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PPP:Drying: The Hidden Step
By Cameron Harman Jr., charman@kilnman.com

DRYING FEATURE, CI MARCH 1999 By Cameron Harman Jr., Ceramic Services Inc., Bensalem, Pa. Ask most potters what the important steps are in making their ware, and they will probably say something like "Throwing, glazing and firing." The exact words and order may vary slightly, but almost no one will mention drying as an important step. The reason is that many ceramic products dry so easily that the drying step is rarely given a second thought. Put simply, the purpose of drying is to remove water from the product. However, the drying process must also remove that water safely, without warping or cracking the ware. The problem with drying defects is that they are often mistaken for firing problems, handling problems, or even "bad clay." In almost every case, however, proper drying will enable the potter to use just about any mix and produce any shape without problems. What is Drying? It is often said in texts that there are three stages of drying: 1) loosely held water, 2) mechanically held water, and 3) chemically held water. The loosely held water is excess water that is on the surface and in the pores. For example, all the water in a water-sand mix is loosely held water. The water comes away easily because the sand particles do not absorb any of the water. In clay ware, the clay becomes more compact as loose water leaves the pores; in other words, the clay exhibits drying shrinkage. When the water leaves, the size of the ware changes proportionally with the amount of water taken from the pores. It is easy to see what would happen if one side were to dry more than another. The side drying faster would change size more than the other and cause either warping or cracking. After the loosely held water is gone, the mechanically held water must be removed. Once this stage has been reached, it is possible to change the different variables to speed up the drying process because there is no more shrinkage involved to cause cracks or warps. At this point, the air speed can be increased, the temperature can be raised or the relative humidity can be lowered. In the final stage, the chemically held water, which is even more tightly bonded, must be moved through the very small space of the pores to get to the surface. Put simply, drying is the process of water being driven to the lower moisture content of the surface, where it evaporates into the surrounding air. Controlling the Drying Rate The rate of drying is controlled by three things: 1) the air velocity and direction, 2) the air temperature, and 3) the relative humidity of the air. It is possible to achieve the same amount of water removal from the surface of the ware by changing any one of these variables and holding the other two. For instance, the travel of water in the ware's interior is affected by the water's viscosity. If the water is "thinner" (lower viscosity), it will move from the center of the ware more easily. Since heat decreases viscosity, a hotter ware (achieved with higher air temperature) tends to dry a little faster. Ware with large pores, however, will dry easily enough without increasing the temperature. Non-clay ware will sometimes dry more quickly because the water is not chemically or electrically held to the particles as it is in clay ware. Hence, better drying is not necessarily achieved merely by adjusting one of the variables. The real technology of drying is to find ways to move the water from the interior of the ware to the surface at the same rate of speed as it is evaporated from the surface. Since water leaves the surface quite easily, this is the rate of water removal that must be adjusted. By controlling the relative humidity of the air, and by keeping the air velocity at a reasonable level, it is possible to slow down the water removal from the surface to the same rate as the water traveling to the surface from the interior. Many of the early studies showed that the movement of water from the ware's center to the surface was constant. However, much of this research did not take into account the size changes in the ware brought about by the drying shrinkage. Drying Defects The secret to success in making ware is understanding that drying is one of the most important steps. Done properly, it can increase the quality of the ware; done improperly, it can lead to numerous problems and defects. Drying defects show up in some of the following ways: Cracking and/or warping from uneven drying Warping due to the release of stresses acquired during improper drying Cracking during firing due to drying stresses induced by improper drying The appearance of cracking during firing, in what is actually the opening of a previously undetected crack The explosion of a part during firing from moisture still inside due to incomplete drying Cracking during firing from the expansion of retained water due to incomplete drying Incomplete Drying. Time alone is not a sufficient indicator of drying. Surprisingly, even if a ceramic is left on the shelf for months or years, there is no guarantee that it is dry. As the water evaporates from the ware's surface, it consumes energy (the latent heat of vaporization). This energy consumption causes the temperature at the surface of the ware to drop. That is why a wet or drying ceramic part will be cool to the touch. Just because a piece of ware ceases to be cool does not mean that the piece is dry; it only means that drying is taking place very slowly or that the moisture in the center of the ware has not yet reached the surface. If the room is subject to normal atmosphere (as opposed to a controlled atmosphere), the part can gain enough hydroscopic moisture from the air to cause drying defects. If the atmosphere in the room is cool and moist, the center of large pieces might not dry at all. Even in a room that seems normal it may take weeks for large parts to dry. The only way to be certain that a piece is completely dry is to weigh it. If it has reached its minimum weight, it is dry; if not, it is still wet. Some ceramics contain fine-grained clay or other fine materials, such as bentonite, which inhibit the release of water. Slow-releasing bodies will dry on the surface and remain wet inside, causing the outside to shrink more than the inside. This results in cracking almost every time unless the drying on