FAQs for rec.crafts.pottery Usenet newsgroup

3. Glaze data for newbies
(compiled by Tom Buck: all rights reserved)

3.1 What is a glaze?
3.2 Are glass and glaze the same?
3.3 What affects a glaze surface?
3.4 How many materials in a glaze?
3.5 What factors go into a glaze?
3.6 How do you design a new glaze?
3.7 What is a Seger/Unity formula?
3.8 What's new in glaze design?
3.9 What glaze faults may occur?
3.10 What causes glaze faults?
3.11 What is the meaning of:
a> Base glaze; b> Flux; b> Colorant; c> Opacifier.

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3.1 What is a glaze?

In plain terms, a glaze is a very thin layer of glass 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 (above 1800 F, 1000 C).
On a typical piece of pottery the glass layer is 1 - 2 mm (0.04-0.08")
thick on average. However, some pots undergo several "glost firings" (ie,
glaze-forming firings) and these pots may end up with glass layers 3 mm
thick. Most pots are fired twice, a few five or six times.
The glaze layer usually starts out as a glaze "recipe", also referred to as
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")
15.8 Custer feldspar (a "potash" feldspar)
21.0 Dolomite (mineral, 50/50 calcium carbonate, magnesium carbonate)
21.0 Bell Dark ball clay
5.2 Gerstley borate (mineral, source of boric oxide)
10.5 Flint (or silica or quartz)
To this mix, which totals 100 weight units, Michelle Lowe adds 6 weight units of
Tin oxide. She then combines the mxied dry powders with an appropriate amount of
water to form a "slurry", 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 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 given 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 glaze designer.

3.2 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 a
special furnace 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 silica sand (silicon oxide),
soda ash (sodium carbonate), and 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 some borax (sodium borate) is
added to the basic recipe, the result is hardened glass suitable for laboratory
glassware.
Despite some limitations, a "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", that is, a mix of fine
powders suspended in water. Then, this mix is transferred to the surface of the
clay pot (usually "bisqued-fired"--see Clay FAQs) by one of three common
methods, dipping, spraying or brushing. The clay-pot's surface has an affinity
for the glaze powders and holds them in place. After the wetted pot has dried
again, it is placed in the kiln and heated ("fired") to the proper temperature.
This two-stage process requires that the glaze will stay on the pot while it is
drying and later, in the kiln, 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 clusters that can form liquid glass. Some
materials also undergo chemical change either by giving off gases or by
re-arranging their molecular shape. If the mix of oxides falls within a certain
range, the materials mix will form glass on the fired pot, that is, become a
glaze.

3.3 What affects a glaze surface?

A glaze surface may be glossy, satiny, or rough (dry matt); the
actual result will revolve around the silicon oxide and the alumina oxide
contents, both in relative and absolute terms. A glass that contains 60% silicon
oxide (silica, flint, quartz), plus or minus 5%, will usually make a good glaze.
However, if the basic oxide mix (the "Seger" 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, most often a
silica-deficient glaze will take up some silica from the body to form a glaze
closer to a good glass, i.e., a balanced glaze. If the silicon oxide content is
70%+ SiO2, the glaze becomes high-melting and may not form glass at the expected
cone/temperature. Then, its surface could exhibit some unwanted effects, eg,
crawling (the glaze clumps in pools).
A good long-lasting glaze surface contains sufficient alumina (Al2O3)
to make the glaze melt stay put (non-runny) and to form an alumino -
silicate polymer that is strong and resists scratching. The ratio of silica
molecules to alumina molecules 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 body/glaze interaction. Above 10 the
glaze will be glossy and perhaps runny, again body interaction being the
deciding factor.

3.4 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 "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 or dolomite, and kaolin or
clay. In the mid-fire range, cone 1 to cone 7 (1160-1250 C, 2120-2280 F), the
ingredients list will usually be expanded to 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 or a
high-boron frit 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 clay).
Why these particular materials? Are there others that could be used?
Potters, over decades, have learned by trial an 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.

3.5 What factors go into a glaze?

Glaze design is both simple and complex; the list of basic 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
with constant composition and behaviour.
Excluding a few exceptions, a typical glaze recipe brings together
ingredients dug from the earth and which thereafter undergo minimal processing
to clean and pulverize them. To keep costs down, suppliers use the least amount
of processing consistent with adequate performance. Also, there are variations,
time to time, in the composition of the material being mined. And, with some
glaze components, there are several 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, time to time.
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.

3.6 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 experiments, 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 or Unity formula; this
is a shorthand statement of the glaze make-up, or a list of "basic 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 basic oxides
that hopefully will turn into highly viscous (non-runny) molten glass on the
surface of the pot. These basic oxides, seldom isolated as such, are contained
within the batch recipe's raw materials.

3.7 What is a Seger/Unity formula?

One way to help evaluate a glaze recipe is through the Seger or
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 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, called
"modifiers", which included the oxides of Aluminum (aka aluminium), Boron,
Iron and Phosphorus, the dominant one being Aluminum.
The kind of glass (quality) is decided by the "relative" amounts of
each type of basic oxide put into the batch, and to make the proportions
easier to recognize, Seger set the TOTAL number of flux oxide molecular
equivalents ("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 basic 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 summary chart called "Flux Unity Formulas". The table thusly cites what
proportions of basic oxides make good glass at specified temperatures.
Such a list of basic oxides, when converted to a mix-batch, are known as
"balanced" recipes.

3.8 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 she/he has:
1. Access to a microcomputer, either an IBM type with a disk operating
system (DOS) 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.

3.9 What glaze faults may occur?

(This section is under development)

3.10 What causes glaze faults?

(This section is under development)

3.11 What does Base/Flux/Colorant/Opacifier mean?

(This section is under development)

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