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glas FAQs

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Old October 16th 03, 07:50 PM
Tom Buck
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Default glas FAQs

Glazes: F A Qs
frequently asked questions
by Tom Buck, B.Eng. (Chem.).

Compiled January 1996, reviewed/revised October 2000
by Tom Buck
Copyright (C) 1996 and 2000, all rights reserved, please
contact the author for permission to issue a copy of this

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.
Tell interested potters to visit this site at mid-month.
Tom Buck, October 2000.

G.0 Glaze data outline for beginners.
G.1 What is a glaze?
G.2 Are glass and glaze the same?
G.3 What is a Seger / Unity Formula
G.4 What affects a glaze's "look"?
G.5 How many materials in a glaze?
G.6 What factors go into a glaze?
G.7 How do you design a new glaze?
G.8 What's new in glaze design?
G.9 What is the meaning of:
a Base glaze; b Flux; b Colorant; c 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 (chemical). She then combines the mixed
dry powders with an appropriate amount of water to form a "slurry"
(colloidal dispersion/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 is a Seger / Unity Formula?

Some popular books on pottery make reference to the Seger
Formula as a way of describing the nature of clays and glazes. Most
such materials, when fired, become mineral (metallic) oxides. Two
such oxides are present in the largest amounts; they are silicon
oxide (silica) and aluminum oxide (alumina). Yet, other elements
also have effect on the final product, particularly the glazes.
We can thank Hermann Seger of Germany for much of our
knowledge of ceramics today. In his efforts to understand these
oxides and to predict the outcome of a firing, scientific pioneer
Seger laid the foundation for today's ceramic activities, including
those of the studio potter.
It was in the late 1800s when Seger came up with a notion by
which he could predict how a mixture of clays and minerals would
behave when formed into a pot and fired to "maturity". His studies
gave us the pyrometic cones we use to track our firings, and also
provided us with a way to compare glaze recipes.
Seger used ideas of molecular chemistry then being actively
studied worldwide. Firstly, he obtained an analysis for each raw
material, namely, which elements were in its chemical makeup.
Secondly, he (and others) deduced a likely structure for the
molecules of each raw material, for example, it was shown that
china clay (kaolin) had a crystalline structure that consisted of
two repeating molecules, alumina and silica, Al2O3 and SiO2. Seger
learned that a glaze was a glass mostly made up of these two
oxides. And, with the help of other scientists, he discovered which
molecules in the raw materials would persist in the glass and which
ones would leave the scene.
Seger arranged glaze components (as molecules) in a particular
order. He called one group of molecules the "Flux" Oxides -- 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 common glasses (and
glazes) consist chiefly of Silicon Oxide. Seger also described a
third group which he called "Modifiers" (now called
"Intermediates") -- in this third group Seger included the oxides
of Aluminum, Boron, Iron and Phosphorus, the dominant one being
alumina, Al2O3.
From Seger's studies we know that a glaze's quality is
decided by the relative amounts of each oxide put into the batch,
and to make the proportions easier to recognize, Seger summed the
flux oxide molecular equivalents (now called "moles") and then
divided ALL molar values by this flux-oxide total, thus arriving at
a "formula" (unity or unified formula) for the recipe.
When the moles of 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 oxides will make good glass at specified temperatures. Such a
list of oxides, when converted to a mix-batch, are known as
"balanced" recipes.
To illustrate, let us examine the glaze cited by Michelle Lowe
of Arizona:
Satin White Glaze, Cone 4-7 oxidation
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 (chemical). She then combines the mixed
dry powders with an appropriate amount of water to form a "slurry"
(colloidal dispersion/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."
All the above ingredients have been analyzed and the published
data entered into a glaze calculation program. Thus, when the
recipe is also entered and converted into a Seger Formula, we get
this result:

Flux oxides Intermediates Glass-formers
CaO 0.41 moles Al2O3 0.44 moles SiO2 2.53 moles
MgO 0.34 B2O3 0.06 TiO2 0.01
K2O 0.09
Na2O 0.16
Total 1.00

By checking this Seger against a Limits Table (Insight's), you
learn: 1) This glaze will likely be a satin matt due to the values
of CaO+MgO and Al2O3 (somewhat higher than the values that produce
a glossy glaze); and 2) the glaze will yield a smooth surface under
suitable firing conditions. And by casting it in the Seger
(molecular) form, there becomes available a reliable way to make
substitutions with different raw materials, if needed.

G.4 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" -- see G.7)
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.5 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/basic 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 mineral borates (colemanite, ulexite,
"frits"), spodumene/lepidolite (lithium feldspar/lithium mica),
zinc oxide, and certain frits -- prefired, special glasses,
In the low-fire range, cone 08 to cone 01 (950-1150 C,
1740-2195 F), a borate mineral or a borate" frit (one
with adequate Boric Oxide, B2O3) 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.6 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 sometimes it is difficult to predict how a given
recipe will behave when fired to "maturity". 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 a 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.7 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

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

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

G.9 Defining Base/Flux/Colourant/Opacifier

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 a 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
The colourant oxide/compound is added to a base recipe
(glaze), and thereby changes uncoloured glass to a specific hue. A
potter may use simple compounds of the above metals, or "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

Copyright (C) by Tom Buck, October 2000.

Tom Buck aa563 at hwcn.org) -- primary address. Tom.Buck at hwcn.org
"alias" or secondary address.
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street, Hamilton ON L8T 3E1 Canada


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