Topic: Pottery Clay

Techno File: Drying 101

A major challenge facing ceramic artists lies in mastering technical details to achieve a specific vision. Drying greenware so it flawlessly preserves its formed shape is one of those challenges.

1–5 Stages of clay drying. Dark lines surrounding each particle indicate a layer of bound water. Small, black speckles represent unbound water molecules. 1 Clay slurry particles suspended in water, freely moving. 2 Plastic clay, where some water has evaporated and there is now some friction caused by the suction seal of water. 3 Leather-hard clay, in which the particles have physical contact along with friction. Water can only evaporate at the surface, thus drying is slowed. 4 Air-dried clay. This is as dry as the clay will get in open air. 5 Bone-dry clay. This occurs in the early stages of firing after the water-smoking period. The bound water remains until the ceramic change at 1112°F (600°C). Image and information: The Potter’s Dictionary of Materials and Techniques by Frank and Janet Hamer.

Science

Cracks and warping found in dry greenware are two faces of the same problem. Part of the formed shape has shrunk more at some point in the drying process than another part. That irregular shrinkage puts stress on the piece. The clay responds either by warping to ease the stress, or where the shape does not permit that, it cracks.


Definitions

Psychrometric chart: A plot of the moisture and energy content of air versus its temperature, typically at sea-level air pressure.

Relative Humidity: The amount of moisture in air as a percent of the amount that air can contain at a given temperature and air pressure.

Tension: A condition in which forces on a material attempt to stretch it by pulling on it in opposite directions


Examples of these faults abound. An S-shaped crack in the foot of a wheel-thrown plate, and a radial crack running from the rim toward the center of the plate are just two examples. Either crack can occur if the rim of the plate dries and shrinks faster than the center of the plate does.

This non-uniform drying puts the entire circumference of the plate rim in tension. That can be called lateral tension. Since the clay at the rim has shrunk more than the clay in the center of the plate, there is also tension between the center and the rim of the plate. This is radial tension.

Brittle materials (all ceramics) are weakest in tension. The radial tension pulling on the center of the plate can cause an S-crack there. Lateral tension can cause a radial crack beginning at the rim. Which failure occurs depends on where the tension is the greatest.

All drying cracks proceed in the same way and from the same cause. Part of the ware dries and shrinks faster than another part and when the tension caused by this irregular shrinkage is greater than the strength of the body, the clay cracks.

Drying cracks are not always easy to see in greenware. Sometimes cracks show up only when the work is fired. Inspecting greenware closely using a low-power magnifying glass can help avoid firing pieces that have cracked in drying.

 

How Industry Dries (Fast and Furious)

Manufacturers in the ceramic industry face all the same technical issues studio artists deal with when drying greenware. Studying industrial drying methods, then, can be very informative.

An industrial ceramic greenware dryer is mechanically simple, but sophisticated in operation. The device itself is basically a closed room suitable to the size of the work and the rate the work moves through the factory.

The room has three important features. First, it must be capable of being sealed against unwanted air leakage, either into or out of the room. Next, it must have a source of heat and humidity. Third, there must be a way to control the humidity and temperature of the room.

In practice the room is filled with damp greenware and sealed. Then the room is heated to near, but below, the boiling point of water (212°F) and humidified to 100% relative humidity. The room is held in this condition until the center and the surfaces of the thickest parts of the greenware have reached the same temperature.

Once that equilibrium is obtained, the humidity of the room is gradually lowered and the ware begins to dry. Because even the center of the ware has been heated, moisture moves toward the surface of each piece and drying shrinkage is relatively uniform. This allows the actual drying process to proceed fairly rapidly without danger of warping or cracking the greenware.

Drying Variables

Many variables contribute to whether ware will crack between when it is formed from wet clay and when it is dry enough to fire in a kiln. The clay body, the forming process, the shape produced, the air in the studio and the drying process all contribute to success or failure when drying greenware.

The higher the percentage of plastic materials (clays) in a body, the stronger the bone-dry greenware made from that body will be. The tradeoff, however, is high-clay bodies have higher wet-to-dry shrinkage. Drying ware from such bodies can be difficult.

In general, bodies with about 50% clay (most porcelains and many low-fire bodies) dry rapidly and produce weak greenware. Grogged stoneware bodies containing 85–92% clay yield stronger greenware and require less care in drying.

Regardless of the forming process used, whenever possible the thickness of the clay should be as uniform as possible throughout the piece. Thick and thin sections on the same piece make successful drying more challenging.

It should be obvious (because all parts of the form are uniformly exposed to air) that pieces with dimensions approaching those of a sphere or cube will be easiest to dry successfully. Shapes that are either tall relative to their width or wide relative to their height may require very special care in drying.

Air movement in a studio, and the air itself, pose the greatest challenges to drying wet greenware. Covering work with porous fabric such as cheese cloth or light plastic sheeting shields drying pieces from air movement caused by doors, windows, and heating and cooling systems. Such covers also reduce the rate of drying, thus allowing moisture to migrate from thicker to thinner parts of ware. Another technique involves inverting work so rims are against a table or shelf while a foot is exposed to the air, which slows drying of the rim while speeding drying of the foot. Where that isn’t possible, rims, handles, or other parts that tend to dry too quickly are often covered with small strips of cloth, plastic, or even wax to allow those parts to dry at a rate similar to the base of the work.

Seasonal variation in drying is always a factor in art studios. Cold air contains little moisture. For example, room-temperature air can hold four to five times as much moisture as air whose temperature is at the freezing point of water.

When cold air is drawn into a building and heated, that air will be very dry. One doesn’t have to read a psychrometric chart to understand this. We sense dry air in winter as dry skin or dry hair. A heating system may warm the cold outside air to room temperature, but that air can have a much lower relative humidity than air at the same temperature in the summer.

Studio artists typically have no more control over seasonal variations in the relative humidity where they work than the weather outside their studios. Arid mountains are just that. Studios at sea level near a coast or lake or in high-rainfall climates tend to be damp. Success in drying greenware requires developing processes that accommodate the conditions encountered.

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