hal mc whinnie on mon 26 jun 00
CERAMIC GLAZE COURSE
LESSON ONE PART TWO
[ note; the text format of these lessons is the result of conversion from a
text file in word, i have decided to use this method since so many have
problems with opening attached files in e-mail.
I would suggest that you might try to convert these files back into word or
just keep them as is. if there is interest at the end of this process I
might prepare a hardcopy which can be purchased.]
THE COMPOSITION OF GLAZES
A glaze is a coating of glass on the surface of the clay body when fired. A
glaze is
a solution with silica as the main glass-forming ingredient. For practical
purposes
consider that all glazes have silica as a base.
Pure silica is very refractory and by itself melts at about 1700 degrees
centigrade
which is well above cone 9-11. Therefore, certain materials are added to the
composition of the glaze, which act as fluxes on the silica and cause it to
melt at a lower
temperature.
Some fluxing materials are strong fluxes and some are weak so that the
proportion of the fluxes in relation to the silica is used in determining
the firing
temperature of a glaze. A low temperature glaze natural has a greater
proportion of the
strong fluxes than doe a high temperature glaze.
Clay is a materials that is added to the glazes in small amounts, Epk or
ball clay
are the e most commonly used and these clays are refractory and the fluxes
act on
them also to cause the melting or fusion of all elements into what is the
final glaze.
Common fluxing materials
Feldspar
Whiting
Zinc oxide
Dolomite
Talc
Spodumene
Barium carbonate
Colemanite
Lead
In summary think of glazes as containing three kinds of materials
Silica
Clay
Fluxes
Sample glaze formula, for a satin Matt glazes at cone 8-10
Soda feldspar 40
Epk 20
Flint 20
Dolomite 20
This is a standard variation of our 40-30-20-10 glaze and its source is
Rhodes, Alfred
University and before that Bernard Leach. It is one of the most famous
glazes of the last
40 years and perfectly demonstrates the basic relationship of the three
parts of a glaze.
THE FLUXES
These are the surface modifiers, which assist in the maturing of the glaze
at a
specific cone level and the overall look of glaze as well as its colors.
A] The feldspars
These are found in almost every glaze and the feldspars provide both silica
and
alumna to the glaze which cause real problems in ceramic glaze calculation
when one
employs, as we shall see later, the seger methods.
For example many raku glazes do not list silica as an element however the
necessary
glass elements are derived from the silica component of the feldspars and
the borons.
On the whole the purpose of the feldspars is to increase the glaze flow and
to prohibit
the flowing of the glaze off the ceramic piece.
2] The refractors
Whiting for matting
Lithium carb for matting and color
Magnesium for matting and glaze fit as well as color effects
Dolomite sources of lithium and magnesium
The purpose of the refractors is to balance the fluxing effects of the
feldspars and to
further alter the service qualities of the glaze. As we saw in our last
lesson, most glazes
are balances of the four basic elements of silica, alumna, feldspar, and a
refractory
element.
MULTIPLYING YOUR GLAZE TEST
I am now going to include a brief description of a method that you may wish
to employ to
test your glazes
This is from the following we b site
http://:www.camasnet.com/~asondam/glazetest.htm
"Glaze tests take a good deal of time to weigh and prepare. Using this
hybridization
method, you can effectively multiply your efforts and generate many new
glazes from
recipes you are trying. Here is the outline of the method.
Measure a standard weigh of glaze tests and add a standard weight/volume of
water to
each and mix.
Make tiles of each original glaze, a,b,c,d
Make tiles of each combination of two by scooping our equal volumes of each
glaze and
mixing
Ab ac ad bc bd cd
For complete information consult the web site.
Another great web site to visit
Carolina clay
http://www.cclay.com/
In our next lesson we shall consider a chemical and a mathematical model by
which
these four major components can be related one to another.
Homework problem
1] Take two of your favorite glazes and try to identify these four main
elements
2] Can you determine any proportions between these basic elements?
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