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FAQ:Intro to rec.crafts.pottery

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Old July 18th 03, 06:05 AM
Mishy Lowe
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Default FAQ:Intro to rec.crafts.pottery

FAQ: Introduction to rec.crafts.pottery

Welcome to the rec.crafts.pottery FAQ. This file is designed to give an
introduction to the group and some resources to help you explore the world
of pottery and ceramics. This newsgroup started in late 1995 so none of the
questions have been asked all that frequently yet. Comments on this FAQ are

You are now reading the FAQ Introduction to rec.crafts.pottery. If you are
new to the newsgroup, please read the part on newsgroup guidelines. If you
are new to pottery, or just interested in seeing what I think is important
for newbies, read the rest of the introduction.


Evan Dresel
Michelle Lowe

Tom Buck

Steve Branfman


We are trying to provide useful information here by balancing clarity with
nuance. We hope we are successful but take no liability for any errors,
omissions, or oversimplifications. All information is provided "as is" and
"buyer beware" without any express or implied warranties. The authors assume
no responsibility for damages resulting from the use of the information
contained herein. Please read critically and be aware that the answers below
will not cover all situations. This disclaimer also applies to the related
FAQs listed below.


Ruth Ballou and Fred Sweet for helpful review and comments.


Last Revised June 15, 1996

The FAQ has been reorganized into separate FAQ "modules". This reflects the
way different authors are responsible for different parts. We post them
separately, anyway, so be sure to look for additional FAQs. We are trying to
work out a system where each FAQ stands alone but the numbering is somewhat
consistent so you can concatenate them all if you so desire. However, I will
no longer attempt to provide a table of contents for each section in this


P1. Guidelines for rec.crafts.pottery

P1.1. What is rec.crafts.pottery?
P1.2. What topics are appropriate to this news group?
P1.3. Is it OK to post for sale messages?
P1.4. May I post binaries to this group?
P1.5. How can I help with the FAQ?

P2. Pottery and Ceramics for newbies

P2.1. What is clay?
P2.2. What is the difference between earthenware, terra cotta,
porcelain, and raku clay?
P2.3. What are slips and engobes?
P2.4. What is terra sigilatta?
P2.5. What are greenware and bisque ware? What does leather-hard mean?
P2.6. What are cones?
P2.7. What temperature should I fire to?
P2.8. What is the difference between oxidation and reduction firing?
P2.9. What are pit-firing, saggar-firing, and sawdust-firing?
P2.10. What is raku?

P3. Potter culture

P3.1 Some of the things that make potters special
P3.2 How do you pronounce "kiln"?
P3.3 Why do potters immediately turn over pots to look at the bottom?
P3.4 Why do many potters grimace at anything blue and white?
P3.5 How can I recognize other potters?


FAQ Pottery Glazes - Email Tom Buck

FAQ Pottery Books and Journals
FAQ Pottery Internet Resources
FAQ Raku Pottery - Email Steve Branfman


P1. What is rec.crafts.pottery?

P1.1. What is rec.crafts.pottery?

rec.crafts.pottery is a news group for discussing the construction,
decoration, and firing of pottery and ceramic art. We try to take a broad
view of the meaning of pottery. Functionality and craftsmanship are not
prerequisites for inclusion in this group. Discussions do not need to be
strictly recreational either -- Issues affecting professional potters are
welcome. We hope that the amateurs (in the sense that an amateur is a
devotee of a subject) have something to offer the professionals. I know the
recreational potters will learn a lot from the professionals and that there

is a broad range of semi-professionals in between. If you are new to
pottery, you will soon discover that potters are by and large nice people
and we place a high premium on being nice in this group.

P1.2. What topics are appropriate to this news group?

Appropriate topics include discussion of construction and decorating
techniques, glaze recipes, firing methods, and other aspects of making
ceramic objects. Questions, answers, and opinions about pottery making
equipment are suitable for this group. Discussions of history of ceramics,
collecting, studying, and where to see pots and potters are also welcome. It
is fine to discuss the business aspects of being a potter, but this is not a
forum for marketing your work.

Inappropriate topics include spamming, blatantly not-ceramic posts, personal
attacks, and commercial activities. Help from manufacturers and suppliers is
appreciated in the spirit of the free-flow of information. However, people
are bound to complain if it sounds like you are fishing for customers. Some
people in the "support" side of the pottery business have generated a lot of
good will by offering assistance and opinions without marketing. It is a
delicate balance and those who might complain about "commercial posting"
should consider that and also realize that there is nothing wrong with
suppliers being proud of their work.

