POTTERY PRODUCTION PRACTICES: Pottery Myths

March 1, 2008
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The making of ceramic objects is, at best, part science and part intuitive art. In the past, the basic science of pottery production was somewhat hit or miss. Without an extensive understanding of the underlying theories concerning ceramic materials, potters were often left with myths and inaccurate information on forming techniques, glaze formulas and firing processes.

For example, if a master potter obtained clay from a particular location and it was able to withstand the temperatures inside his kiln, he might assume that the clay was the central factor in the fact that his pots did not blow up during firing. While this might be true, other factors could also be at work, such as a slow heating rate within the kiln.

Without an understanding of how ceramic materials react in the firing process, a rote set of procedures was the best course to follow. If the apprentice potter changed one variable, he risked losing a whole kiln load of pottery. The uncertainty of results often called for reassuring myths and dogma.

Some myths are related to various studio techniques that are handed down from misguided teacher to student or are transferred among potters. Since pottery is an endeavor that involves many steps-any of which can cause total failure-potters often misguidedly cling to a set method or technique that they think will provide a guaranteed result. Often, these tips or dictums are based on incomplete information or a misunderstanding of how ceramic materials function. Furthermore, what worked for one potter might not work for another.

Today, due to a greater range of experience with ceramic materials and a more scientific approach to pottery production, the blank spaces in our knowledge have shrunk. Unfortunately, some areas are still subject to unsupported speculation. Following are some of the many myths that still circulate in the pottery community.

Economic Myths

Economic myths concerning the production of pottery are the most costly in terms of a potter’s time and labor, and are often the leading causes of pottery business failures. Most myths regarding economic factors originate because potters do not have an understanding of standard business practices and the forces that influence the pottery buying market. It is often said that potters without business skills do not do as well as business people who become potters.

Potters often do not take into consideration their time and labor, or they underestimate it. As a general rule, the more you touch the pots, the more they cost to make. Efficiency is critical, as the “handmade” aspect of pottery will eat up any potential profit. The profit margin just isn’t wide enough to allow for bad business practices, no matter how excellent the pottery may be. The failure of many pottery operations is caused by inefficient production practices and bad business practices, not the lack of aesthetic quality of the pottery. The most effective advice for a potter who wants to sell their work is to start by taking business courses.

A common misconception when selling pots to relatives, friends and neighbors is that sales will continue to expand at the same rate. After this initial period of easy growth, the next step relies heavily on developing business skills, not just the ability to make more pots. The myth that selling pottery is easy soon turns into the realization that no matter what you sell, it requires bookkeeping, advertising and market research skills to grow the business.

DIY and Save Money

A lot of potters think that saving money equals low-cost equipment, tools and supplies. However, no true cost savings will result from building a piece of equipment that will be less efficient and of lower quality than one you could buy. In addition, buying a less expensive potters wheel or kiln that does not meet your production needs can be very expensive in the long run.

One classic cost-cutting scenario is when a potter decides to purchase less expensive moist clay and have it delivered to the end of their long driveway. The time spent carrying the clay to the studio does not outweigh the extra cost for in-studio delivery.

Some potters decide to build a wood kiln because they have an inexpensive source of wood. However, most wood kilns take one to four days to fire and cool, and the real cost involved is the time and labor required to constantly load wood into the firebox for the entire firing. Potters should ask themselves how many pots they could make in the time it takes to stoke the kiln. If the pots can be sold for significantly more money because of the wood firing process, then the extra time and labor required for their production might be justified.

Occasionally, a potter will decide to make their own brick to build a kiln. On first hearing this idea, it sounds absurd that many potters still do not recognize how expensive their own labor is. Aside from the fact that commercially produced bricks are more accurate and exactly formulated to withstand high-temperatures, who has the time to make thousands of bricks? The same principle applies to those who wish to make their own kiln shelves or posts.

Another myth is that it is less expensive for potters to mix their own clay. Except in special situations, leave the clay making to ceramic supply professionals-they have the machinery, expertise and track record to produce consistent moist clay. Making your own clay necessitates purchasing and storing raw materials, and setting aside time to mix the clay. Aside from buying a clay mixer, a pug mill, performing maintenance and tying up studio space, why start a business (clay mixing) to supply your original business (making pottery)?

Other potters want to mine their own clay to save money. As with the other economic false steps, this idea involves extreme expenditures of time and labor to arrive at a product (clay) that can vary in particle size, chemical composition, organic content and possibly be limited in deposits. Mining companies spend millions of dollars in research and development to achieve constant, reliable clay. The individual potter cannot compete. Starting further back in the pottery production cycle is always time consuming, inefficient and expensive.

