
ze.
(And if you hate the whole business, or lack time, space, and energy, the=
n
go to commercial glazes. )
Lili Krakowski
Be of good courage!
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