Some questions posted to the group are likely to be met with at best a stony
silence. The poster may be left wondering at this slight. Although some
posts are difficult or weird enough that no one has a good answer, more
commonly this is because the question is overly broad, uninteresting, or can
be answered easily by checking in a text book or two. Requests such as,
"Please send me some nice cone 6 glaze recipes" fall in this category. The
best response to that request is, "Check out the glaze files on the SDU
ceramic gopher, the ClayArt archives, any of a number of books, and be
prepared to test your glazes. The success of a glaze depends not only on the
formula, but on the clay body, the firing cycle, and the skill of the

Questions on techniques are fine, but some are either too basic or not
amenable to answering on the Internet. I doubt anyone can really describe
how to pull a handle using only ASCII characters. Get a book or better yet,
personal instruction.

P1.3 Is it OK to post for sale messages?

By straw poll, short "for sale" messages by individuals are OK. Commercial
for sale messages by ceramic suppliers, galleries, and others "in the
business" are not. Posting of workshop and other educational opportunities
are welcome in moderation. By moderation, I mean special events of general
interest such as a recognized potter in for a 3 day workshop. Your complete
schedule of beginner pottery classes would not be appreciated by the
majority of readers. If you post a for sale notice please include "FS" in
the subject line and try also to identify the geographic location. The
Internet covers the globe but people are not likely to be interested in
something that weighs a ton and is located half-way around the world. An
example of a useful subject line would be:

"FS: 20 cu ft gas kiln, Bay Area, California, USA"

P1.4 May I post binaries to this group?

Some sites do not have storage for binaries such as pictures and programs,
so do not appreciate binaries in rec groups. Please post binaries elsewhere
and describe where to find them, here.

P1.5 How can I volunteer to help with the FAQ?

Write up the answers to some questions. Better yet pick a topic to help
maintain. Send or post questions of general interest. Send constructive
comments. We are new at this so appreciate your interest. The author list

Evan Dresel

Michelle Lowe

Tom Buck

Steve Branfman


P2. Pottery and Ceramics for newbies

P2.1 What is clay?

Clay is the stuff we shape into pots. The definition gets a bit more
complicated when you look at what makes this earthy material so unique and
wonderful. The word "clay" has several meanings and all aspects contribute
to the craft of pottery.

To a mineralogist a clay mineral is one of a group of silicate minerals with
a flat sheet-like structure. Clay minerals are mostly made up of atoms of
silicon, oxygen, and aluminum. Other substances such as magnesium, iron,
potassium, and calcium are usually present in smaller amounts. Clay minerals
are "hydrated" -- that is they contain hydroxyl ions (OH) chemically bound
within the structure. (You can think of clay as being made of silica (SiO2),
alumina (Al2O3), water molecules (H2O), and some other stuff). The hydrous
nature becomes very important during firing of the clay and distinguishes
clay from other sheet structure aluminosilicates like micas. The composition
and plate-like structure of clay minerals is largely responsible for the
properties like plasticity which make for good pot-making materials.

Clay minerals typically exist as extremely small particles, which leads to
the second meaning of the word "clay" which is a mineral particle sized
between 0.001 and 0.15 millimeters in diameter. By this definition not all
clay-sized particles are clay minerals. However, clay minerals not only are
found as tiny particles, but the flat structure of the minerals tends to
align like a deck of playing cards giving the cohesion and workability of
the material. If you chew on a bit of clay between your front teeth you
won't be able to feel any grittyness because the particles are too small.
Silt sized particles will feel gritty.

Finally "clay" is often used to mean a "claybody" which is the stuff we use.
Claybodies usually contain a variety of clays an other minerals in order to
promote the right properties for working. Often claybodies contain materials
such as sand or grog (ground up pre-fired clay) in order to add strength and
resistance to thermal shock. Other minerals often used in clay bodies
include flint (microcrystalline quartz) which helps resist warping and
strengthens the fired material, feldspar and nephelene syenite which act as
fluxes to lower the maturation temperature, and talc which acts as a flux
and helps resist thermal shock.

P2.2 What is the difference between earthenware, terra cotta, stoneware,
porcelain, and raku clay?

