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 welcome.
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.
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 introduction.
P1. Guidelines for rec.crafts.pottery
What is rec.crafts.pottery?
P2. Pottery and Ceramics for newbies
What is clay?
P3. Potter culture
Some of the things that make potters special
RELATED FAQs POSTED SEPARATELY
P1. 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.
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 potter."
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.
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"
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.
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 includes:
P2. Pottery and Ceramics for newbies
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.
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.
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.
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.
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".
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.
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.
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.
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.
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
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".
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.
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.
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.
By the glazed look in their eyes. 8-)
Background Image; Chocolate Pie Plate by Mishy
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