The Black Art of Drying Ceramics Without Cracks
Section: Clay Bodies, Subsection: General
Description
Anything can be dried if it is done slowly and evenly enough. To dry faster optimize the body recipe, ware cross section, drying process and develop a good test to rate drying performance.
Article
I admit it again. I blissfully ignore countless warnings and suffer endless problems. Eventually, however, I get the point. When it comes to drying ceramics, my learning has been true to form. I have an uncanny capacity to break the rules and get away with it, then act surprised when problems strike! I heard an appropriate quotation that applies well: “I have failed before. And I can do it again!”. Anyway, I have compiled a few points about drying. They might seem simple, but that doesn’t seem to stop a lot of others from tripping over the snags the way I have.
Ware shrinks when it dries. In most cases cracks happen when the shrinkage occurs unevenly enough within a piece to overcome its inherent strength to resist. So, it is not the speed of drying, but the unevenness of drying that results in ware either cracking or harboring residual stresses. The influence of evenness of drying is well demonstrated by considering that one studio or plant may dry ware in a uniform manner without failure in half the time another is doing it with high losses. If you are a small production company or potter you may think that you can dry functional ware, for example, out in the open if you turn and rotate it often enough. Unless you live in a damp swamp and use a non plastic clay, I seriously doubt you will get away with this for long. Unless you have a robot you simply cannot turn it often enough to keep the drying even. For complicated shapes certain sections will dry before others no matter how you orient it. For smaller production, why not simply cover the ware well in cloth (use a fabric that drapes well and does not let air blow through it).
How can you get more uniform drying? Consider these suggestions.
Isolate the stage in the process where variations in water content within a piece (drying gradients) are first introduced (i.e. an early unshielded draft, ware left out too long). Plastic clay easily absorbs stresses created by a 3% spread in water content between the driest and wettest sections of a piece. Once introduced, however, this spread tends to remain throughout the rest of the drying process. In later stages, it can put great stress on the stiffening piece, inviting weaker sections to seek relief.
The “nirvana” of drying is a humidity controlled batch or continuous drying chamber. Surface water is removed from ware by a hurricane of damp air in an atmosphere of slowly declining humidity. Unlike a gentle breeze of dry air that only reaches more exposed sections and dries them first, a brisk humid draft is more likely to reach all of the surfaces and sides (including the recessed nooks and crannies). It removes the same amount of water but does it evenly. If you lack such a facility, a special plastic-enclosed area of the studio or plant (a humidity chamber) where racks of ware can be stored is excellent (humidity is a natural consequence of enclosing the wet ware). For small scale operations drying ware slower under cloth and plastic is another option. In the final analysis, any ceramic item made using reasonable construction techniques can be dried given the necessary time to make the process even enough.
Give consideration to each type of piece being dried, think of its special drying needs. Potters and sculptors, for example, must do this all the time. In complex or unusual shapes, slow drying sections can be accelerated by a different orientation (e.g. turning bowls over as soon as possible), placement (e.g. putting mugs in circles with handles at the center), or air flow adjustment or shielding (e.g. slowing fast drying portions by special covering or application of wax resist or latex which is burned off in firing).
Ware needs adequate air flow, temperature, and time to give up remaining pore water that is not released at room temperature (or water introduced during raw glazing). In the absence of a +100°C drying chamber, your kiln is the final stage batch drier. If you are using an electric kiln, for example, that lacks air flow to carry away surface water, then consider that a kiln venting system is not only an investment in safety, but in proper drying. Vent or no vent, it is vital to provide a slow heat up around boiling point to give opportunity for all pore water to escape before firing proceeds. If not, ware will either explode or fail at sites of residual drying stress. If necessary leave the kiln on low overnight before beginning the firing in the morning.
Highly grogged clays of low plasticity have superior drying properties not only because of the obvious lower drying shrinkage, but because permeability is much improved and water is more easily able to channel to the surface without obstruction. Efforts to even out the drying process are therefore much more fruitful with lower plasticity bodies. Grogged clays not only vent water better by providing a network of interconnecting pores, but the grog particles act to terminate cracks in their early microscopic stage.
Drying can be a batch process or a continuous one. If you need fast production drying and you must do it as a batch process then you need a chamber with equipment that can control the temperature and humidity on the inside so that the ware goes through a cycle starting with high humidity cool air and ending with high temperature dry air. High velocity flow is needed throughout the cycle so that all surfaces are visited by moving air. The airflow should be constant with the humidity and temperature changing as the cycle proceeds. Humidity control can be as simple as controlling how much outside dry air is allowed in and damp re-circulating air is allowed out. If you are content with slower drying, then ware will eventually dry enough in a simple humidity chamber so that all shrinkage has occurred (final drying will need to be in a slowly fired kiln or out in the open).
For a continuous process, a tunnel accessible at both ends is best, with carts that push in one end and come out the other. The full heat of the furnace should blow near the exit of the tunnel where outside dry air is also introduced. The flow of air should proceed toward the entrance where the lowest heat and maximum humidity are present. Vents along the tunnel can be used to introduce outside fresh air or expel humid air, as needed (to fine-tune the process).
Drying difficulty increases exponentially with ware size and weight. So do not underestimate the escalation of problems in drying ware that is larger than what has been made in the past. For example, while 10 inch plates may have presented few problems for years, going to the production of 12 inch ones could multiply drying loses tenfold. Some shapes of widely varying thickness and angular contours are particularly difficult. Large plates, especially porcelain ones, can be a real challenge. Many companies are very mindful of the influence of shape and contour on the incidence of drying cracks and they take this into account when tooling up for production.