Earthenware, terra cotta, stoneware, porcelain, and raku clay are different
types of claybodies. Earthenware is a clay body that is fired to a low
temperature (approximately 1000 C or 1800 F). The resulting material is
still very porous so it will absorb water or even leak if not glazed. Terra
cotta is generally used to mean a red earthenware. Stoneware is a generally
buff, tan, or brown clay which is matures at high enough temperature
(approximately 1200 C or 2200 F) to form a dense ceramic having a low
porosity and which will not leak, even if unglazed. Porcelain is a very
white clay body which is fired to the point of becoming almost totally
vitreous (glassy) so it is translucent when thin. You will often see almost
any white clay body referred to as porcelain but many are better termed
white stoneware. True porcelain is fired to cone 9 to 12 (see below). Raku
clay is a clay body designed to take the stress of raku firing process. Raku
clay typically is white and contains a lot of sand, but see the FAQ Raku for
a more complete explanation.

In the past, potters had to dig clay and prepare their own claybodies. Now,
in industrialized countries, potters are fortunate to have suppliers which
provide an almost overwhelming choice of claybodies. Even potters who mix
their own claybody can purchase raw materials, avoiding much of the labor
and quality problems associated with digging their own clay.

The variety of claybodies available can be confusing to the potter just
starting to work on his or her own. You may be best off exploring a couple
of bodies in the firing range of interest at a time, rather than trying too
many kinds at once. It sometimes takes a while to get to know a body and how
to work with it. Try a variety of forms and forming methods. Glazes will
look and act differently on different claybodies, so test-tiles for testing
your glazes on a new body are very useful. Conversely your glazes may need
some modification so that they fit the body.

P2.3 What are slips and engobes?

Slips and engobes are colored clay slurries used to decorate pots. A simple
slip may be made up of a clay body mixed in water. Engobes may mean colored
slips or may be used to describe a slip modified to melt slightly producing
a surface somewhere between a slip and a glaze. Slips and engobes may be
covered with glaze to produce an "underglaze" decoration. Slips and engobes
are generally applied to greenware (see below) but may be applied to
bisque-ware (also below) if formulated to shrink less during firing.

P2.4 What is terra sigilatta?

Terra sigilatta (or terra sig. to cool potters) is a slip made of extremely
fine clay particles. The terra sig is applied to the "leather-hard"
greenware, or even bisqued pot and when mostly dry can be burnished (or
polished) with a stick, smooth pebble, back of a spoon, or whatever works.
The resulting surface is rich and shiny. Burnishing the surface was
important in many cultures because the resulting surface made the pot more
waterproof when fired to low temperatures. This is because the burnishing
lines up the flat clay mineral particles along the surface of the pot. The
alignment minimizes the porosity of the surface. Burnished pots must be
fired to only very low temperatures to maintain the shiny surface because
the clay minerals begin to recrystalize at higher temperatures and the
structure is lost leading to a dull surface.

P2.5 What are greenware and bisque ware? What does leather-hard mean?

Pots that have not been fired are called greenware, particularly when dry
enough to load into a kiln. Pottery is at its most fragile at this stage, It
has lost its plasticity but has not attained the strength of a ceramic.
Generally pots are "bisque" fired to a low temperature (approximately
950-1050 C, 1750-1950 F or cone 08 to cone 04) prior to glazing. These
bisque-ware pots (or biscuits) are porous but fairly strong. They have
already undergone much of the shrinkage expected during firing. Bisque pots
are soft so be careful not to chip or scratch them. The pots are then
re-fired after glazing.

Leather-hard is the stage in drying a pot where the pot will hold its shape
but has not lost all its water. This is the stage for applying handles,
using terra SIG, or doing a variety of other tasks in making pots. When pots
are as dry as they get, prior to bisqueing they are called "bone dry".

P2.6 What are cones?

When clay and glazes are fired the result depends on the rate of temperature
rise and the time it is left at high temperatures as well as the temperature
reached. If you just measure the temperature with a thermocouple, that does
not tell you enough to get consistent results. Cones are specially
formulated ceramics formed into triangular, pointy shapes. When placed in a
kiln they will soften and bend (or "mature") at a specific temperature **
when the temperature is increased at a specific rate **. If fired faster or
slower, the cones will mature at a different temperature. Cones thus account
for the firing history as well as the temperature.

Cones with different numbers are made to mature at different temperatures.
From lowest to highest temperatures the cones count from approximately
022, 021, 020 ... 03, 02, 01 then to 1, 2, 3, and on upward. The "0" at the
beginning is like a negative sign although there is no 0 cone.

