Compiled January 1996, reviewed/revised February 1999
by Tom Buck (tom.buck@hwcn.org)
Copyright 1996 and 1999, all rights reserved, please contact
the author for permission to issue a copy of this document.
This FAQ reproduced on the Astbury Web site with the kind permission of the author, Tom Buck.
This brief summary of glaze technology is provided solely to
the INDIVIDUAL potter who is now reading this notice. Please do
NOT copy and distribute to others, either directly or in a
classroom situation, without my personal agreement/permission.
Tom Buck, April 1999.
G.0 Glaze data outline for newbies
G.1 What is a glaze?
G.2 Are glass and glaze the same?
G.3 What affects a glaze's "look"?
G.4 How many materials in a glaze?
G.5 What factors go into a glaze?
G.6 How do you design a new glaze?
G.7 What is a Seger Unity Formula?
G.8 What's new in glaze design?
G.9 What is the meaning of:
1> Base glaze; 2> Flux; 3> Colorant; 4> Opacifier.
G.0 A glaze overview
This article covers glaze-making and glaze-using from two
viewpoints:
1) The nature of a glaze, both physical and chemical; and
2) Some aspects of glaze application and performance.
I have tried to keep the discussion on a "simple as possible" level. But
now and then, the notion of atoms and molecules must be examined.
G.1 What is a glaze?
Usually, a glaze is best described as a very thin layer of
glass (itself a complex material) formed on a clay pot during the
firing processes, that is, during one of the times that the
fragile clay vessel is heated to a high temperature (usually
above 1000 C, 1800 F). On a typical piece of pottery the glass
layer has a thickness of 1 mm or less (0.04"). However, some pots
undergo several "glost firings" (ie, glaze - forming firings) and
these pots may end up with a glass layer as thick as 2mm (0.08").
The glass/glaze layer usually starts out as a "recipe",
unfortunately also called a "glaze" in short-hand talk. This
recipe is a mix of "chemicals" and minerals, all as fine powders
(pulverized).
A typical glaze recipe reads as follows:
Satin White Glaze, Cone 4-7 oxidation
(submitted by Michelle Lowe)
26.5 Nepheline syenite (mineral, source of "soda", others)
15.8 Custer feldspar (mineral, a "potash" feldspar)
21.0 Dolomite (mineral, calcium and magnesium carbonates)
21.0 Bell Dark ball clay (mineral, alumina-silica mix)
5.2 Gerstley borate (mineral, source of boric oxide)
10.5 Flint (mineral, aka silica or quartz)
To this mix, which totals 100 weight units, Michelle Lowe adds 6
weight units of Tin oxide. She then combines the mixed dry
powders with an appropriate amount of water to form a "slurry"
(colloidal suspension), and dips a bisqued pot in the slurry.
When dry again, the pot (with others) is placed in a kiln and
heated to "Cone 6" (1230 C, 2250 F).
She says this glaze recipe produces a "very smooth even white,
really nice with stains on top, or with other glazes splashed on
for decoration."
Glaze recipes are often given in books and magazines, and
sometimes a particular recipe will yield a pleasing result on a
particular claybody, and sometimes the result is disappointing.
To be able to predict a likely result requires that the potter
study the properties of ceramic raw materials and their behaviour
when heated in a kiln.
Because glazes are more complex than simple glasses,
chemists tend to stress fine structure to explain performance.
Atomic and molecular notions are used to interpret the results of
experiments in the fields of crystallography and spectroscopy.
Such work can give us a picture of glass on a microscopic level,
but such fine detail is beyond the needs of a working potter or
indeed a glaze designer.
G.2 Are glass and glaze the same?
Sometimes, but not often. Glass itself dates back at least
50 centuries. Mix sand and some other dry minerals, heat in a
fireplace, and you obtain a material often called a "network
polymer" (a "mer" is a unit molecule and "poly" means many,
joined together). Today, most glass is made in special furnaces
from four elements: Calcium, Sodium, Silicon, and Oxygen (from
air). Of these, a combination of Silicon and Oxygen serves as
the "backbone" of the network polymer.