Frozen Clay is Ruined

Moist clay contains approximately 20 to 28% water, depending on the particular clay body. Some potters store their clay in conditions that cause it to freeze. Frozen clay looks rock solid; if it has thawed, fissures and cracks might appear. The clay has not been damaged, however. It has simply gone through a change that it has experienced countless times in nature. Simply warm the clay and wedge it to the appropriate consistency.

Frozen Glaze is Ruined

As with frozen clay, liquid glaze that has frozen can be thawed out and used successfully. It is always best to run the liquid glaze through a sieve to evenly distribute its raw material content.

When your favorite flower pot is left out in the winter and cracks, strong forces are at work.

Freezing Conditions Do Not Affect Fired Clay

After the clay has been fired, another type of freezing condition has to be considered if the object is to be placed outside. As some potters have already discovered, glazed or unglazed ceramic forms can fracture and spall (chip due to internal stress) when placed into freeze/thaw conditions. When your favorite flower pot is left out in the winter and cracks, strong forces are at work. Not all clay bodies can withstand freeze/thaw conditions, no matter what temperature they are fired to.

Most materials shrink when frozen. Water, however, expands due to the formation of ice crystals. The open pore structure of fired clay traps moisture in the form of rain, snow and humidity by capillary action. Upon freezing, ice crystals expand in the unyielding pore structure of the clay, causing cracking or chipping.

Any clay body that is to be placed outdoors in freeze/thaw conditions should be tested in accordance with the American Society for Testing Materials, Designation: C373-88, Standard Test Method for Water Absorption, Bulk Density, Apparent Porosity and Apparent Specific Gravity of Fired Whiteware. The freeze/thaw test measures the percentage of water entering a fired clay body when it is soaked in water for 24 hours and then boiled in water for five hours, which conforms to the American Society for Testing Materials test procedures. The test method replicates conditions present in the natural process of the clay body in actual freeze/thaw environments.

Slow Drying Prevents Cracks

Slow drying of leather-hard pottery or sculpture is often thought to prevent cracks. Many potters dry their work under plastic or water-saturated cloths wrapped in plastic. However, when this type of drying technique works, it is more likely due to even drying conditions. Pottery can be dried very fast if it is dried evenly. Slow drying often wastes time and can delay the onset of a crack that is already present due to improper selection of a clay body, incorrect forming technique or excessive use of water during the forming process.

Pottery or sculpture can be turned over every other day under a lightweight plastic sheet. In most instances, it can be uncovered during the day, covered with plastic at night, then uncovered the next day and rotated. The process can be repeated until the pottery dries evenly. Forms that cannot be turned upside down can be supported with foam rubber to hold them uniformly. Even drying insures that all parts of the ceramic form undergo shrinkage at the same rate.

Sharp edge cooling crack in ware caused by cristobalite inversion in the kiln or very hot kitchen oven.

Slow Drying Equals Safe Drying

It is not unusual for potters to dry pottery in their studios for weeks, months and even years, and then believe that it contains no moisture. When the “dry” pottery is bisque fired, however, it sometimes blows up or cracks. What potters do not understand is that the pottery still includes the moisture content of the studio when it was placed in the kiln. The pottery contains mechanical water that has to be driven off from 212 to 392ºF. In addition, chemical water is released from 842 to 1112ºF and has to be dissipated by a slow temperature increase or the pottery can crack.

Published Information is Always Accurate

Ceramic supply companies frequently list the shrinkage and absorption percentages for the moist clays they sell. While these figures are useful when comparing one clay body to another, they can be inaccurate when compared to other suppliers’ clay bodies or an individual potter’s expectations in their own kiln. Various test procedures can be used by the ceramic suppliers to produce the results, which can affect the outcomes.

Air Bubbles Always Ruin a Piece

One of the main reasons given for wedging clay is to remove air pockets. Wedging also distributes moisture uniformly and deposits raw materials evenly throughout the clay mass. Any air pockets still trapped in the moist clay will not cause it to blow up or crack if the kiln is fired at a slow rate of heat increase. However, fast firing in the initial stages can cause high-pressure steam to expand in the clay, causing the pottery to fail.