When firing by cones, typically three cones are used. The guide cone is one
or two numbers lower than the desired end point. When it bends over, you
know you are close to being done. The firing cone bends over at the desired
end point. The guard cone is a cone higher than you want and lets you know
if you have over fired. The cones are placed in front of a peep-hole in the
kiln and monitored periodically when the end of the firing is near. It is a
good idea to put cone packs in different parts of the kiln so you can see if
the firing was even (after the kiln cools).

On the net people will either spell out cone or use the ^ symbol to
represent a cone (e.g. ^06).

Cones manufactured by the Orton Foundation are the most commonly used in
North America. Large cones are the standard cones used by potters. Orton
provides a chart of maturation temperature for the large cones when the rate
of temperature rise is 150 C (270 degrees F) per hour. The chart for small
cones is for a temperature rise of 300 C (540 degrees F) per hour and the
maturation temperatures are higher than the large cone of the same number. I
would include a cone temperature equivalent chart here, but I'm too lazy to
type it in. They are available in many books and pottery supply catalogs,
and often come with a box of cones.

Generally the small cones are used in kiln sitters which are devices which
automatically shut of the kiln at the proper cone. The small cone sits
horizontally on top of two metal bars and holds a trigger bar up. When the
cone melts it bends, the trigger drops, releasing a weight on the outside of
the kiln. The weight hits switch which shuts off the electricity. Rube
Goldberg would be proud. Kiln sitters are reliable but not infallible. Back
up timers are a highly recommended option on your kiln sitter. Once you know
how long a firing is expected to take (from experience and careful records
of your firing schedule) you can set the timer for a little longer time. If
the sitter fails, the timer then will shut off the kiln before a major
melt-down occurs. The best backup, however, is a set of large cones visible
through a peephole and yourself baby-sitting the kiln, particularly near the
end of the firing, (I use a sitter/timer and hang around at the end of the
firing). Most people find that they need to use a small cone in the sitter
which is one number hotter than the large cone target temperature.

P2.7 What temperature should I fire to?

Obviously this is a matter of personal choice but the firing range will
affect your pots greatly. Remember different clay bodies are designed for
different ranges of temperature and if you get them mixed up you risk a real
mess in the kiln! Earthenware clay will slump and "melt" into a disgusting
heap at stoneware temperatures.

Earthenware is generally fired to cone 06 to 02. One common technique is to
bisque fire to cone 04 and then glaze at cone 06. Firing to cone 03 or 02
will promote the formation of mullite in the body which can help the glaze
fit the pot properly. Firing to earthenware temperatures (or lower) is
called "lowfire".

Stoneware is traditionally fired to cone 9 or 10 which is called "high
fire". However, in the USA, cone 6 claybody is usually referred to as
stoneware and is frequently fired in an electric kiln. People in other parts
of the world may refer to cone 6 as "mid-fire".

Porcelain is usually fired to cone 10 or 11. Although there are white
lower-maturing clay bodies, you may find that the body is not truly dense at
cone 6 and may not even hold water.

P2.8 What is the difference between oxidation and reduction firing?

Pottery may be fired in oxidation or reduction. Oxidation firing takes place
in air with its 21 % oxygen content while during reduction firing excess
fuel is introduced at times to consume the oxygen. The reducing agents
present during reduction firing react with the claybody and glaze. The type
of firing chosen has a huge impact on the final appearance of the pottery.
Many glazes such as copper reds and celadons only work in reduction.
Oxidation firing often promotes brighter colored, glossy, and some would say
harsh surfaces. Reduction firing promotes warmer clay body color and often
softer, more visually textured glazes. Oxidation can be achieved in any type
of kiln where the atmosphere is not depleted in oxygen through the
combustion process. Electric kilns, for all practical purposes fire in
oxidation. Fuel burning kilns may be fired in reduction by making the flame
richer at certain times during the firing by adding more fuel, cutting the
oxygen to the burners, and mainly by partially closing the damper. The
oxygen starved atmosphere in the kiln reacts with iron and other elements in
the clay and glaze causing chemical changes that affect the appearance.
Reduction firing involves the potter in the process more, is more involved,
and may be less predictable. These traits appeal to many potters who value
the variety and "life" produced in pots which are fired in reduction.

P2.9 What are pit-firing, saggar-firing, and sawdust-firing?