Don’t underestimate the effects of weather changes on drying patterns. Long cold damp periods that culminate in a dry spell can have a profound effect on drying, especially if the conditions have encouraged you or your workers to get sloppy. Cold winter months also often mean low humidity. Adjust your methods or make the drying environment as independent of these changes as possible, or adapt.
Clay is composed of microscopic flat crystals. When they are randomly oriented in the plastic state, shrinkage is the same in all directions and no stress occurs during uniform drying. However, when they align, shrinkage is less along their length than width and cracking could thus occur even if drying is even. Particle orientation across the bottoms of wide thrown pieces, for example, tends to be more random than up the walls. So if you are hand throwing such ware, compress the bottom to try to line them up parallel to the surface to reduce stress during drying. Consider also how your forming equipment might be aligning particles in an uneven way.
It is possible that the cause of cracking can be traced to a body that is too sensitive to material changes, or a drastic material change has affected a normally stable body. You may need to respond to such changes with a recipe adjustment. Never view the clay recipe as somehow sacred, shying away from adjustments to simple characteristics like shrinkage, permeability, and plasticity to maintain properties important for drying. On the other hand, don’t be too quick to blame the body either. If a sanitary ware manufacturer can dry a 50 pound porcelain toilet and you are having trouble with a 2 pound stoneware plate, there is a good chance your process could be adjusted to solve the problem. If your body contains bentonite, for example, you could safely reduce it by 1 or 2 percent without having any impact on fired properties. This will increase permeability (so water can escape faster) and it will reduce shrinkage. Another approach would be to exchange some of the ball clay for a less plastic type of ball clay (or kaolin for a less plastic kaolin). This will have an effect similar to reducing the bentonite. A more extreme change (but still potentially safe) would be to replace some of the ball clay for normally much less plastic kaolin. Be aware of course, that a body of lower plasticity won't form quite as well, however you might find it easier to adapt to a plasticity loss than deal with drying cracks. Watch for any changes in body maturity and adjust with a slight drop or increase in feldspar if needed. One more point: don't trust other peoples opinions about which kaolins and ball clays are more and less plastic, do drying shrinkage tests yourself to confirm.
A very important aspect of optimizing your drying is having an effective way to test the drying performance of a clay. What is needed is a test that produces real numbers you can write down; a test that is sensitive to changes in a clay body’s drying characteristics. Some labs have complex instruments and generate tons of numbers and reports but if you ask them to talk about a clay materials drying performance they are sometimes at a loss for words. Knowing particle surface area, particle size, shrinkage, etc. are all fine, but drying performance is a complex dynamic that also involves particle shape, identity and the nature of particle-particle and particle-water interactions. We need a simple test that displays before us drying performance. Digitalfire FORESIGHT software ships with a predefined test named DFAC (or Drying Factor) that enables you to generate real numbers that represent this property.
Be Realistic When Using Porcelain
Do not expect a porcelain to dry as well as a stoneware or earthenware. Do not expect to be able to make any kind of shape and size using a porcelainous body, you need to be realistic. If ware has drastically differing thicknesses along its cross section and has angular contours then it will not be possible to make using certain bodies. At a minimum you need to be willing to do things differently than you have done. While industry cannot afford to have time or labor consuming drying scenarios, individual potters can nurse finicky clay bodies through the drying. For example, porcelains can lose water very quickly (and therefore shrink a lot) during early minutes and hours of drying. It might be possible to successfully dry otherwise troublesome pieces by covering them almost immediately after forming and leaving them as much as a few days to slowly descend through initial drying. Actually, in industry humidity drying chambers can be set to any needed schedule so this same approach might be needed for some circumstances.
Establishing a Drying Factor for a Clay BodyThe DFAC TestThe DFAC test is valuable because it does not require any specialized equipment (other than a drying cabinet that can maintain 80-100°C). Care is required to prepare the disk from well wedged clay using the same movements and water content each time and not handling or bending the disk more than absolutely necessary (the relevance of this test depends heavily on your ability to maintain a consistent method of disk preparation). Don’t live in the past, control the drying process now. It is true, maybe it did work before. We must respond to ‘now’ and be open to the possibility that past success was luck, a position on the edge of a precipice from which you have now fallen. Thus a 5% cracking loss today on ware that all receives the same drying treatment could mean 50% tomorrow (depending on how precarious your process is). Take cracked ware seriously, even a few pieces can betray a flawed process and it could be just luck that has spared the rest. Pretest your clay so you know exactly where you stand. The drying process is not really a black art. Just work toward the goal of being able to pick up a piece of ware at any stage of drying with confidence that water content tests taken anywhere on the piece will yield similar results. Yes, it is fine to keep making all those mistakes and learning what does not work. But at the same time, don’t forget the things that do work! |
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Soluble SaltsFORESIGHT defines a related test it abbreviates ‘SOLU’. This test deals with the measurement of the incidence of soluble salts in clay bodies and materials. This test employs the specimens made for the DFAC test. You can find information elsewhere here on the theory of what soluble salts are (see article on use of barium carbonate in clay bodies), their potentially detrimental effects and how this test can be performed with simple equipment. Soluble salt surface depositing (or efflorescence) is a phenomenon not understood my many ceramists but one that will one day make itself known to you if it has not already done so. It is something that can affect any type of clay body, from porcelain to brick clay (the brick industry knows all about it!). |
The photo shown here displays some ball clay drying disk fragments. Notice the dramatic effect on surface coloration these salts can have. As noted, barium carbonate is commonly used to precipitate salts so they do not migrate to the surface with the water during drying. •
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Authors
- Tony Hansen (Owner)
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