Glaze Coatings Prevent Leaking

Glazes serve two basic functions. They produce smooth surfaces that can be easily cleaned, and they add an aesthetic quality to the pottery. Glazes can never be considered sealants. If the clay body is absorbent, liquids or solids can penetrate slight imperfections in the glaze layer. This effect is often observed when water permeates a glazed vase and leaves a water stain on furniture.

Test Kilns Yield the Same Results as Production Kilns

It is unrealistic to assume that a clay body or glaze will react the same when fired in a small test kiln as in a larger kiln, even when fired to the same temperature. Larger kilns have greater thermal mass as a result of bricks, shelves, posts and pots, and small test kilns can also heat and cool faster than larger kilns. These factors all affect the melting characteristics of glazes and clay bodies, which react not only to absolute temperature but the time it takes to reach that temperature. In small test kilns, the clay body and glaze do not remain in their maturation range as long as they would in larger kilns.

The different firing characteristics of each kiln size can sometimes be observed when a small kiln produces a satin matte surface glaze. The identical glaze fired to the same temperature in a larger kiln might become a transparent gloss due to the increased heat work acting on the glaze. A more subtle defect can occur when the clay body does not reach maturity in the small kiln. While it might appear dense and hard, it can actually have a higher absorption rate and be less durable in the small kiln than it would in a larger kiln. Use the same production kiln for any clay body and glaze testing for best results.

Electric Kilns Can Handle Reduction Firings

Pottery kilns heated by electricity should not be exposed to reduction atmospheres caused by hydrocarbons like wood, paper or moth balls. Such practices can shorten the life of electric kiln elements.

All Lead Frits Are Safe

Frits are combinations of ceramic oxides such as potassium, sodium, calcium, alumina, silica and sometimes lead. The oxides are heated to a liquid state, fast-cooled and ground into a fine powder. The goal is to remove volatile components and render soluble materials insoluble.

Even after calcining, some frits can be slightly soluble. If lead is not safely tied up with the correct ratio of silica and/or other oxides, such as alumina, it can be released from the dry powdered frit or when the frit is used in a glaze. When in doubt, a glaze should be tested by a professional laboratory to ensure its safety.

Bisque Pottery Must be Washed

The time-consuming and labor-intensive practice of washing bisque ware is believed to remove dust. Washing bisque pots is also supposed to make them less absorbent, slowing down the drying time when glazes are applied. High absorbency in the ware can cause the glaze to pinhole as it is wicked into the bisque surface.

Unfortunately, wetting a sponge and running it over a dusty bisque surface can have the effect of moving dust from one area to another while depositing debris from the sponge or water. In addition, the bisque surface is fragile and any disruption can destabilize the glaze adhesion.

It is more effective to blow dust off bisque pottery. If the bisque is in the studio for more than one or two days, a covering of lightweight plastic will prevent contaminants from coming into contact with the pots. Also, firing the bisque kiln one or two cones higher can often solve the problem by slightly fusing bisque dust to the surface of the pot.

A higher bisque temperature reduces the absorbency of the bisque pottery while lower temperatures increase absorbency. Gums such as CMC or Vee Gum CER (¼ to 2% based on the dry weight of the glaze) can be added to the glaze to slow down the drying time and increase the durability of the raw glaze surface. These techniques are more effective and less labor-intensive than wetting the bisque pottery.

Bone Ash Can Lead to Mad Cow Disease

Some potters believe they can contract Mad Cow Disease (Bovine Spongiform Encephalopathy) when using natural bone ash made from cattle bones (calcium phosphate) ground into a powder. However, the bones are fired to temperatures above 2500ºF, which eliminates any possible contamination, and they are chemically inert and free of organic matter like DNA.

In the past, bone ash was imported from England. Since the Mad Cow outbreak, however, the U.S. Department of Agriculture (USDA) has required many stipulations and lengthy procedures before it will allow bone ash into the country. The USDA unfortunately confuses bone ash with bone meal (ground animal bones that are not calcined). For potters who still think there might be a slight chance of catching Mad Cow Disease, they can use synthetic bone ash (tri-calcium phosphate), which in many instances is an acceptable substitute.

Sunglasses Can Be Used When Looking Into a Kiln

In the past, “glassblowers ailment” was a common problem among people who worked with hot glass and were subjected to infrared and ultraviolet light when looking into high-temperature molten glass tanks. The cumulative effect after years of unprotected exposure caused cataracts to develop in their eyes. In some instances, the exfoliation of the eye lens was a gradual process that went unrecognized until the damage was severe. Today, the cause of glassblowers’ cataracts and the protective methods to prevent eye damage has made this condition a historical curiosity.