Pit-firing, saggar-firing, and sawdust-firing, are types of low-temperature
firing where the pots are stacked in combustible material such as sawdust,
dung, or wood shavings which produce strong reduction during the firing.
This is one of the earliest firing techniques and often lends a "primitive"
quality to the pots. In pit or sawdust firing, the material around the pots
is ignited to produce the heat source. In saggar firing, a large covered pot
called a saggar is loaded with the organic mixture and one or more pots then
placed in a conventional kiln for firing. (Saggars were originally used to
fire pots prior to the development of kiln shelves. This allowed glazed pots
to be stacked and provided protection from wood ash in the kiln.) Modern
potters may add a variety of material such as sea weed, orange peels, copper
wire, or oxides in order to produce different colors and effects. Pit firing
is a good technique to explore if you have some space where you can fire but
only have minimal equipment.

Pit, sawdust, and saggar firing are usually to very low temperatures and the
soft, porous pots that result are mainly decorative rather than functional,
although these techniques have served the functional needs of some cultures
for thousands of years.

P2.10 What is raku?

Raku is the name given to a firing technique, where the pots are removed
from the still-hot kiln when the proper temperature is reached. Commonly
raku involves post-fire reduction where the pots are immersed in a can
containing organic material (e.g. sawdust) and covered so that reducing
conditions are attained. The clay body turns black, the glazes craze (or
crackle), copper bearing glazes are often reduced to produce metallic
lusters. This is an exciting process often leading to surprise results.

The origin of raku was in Japan where it has been practiced by the Raku
family of potters since the 16th century. The technique is somewhat more
subdued producing black raku and (less commonly) red raku ware. Some of the
most famous tea-bowls are raku.

Sometime in the 1960's Paul Soldner, Robert Pippenberg, Hal Riegger, and
other American potters got into the act and produced innovations in
post-fire reduction. The term raku has stuck for their techniques in spite
of vast differences from the Japanese origins.

The handling of hot pottery and rapid cooling required by raku firing means
that the clay body must be able to withstand a lot of thermal shock.
Typically raku involves a white body that contains a lot of sand for
strength. Also the body is general fired only to an open porous ceramic that
helps it withstand thermal shock. For example the raku body used at Penn
State, when I was a student, made a fine cone 10 white stoneware body even
though raku is typically fired to about cone 011 to cone 06.

Because of the porous, fragile, nature of the less-than-mature raku pot,
raku ware should generally be considered decorative rather than functional.
Also some raku surface decorations will fade with time unless preserved with
non-ceramic coatings such as acrylic spray, or wax.

See the FAQ on raku pottery and the pottery balkiest FAQ for further
information on raku.

P3. Potter culture

P3.1 Some of the things that make potters special

Potters and ceramic artists, like any other group, develop their own way of
communicating, their own approach to things, and other markings of a
subculture. This isn't to say that potters are not a diverse lot, but
certain things can seem strange or bewildering to beginners. Clay isn't the
basis for a secret society, but is is a way of life for many of us so here
are a few things to help put you "in the know".

P3.2 How do you pronounce "kiln"?

Kiln is either pronounced the way it is spelled "kiln" or the final "n" may
be silent so it is said "kill". Is this loss of the final consonant a
holdover from our rural potter roots, a habit from long time, or a sign that
you are a cool potter in the know? Probably all of the above in various
instances. I'm open to suggestion.

P3.3 Why do potters immediately turn over pots to look at the bottom?

The foot of a pot tells much about the potter. It is this humble area that
often shows the individual style of the maker. The line of the cutoff wire,
the angle of the trimmed foot, and even the transition from glaze to raw
clay are the intimate details which make a pot a unique thing. Besides the
base is where you can usually find the potter's mark and the price tag.

P3.4 Why do some potters grimace at anything blue and white?

Cobalt blue decoration on a white background is thousands of years old and
there is no sign of it going out of style yet. Cobalt is a reliable and
intense colorant for a wide range of temperatures. It's an easy seller, and
very common. This isn't to say that there is anything wrong with blue or
white, and there may well be much wonderful work to be made in the style.
Some of us do wish the public would be more receptive to some of the pieces
which show some of the other wonders of pottery. Some of these other pieces
are "potters pots". They attract other potters but don't sell well. By the
way, green glazes have a reputation for not selling but apparently is
becoming popular, in the USA at least.

P3.5 How can I recognize other potters?

By the glazed look in their eyes. 8-)

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