The batch recipe usually lists 75 weight percent silica sand
(silicon oxide), 20% soda ash (sodium carbonate), and 5% lime
(calcium oxide, but limestone or calcium carbonate may also be
used). These three ingredients, each at high purity, are fed into
the furnace in precise proportions to make a batch of container
glass (or window glass or specialty glass). Also, if a small
amount of borax (sodium borate) is added to the basic recipe, the
result is a low-expansion glass used for laboratory glassware.
Despite some limitations, "container" glass, for example,
has great versatility. A major virtue is its capability to be
recycled many times; it can be made into a jar to hold say,
pickles, then remelted and made into a wine bottle, remelted
again, and again, before the glass gets degraded by contamination
and becomes too costly to salvage. A glaze, however, differs
markedly from the common glass jar. Once formed on a clay pot,
the glaze is fused to the rock-hard ceramic and cannot be
economically separated for recycle.
For most potters, a glaze starts off as a "slurry", (aka
"slop") that is, a mix of fine powders suspended in water. Then,
this mix is transferred to the surface of the clay pot by one of
three common methods, dipping, spraying or brushing. Before
glazing, however, most potters heat the dried, raw-clay pot to a
"bisque" temperature using a pyrometric cone as guide; this makes
the pot safe to handle during application of the glaze mix.
The surface of the "biscuit" (fired clay-pot) has an
affinity for the glaze powders and holds them in place (sometimes
an adhesive is also used). After the wetted biscuit has dried
again, it is placed in the kiln and heated again ("fired") to the
proper temperature/cone.
This two-stage process requires that the glaze will stay on
the pot while it is drying; and later, in the kiln, the process
demands that the glaze ingredients will form a viscous glass on
the clay surface as the pot itself also undergoes change. During
the firing of the kiln, almost all glaze materials undergo
physical change, going from individual particles to large fusing
clusters that turn into liquid. Some materials also undergo
chemical change either by giving off gases or by re-arranging
their molecular shape. If the mix of oxides is "balanced" (has
well-tested proportions), this mix of materials will form glass
on the fired pot, that is, become a glaze.
G.3 What affects a glaze's "look"?
A glaze may be glossy, satiny, or rough (dry matt);
the actual "look" comes from several factors but the amounts of
silica (silicon oxide, SiO2) and alumina (aluminum oxide, Al2O3)
play dominant roles. A glass that contains 60% SiO2, plus or
minus 5%, will usually make a good glaze. However, if the oxide
mix in terms of molecules (the "Seger Unity Formula") shows less
than 55% SiO2 then the glaze is not "balanced" and will likely
not form a "coherent" glassy material and therefore will have a
non-uniform surface. Yet, because a glaze-mix coats the surface
of a claybody that contains a lot of SiO2, sometimes a silica -
deficient glaze will take up enough silica from the body to form
a glaze closer to a good glass, i.e., a balanced glaze.
If the silica content is 70%+ SiO2, the glaze becomes
high-melting and may not form glass at the expected cone (or
temperature). Then, its surface could exhibit some unwanted
effects, eg, crawling (the glaze shows bare spots, and may show
thick clumps of immature glass), and some other faults may occur.
Also, a good long-lasting glaze layer must contain
sufficient alumina (Al2O3) to:
1) make the molten glaze stay put (be non-runny); and
2) form an alumino - silicate polymer that is strong and resists scratching.
The ratio of silica molecules to
alumina molecules (SiO2 moles divided by Al2O3 moles) gives an
indication of how the new glaze will behave: at a ratio of 10, a
uniform glass is formed; it will have a glossy surface. Between 5
and 10, the surface will go from dry matt to glossy, the actual
transition point being quite variable and dependent on the
precise mix of materials, and a possible body/glaze interaction.
Above 10 the glaze will be glossy and perhaps runny.
G.4 How many materials in a glaze?
If one examines many glaze recipes, one soon realizes that
most of them contain ten ingredients, or less. These, in turn,
are used repeatedly in different recipes for a given firing range
(low-fire, mid-fire, or high-fire). Each raw material introduces
certain "essential oxides" into the glaze mix. Combined into a
batch recipe, the materials when fired will yield a certain mix
of these oxides. If these are in the correct proportions, the
result will be a glass with known properties, i.e, a glaze.