Potters also mistakenly think that looking into a low-temperature bisque kiln does not require eye protection. Ordinary sunglasses will not protect the eyes from the infrared and ultraviolet rays emitted from a firing kiln. The use of proper eye protection is highly recommended. When considering glasses or goggles, make sure the product meets the American National Standards Institute’s ANSI Z87.1-1989 standards.

Pyrometric cones are precise blends of ceramic materials designed to deform at specific rates of time, temperature increases and atmospheric conditions during a firing. Cones will remain usable indefinitely if they are kept dry.

Pyrometric Cone Myths

Several myths and false assumptions surround pyrometric cones and often lead to inaccurate firing results, so here are some facts. Pyrometric cones are precise blends of ceramic materials designed to deform at specific rates of time, temperature increases and atmospheric conditions during a firing. Some potters paint the cones with iron oxide for easier viewing in the kiln. Iron oxide, or any material coming into contact with the cones, can drastically change the accuracy of the cone reading.

Cones can also flux prematurely if not protected from the vapors of wood, salt or soda firing kiln atmospheres. Cones cannot be reused after they are subjected to heat in a kiln. Small pyrometric cones (11/8 in. tall) will not give an accurate reading when placed on the kiln shelf, as they are calibrated to bend accurately only when placed in a kiln sitter.

Large self-supporting cones and large non-supporting cones placed in a cone pack will not give the same reading if they are not set at the same height. Pyrometric cones will remain useable indefinitely if they are kept dry. Pyrometric cones are not evenly spaced by temperature and can exhibit a 62 to 77ºF difference from one cone to the next.

Cracks Should Always Be Fixed

Most cracks stay the same or get worse in the drying, bisque firing or glaze firing stages. Any time spent trying to fix a crack has to be weighed against the odds of the repair succeeding. The minor exception to this rule is if the crack occurs when the clay is soft, pliable and fairly wet to the touch. Fixes at this stage are often successful. A better strategy is to work in multiples-if one pot fails, it is more efficient to proceed to the next piece without wasting the time and labor to fix something that might remain cracked.

Water Type in Glazes Causes Defects

As a general rule, if you can drink the water, the chance that it will cause a glaze defect is minimal. A potter will often waste time trying to track down a glaze defect by changing the water source, even to the extent of using distilled water in their glazes. While alkaline or acidic PH water levels can change the viscosity of the wet glaze, the type of water used is rarely the source of blistering, pinholing, crazing or shivering defects.

Crazing is a fine network of lines in the fired glaze surface. While craze lines can be aesthetically pleasing, they can weaken the durability of the pottery.

Crazing and Shivering Myths

Crazing is a fine network of lines in the fired glaze surface that occurs when the glaze cools under tension over the clay body. Rubbing carbon black into the craze lines can accent their appearance, especially on a white or light-colored glaze. While craze lines can be aesthetically pleasing, they can weaken the durability of the pottery. Craze lines also allow bacteria or mold growth to take hold in the underlying absorbent clay body. Reformulating the glaze to bring it under a slight compression will stop crazing. For functional pottery, a nonabsorbent clay body will ensure strength and increase the potential for a stable glaze fit.

Shivering occurs when the fired glaze chips off the clay body.

Another less prevalent clay body/glaze mismatch is shivering, where the fired glaze chips off the clay body. In this situation, the glaze cools under too much compression. Some potters believe it is an isolated event if a glaze is shivered or crazed. In either type of defect, the clay body and glaze are under extremes in tension or compression-whether they show it at the time of the kiln opening or at a later date.

All of the pottery using the same clay body and glaze combination must be suspected of having identical internal forces that can produce the same defect at a later date. Whenever crazing or shivering is present, it is best to recalculate the glaze to ensure the correct fit with the clay body.

Subjecting the finished pottery to extremes in heat and moisture, such as in a dishwasher, will often reveal potential crazing in the glaze. An autoclave that produces higher heat and pressure is also often used to test glazes. Shivering can sometimes be detected by tapping the edges of the pottery with a metal tool like a screwdriver.

Fast Firing Decreases Production Costs

While firing the kiln faster can cut energy costs, it can also leave the clay body and glaze immature. Decreasing the firing time can also produce less-durable pottery. Clay and glazes not only need an end point temperature for maximum strength and durability, they also require the appropriate time to reach that temperature. Any cost savings in fuel can be more than offset by functional pottery breaking or glaze surfaces being prone to abrasion and chemical attack from food/drink or everyday cleaning detergents.

