hal mc whinnie on thu 22 jun 00
CERAMIC GLAZE COURSE
LESSON ONE
PART ONE
WHAT IS A GLAZE?
A ceramic glaze is basically silica or glass, which is designed to melt at a
certain
temperature. If we fired kilns to more than 3000 degrees we could probably
use pure
silica to create our glazes. The basic problem in glaze formulation it to
design a glaze
that will reduce the melting points of the silica to that at which one
fires.
The development of ceramic glaze theory was necessary to develop a reliable
ceramic glaze that could be fired at a specific temperature that is lower
then the
melting point of pure silica. A second critical issue in glazemaking was to
also develop a
mixture that would dry and remain on the ceramic ware until fired.
At the end of the 19 Th. Century Herman Seger a German chemists formulated
a method of mathematical model for the description of a ceramic glaze.
Glazes
predictable by a Seger model were developed centuries earlier by the chines
and other
oriental potters and transferred to Western Europe where they were modified
b
To specific local conditions and materials. Thus ceramic science as we know
it today had
its beginning.
The development of this online glaze course has been a challenge and has
led me
to many sources on the Internet, which I will include at various points in
this series of
lectures. I am amazed with what is available on the web, 600,000 matches to
my glaze
searches. Of course the problem is to really find what is important and I
hope that some
of what I have found in this adventure shall be useful to everyone.
A ceramic glaze consists of several important materials, which are:
A glass former the silica
A clay the alumna
A feldspar to modify the melting point of the silica
A refractory generally whiting used to complete the melting of the glaze
for a
specific temperature level.
There are certain rocks, generally the granites that when ground and refined
make
perfect glazes for the high temperatures. Curbstones in many New England
towns are a
perfect blend of silica, clay, feldspar, and chalk to be a cone 10 glaze.
The early Chinese
potters also had natural stones that when ground would form perfect glazes
according
to the Seger model.
A classic Chinese cone 8-10 glaze
Silica or flint 40
Feldspar 30
Whiting or chalk 20
Clay 10
The above describes a 40-30-20-10 proportional glaze which Bernard Leach
first
introduced into western ceramics in the early 1920"s. We shall return to
Leach and this
proportional glaze several time during the duration of this course. It is
probably the most
famous glaze in the history of 20 Th. Century ceramics. The proportional
method we
shall explore as a useful alternative to the Seger system.
The basic problem for the glazemaker is far more then to simply find a
proportional system to melt the silica. A final glaze has color, texture,
surface qualities,
and a crystalline structure. It can be very Matt and opaque or highly gloss
and
transparent. All of these qualities can be varied with the simple four
chemical elements
in our sample glaze, if we employ specific amounts of certain glaze making
materials.
There is the basic problem for all of us, the unending quest, what
proportions and which
materials for the perfect glaze.
Glaze a glaze b glaze c
Silica 40 silica 30 silica 25
Feldspar 30 feldspar 25 feldspar 20
Whiting 20 whiting 40 whiting 50
Clay 10 clay 5 clay 5
Clear gloss clear satin Matt Matt
Cone 9-10 cone 9-10 cone 9-10
The above demonstration shows that by a variation of one or two elements
the
basic proportions can be maintained but the surface qualities altered, in
this case the
reduction of the silica content as well as an increase in whiting which is
refractory
In our next lesson we shall lean more about how these elements work.
Homework
1] Take three of your favorite glazes and try to find their basic
proportional structure
2] How similar is this structure if you were to remove the colorants,
opacifiers, and
other small elements?
3 do any fit into our 40-30-20-10 framework?
Each of the 12 lessons is included in two parts. These will be attached as
files. In
addition each lesson will appear as an email message. I send the attached
files in html
format because one may wish to use as a part of a web site or print out as
text for use
with student's etc.. The course shall run for 12 weeks and it will be
repeated in April and
in Oct 1999. If you have students or fellow potters who would be interested
tell them to
send me an email and they will get of the list for the next course.
After you finish the course you shall continue to get the glaze of the
week. I plan
on having an advanced course sometime late in fall 1999. At some point these
lessons
will be printed as a book for sale but you are free to copy and use as you
see fit.
FIRST WEB SITE TO VISIT
http://www.digitalfire.com/
Chem glaze software
http:/www.mdc.net/~rjwilt/software/glazechem.htm
You may wish to get your own computer program as we continue.
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