In the high-fire range, cone 8 to cone 11 (1260-1320 C,
2300-2415 F), the recipe usually contains a feldspar, flint,
whiting/dolomite, and kaolin/ballclay.
In the mid-fire range, cone 1 to cone 7 (1160-1250 C,
2120-2280 F), the recipe may include raw materials that melt at a
lower temperature, such as colemanite/gerstley borate,
spodumene/lepidolite, zinc oxide, and certain "frits" (prefired,
special glasses).
In the low-fire range, cone 08 to cone 01 (950-1150 C,
1740-2195 F), gerstley borate/colemanite or a "borate" frit (one
with adequate boric oxide) is usually the main ingredient with
the rest being chosen from those already mentioned above. Also,
some materials with very low melting points are often used,
including lithium carbonate and clays with high iron content, eg,
barnard (blackbird) clay.
Why these particular materials? Are there others that could
be used? Potters, over decades, have learned by trial and error
which low-cost materials will form good glazes on their ware. A
feldspar, for instance, is the chief ingredient of high-fire
glazes. But not all feldspars are created equal; there can be
considerable variation in feldspars mined in different places
throughout the world. Furthermore, there are indeed many other
glass-forming raw materials available to the glaze-maker; the
actual choice of a given set of equivalent materials will vary
with cost, with availability, and with a potter's preference.
G.5 What factors go into a glaze?
Glaze design is both simple and complex; the list of needed
oxides can be expressed in simple chemical terms, but the
interaction of the usual ingredients (up to 10) is most difficult
to describe and even more difficult to predict with confidence.
Further, the ingredients used in glazes are seldom pure
substances but rather are materials (minerals and partly
processed chemicals) that may undergo change month-to-month.
With few exceptions, a typical glaze recipe brings
together materials dug from the earth which thereafter
are only "cleaned" and ground to fine powder. To keep
costs down, suppliers use the least amount of clean-up that
allows adequate performance. Also, from time to time, the ore
being mined may be quite changeable. And, with some glaze
components, there are many mines being worked at any given time.
As result, a specific glaze mix (with some exceptions, eg,
tenmoku or temmoku) will yield different results, place to place,
month to month.
But still, the idea of calculating a glaze design has
merit, for two reasons:
1) The chemistry of glazes can be simplified and hence
readily grasped by the interested potter; and
2) By starting with a known design (Seger formula) one can
more easily fine-tune the mixture and more quickly make
adjustments for irregularities in ingredients, in glaze/body
interactions, and in kiln performance.
G.6 How do you design a new glaze?
It may sound like magic but to design a new glaze
successfully requires no mysterious chants, just a thorough
understanding the factors involved in the process. There are two
main ways to develop a new glaze:
1. Choose suitable raw materials (mostly those that have
worked before) and mix them in various proportions to meet a
planned series of glaze tests; or
2. Choose an appropriate "formula", based on previous
practice, and derive a "mix-batch" recipe for testing, etc.
In either case, one needs to know detailed particulars about the
raw materials on hand.
Other factors being equal, Step 1 may take many tests before
an acceptable result is obtained. Just how many tests is
uncertain; personal choice becomes a deciding issue. So mix/try
testing may continue for many firings (10?, more?) to achieve a
new glaze recipe.
Some glaze designers use step 2. They choose a Seger Unity
Formula; this is a shorthand statement of the glaze make-up, or a
list of "oxides" (essential components) on a "molecular level".
The Seger formula "looks" at a glaze's batch recipe from the
"inside", and reports the mix of such essential oxides that
hopefully will turn into highly viscous (non-runny) molten glass
on the surface of the pot. These essential oxides, seldom
isolated as such, are contained within the batch recipe's raw
materials.
G.7 What is a Seger Unity Formula?
One way to help evaluate a glaze recipe is through the Seger
Unity Formula named after Hermann Seger who a century ago
arranged glaze components into a particular order. He called one
group the "flux" oxides -- usually the oxides of Lithium, Sodium,
Potassium, Magnesium, Calcium, Strontium, Barium and sometimes
Boron, Zinc and Iron. In another group, called "glass-formers",
he placed the oxides of Silicon, Boron, Phosphorus and Titanium,
although most glazes consist chiefly of silicon oxide. Seger also
described a third group which he called "modifiers" (now called
"intermediates"); Seger included the oxides of Alumin[i]um,
Boron, Iron and Phosphorus, the dominant one being Al2O3 or
alumina.