Raku Pottery Can Be Used for Dinnerware

The Raku firing process starts when bisque pottery is glazed and placed in a fast-firing kiln. While still red hot, it is taken out and fast-cooled. The finished pottery is decorative and somewhat functional, but it cannot be considered durable and long-lasting for eating and drinking vessels. The fired clay is open and porous to allow for the thermal shock of fast firing and cooling, and low-temperature glazes often lack enough silica and alumina to ensure a durable, hard surface. In addition, many low-temperature glaze materials such as Gerstley borate produce “soft,” easily abraded and soluble glaze surfaces when fired.

Materials Are Always Consistent

Just because the name on the moist clay bag or raw material stays the same, the product inside does not. Moist clay produced by ceramic suppliers can be subject to a number of variables that the potter might not be aware of until the finished piece is taken from the kiln. In some instances, ceramic suppliers will substitute a clay or raw material in the moist clay formula that alters its forming and firing characteristics.

Raw glaze materials can also change over time. In some situations, a ceramic supplier will obtain the same generic material from another supplier that can have a different particle size or trace material content. For example, whiting can be processed in several different mesh sizes, all of which look and feel like white powder. However, a coarser mesh size can cause glazes to settle faster in the bucket, while a fine mesh size results in greater surface area and causes a glossier or less opaque glaze surface. When ordering raw materials, purchase the mine or processors’ entire bag, which should include the company name and product code. Use the same information when reordering.

In addition, potters often refer to a glaze by its common name, such as Randy’s Red or Val’s Green. However, the glaze formula might have been changed several times since it was first developed. Potters frequently exchange one feldspar for another or drop or add other materials while still keeping the same glaze name. Such adjustments can often lead to disappointing results when the glaze is used by others.

Raw Materials Change in Storage

While some raw materials used in clay and glazes can change from alterations of natural deposits or variations in processing, raw materials in the studio do not change. It is possible to use a feldspar or clay that has been stored for years. However, soluble materials such as borax, Gerstley borate, soda ash, colemanite and potassium carbonate (pearl ash) should be kept in waterproof containers, as they are hygroscopic and take on and retain moisture in storage.

All Feldspars are Alike

Feldspars used by potters fall into three basic groups: potassium-, sodium- and lithium-based. Each group has distinct characteristics when used in a clay body or glaze formula. It is always best to substitute feldspars from within the same group.

Earthenware Pottery is Food Safe

While pottery fired at c/06 (1828ºF) to c/04 (1945ºF) can use food-safe commercial glazes, the underlying clay body is often porous, trapping organic material and creating the ideal surface for bacterial growth.

Mold Growth in Moist Clay is a Problem

Under certain conditions of humidity and warmth, mold can grow on the surface of moist clay. A common misconception is that mold will harm the clay in the forming process. Actually, most types of mold growth increase the plastic properties of clay.

Myths, Misinformation-and Mistakes

While pottery myths can be fanciful and appealing, they often contain misinformation regarding raw materials, forming techniques and kiln firing. Oddly, some myths include accurate pieces of information but transmit an incorrect understanding of the ceramic process. If given credence, these myths can lead to wasted time and effort.

Myths lose their power to lead potters astray when accurate information is at hand. It is only through an extensive and diverse ceramic education that misinformation can be challenged and overcome.

Author's Acknowledgements

I would like to thank the following people for supplying information for this article:
  • Jim Fineman, professional potter, P.O. Box 901, Manteo, NC 27954, helped with technical issues.
  • Bone ash information supplied by Hammill & Gillespie, Inc., 154 S. Livingston Ave., Livingston, NJ 07039; (973) 994-3650.
  • Pyrometric cone information supplied by Tom McInnerny, Orton Ceramic Foundation, P.O. Box 2760, Westerville, OH 43086; (614) 895-2663.
  • Raw material information supplied by Christine Winokur, Kickwheel Pottery Supply, Inc., 6477 Peachtree Industrial Blvd., Atlanta, GA 30360; (800) 241-1895.
  • Raw material information supplied by Jon Pacini, Laguna Clay Co., 14400 Lomitas Ave., City of Industry, CA 91746; (800) 452-4862.
  • Kiln information supplied by Mike Swartout, Continental Clay Co., 1101 Stinson Blvd., N.E., Minneapolis, MN 55413; (612) 331-9332.

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