The kind of glass (quality) is decided by the amounts of
each type of essential oxide put into the batch, and to make the
proportions easier to recognize, Seger set the TOTAL number of
flux oxide molecular equivalents (now "moles") equal to unity.
This is done by summing all the flux oxide moles, and then
dividing all numbers by this flux-oxide total, thus arriving at a
"formula" (unified formula) for the recipe.
When the essential oxides are so arranged, direct
comparisons become of value, especially when other factors are
concurrently interpreted. Over the years successful recipes, in
Seger form, have been collected and arranged in a table/chart
called "Flux Unity Formulas" or Limit Table. The table thusly
cites what proportions of essential oxides make good glass at
specified temperatures. Such a list of oxides, when converted to
a mix-batch, are known as "balanced" recipes.
G.8 What's new in glaze design?
With the advent of the home computer, doing a glaze design
no longer involves lengthy, tedious calculations by hand.
Nowadays, any potter can undertake glaze design providing they
have:
1. Access to a microcomputer, either an IBM type
(DOS/Windows) or an Apple "Macintosh" type with its "pull-down"
menus; and
2. Access to specialized computer programs that simplify
glaze calculation and analysis. For most, this involves acquiring
their own, personal legal copy of the glaze-calculation program
and thereafter receiving improvements ("updates") and advice on a
regular schedule.
The running of a glaze program on a computer allows "follow
- your - nose" adjustment of glaze recipes, instantly analyzing
them to permit comparison between the original recipe and known
standards.
G.9 Defining Base/Flux/Colourant/Opcifier
Glaze-makers uses several terms to describe in short form
the type of mix being presented.
1) Base glaze - a list of raw materials totallling (usually)
100 weight units. This mix if well-designed will form "good"
glass on a pot at the specified firing cone (& temperature).
2) Flux or flux oxide - the oxides of the alkali metals
(Group IA of the Periodic Tabble the Elements), Lithium, Sodium,
and Potassium; and the oxides of the alkaline earth metals (Group
IIA), namely, Magnesium, Calcium, Strontium, and Barium. (Barium
is in declining use because of its potential health hazard).
Besides these chief flux oxides, some other metal oxides
will perform a fluxing action (cause melting) under some
conditions. These include oxides of Boron, Iron, Zinc, and Lead
(now highly restricted in some jurisdictions). And the Colourant
Oxides below sometimes will act as "secondary" fluxes.
3) Colourant - A metal oxide that imparts colour to a
glass/glaze, usually "transmission" colour or reflected-light
colour. Oxides most often used are those of: Iron, Copper,
Cobalt, Chromium, Manganese, Titanium, Tin, and Zirconium.
Less-used colourants are: Vanadium, Nickel, Bismuth, Gold, and
Silver. In most glazes, each of the above metal oxides will yield
a specific hue (intensity varies with amount added to the glaze).
But in certain recipes the actual colour may change, eg, copper
is green in most glazes, yet will become blue or red in special
circumstances.
The colourant oxide/compound is added to a base glaze, and
thereby changes uncoloured glass to a specific hue. Besides
simple compounds of the above metals, a potter may use "stains"
and "pigments"; these are made by chemical companies for the
ceramic industries. The colour spectrum of glaze colourants is a
study all by itself.
4) Opacifier - a material that produces a light-blocking
effect in a glaze, yielding an opaque (white mostly) glaze. The
materials that achieve this are Tin Oxide and Zirconium Silicate,
with Zinc Silicate and Calcium/Magnesium Aluminates providing
opaqueness under very specific conditions/circumstances. A set of
compounds, "spinels", made for wider use than pottery, may also
serve as opacifiers in some instances. Spinels are stable, very
high-melting materials, that affect the colour of glazes by
virtue of being suspended in the molten glass as it forms on the
pot.
Copyrighted by Tom Buck. All rights reserved. April 1999.
Tom Buck (Tom.Buck@hwcn.org) tel: 905-389-2339
& snailmail: 373 East 43rd St. Hamilton ON L8T 3E1 Canada
(westend Lake Ontario, province of Ontario, Canada).