the surface can be slowed enough for the water to get out of the inside of the body. Uneven Drying. Another major cause of drying defects is uneven drying. When one side of a flat piece dries faster than the other, the piece will warp. When a more solid piece, such as a bowl, is subjected to uneven drying, the piece will often crack to relieve the stress of one side shrinking more than another. When one side of a part is placed on a solid surface and the rest is exposed to air, the drying will be uneven since the solid surface cannot take up moisture. Even if the solid surface is absorbent, the piece is still subject to uneven drying since the evaporation rate will be different on the different sides. Some people place flat ware between pieces of plaster to dry. However, since the drying process is uncontrolled, the water might leave the surface more quickly than it can be replaced from the center of the ware. This causes the surface to shrink while the center of the ware does not, often causing microcracks that open during firing. This is often the problem with a part that looks good after drying but cracks during firing-it was already cracked when it was put into the kiln. Uneven air movement around the part in a dryer or drying chamber is another cause of uneven drying. It is common for parts sitting on a shelf in a drying room to dry on the outside first. Sometimes this problem occurs when newly made part sits in a room or on a rack waiting to be moved to a dryer. In a warm, dry season, the outside of the part can dry more quickly than the moisture can be transported to the surface, creating another common form of uneven drying. Induced Stresses. Some drying techniques cause an induced stress that shows up as cracks or warping during firing. The practice of placing flat ceramic pieces between pieces of plaster can place stresses on the ware due to the weight of the plaster and the uncontrolled water removal. The pieces look flat after drying, but the stresses are relieved during firing, and the parts come out cracked or warped. Sometimes the body mix can tolerate the stresses and cracks or warps don't occur. Other times the pieces don't crack until after they are glazed. In every case, however, a controlled drying method can eliminate induced stresses and other drying problems. Successful Drying People dry their ware in a variety of ways, but there is only one successful method to dry any ceramic: The parts must be placed into a controlled environment immediately after manufacture, and they must not be exposed to the open air while waiting to be transported. The controlled environment must move air in a uniform manner over all parts of the ware at the same time. The environment must have a programmed temperature and relative humidity profile. The parts should not be removed until they can be placed directly into the kiln. Often people will ignore these rules because for one reason or another they have previously been able to get away with doing so. However, it is very common to hear complaints that someone's ware dried perfectly for a long while, then suddenly began showing defects. Usually the reason for the sudden change was a change in local climate (such as a seasonal change) or a minor change in a raw material. But the change simply revealed problems that had existed for a long time. Sometimes people are convinced that what they are doing works fine even though they lose 10% or 20% percent of their ware. A well-designed dryer, even a very simple one, will adhere to all of the above rules. Dryer Design It should be noted that a good dryer does not need a fancy exterior. Very good dryers have been made by using a group of offices as drying chambers. What matters is the way in which air is moved by the system, and that the ware is placed in such a way as to receive the most uniform flow of air. Control of the temperature, humidity and velocity of the air is also crucial. Airflow. A good dryer is designed so that the air enters the chamber along a long dimension and passes over the ware in a very short direction to the exit. The best design is a straight line across a shelf of ware. The shelf should be as small a dimension as practical. The air entering the chamber should pass through a slotted or perforated plate and leave the same way. The purpose of the perforated plate is to provide a uniform flow directed across the chamber. It also provides the pressure drop necessary to make the flow uniform across the plate. Behind the perforated plate is a plenum into which air is pumped by a blower. The perforated plate allows the air to be put in at one spot and still be delivered evenly through all of the perforations. The perforations exist in the exhaust plate for the same reason: they allow the suction side of the blower to be connected at one spot and have an even pull all over the face of the plate. Ware Placement. Ware placement can contribute greatly to the uniformity of drying of each part. The ware should be placed so that the air flows as evenly as possible across each piece. Unfortunately, it is not possible to place ware in a way that allows both the front and the back of a given piece to have that same airflow. Therefore, it is often necessary to use a lower volume of air or a higher relative humidity at the outset to keep the exposed face of the ware from drying faster than the hidden face. For specially shaped ware, it is often necessary to construct the dryer with unusual air patterns to accommodate the shape. Dryer Construction Most dryers can do their job at temperatures below 120(F. This is essential for drying plaster molds and for drying clay-based parts. Clay permanently loses its plasticity at temperatures above 125(F or 130(F. If the clay is intended to be recycled, it is best to keep the temperature no higher than 120(F. In some drying, much higher temperatures are required because the size of the load to be dried is so large that the air movement is negligible. An example of this is a kiln car loaded with face brick. In this case, the ware can be subject to a room temperature of around 400(F. At that temperature, the center of the brick located in the middle of the load probably won't get above 200(F, but this is high enough to aid in moving the water from the center of the ware to the surface. In most dryers with a temperature limit of 120(F, the insulation of the drying chamber is relatively simple. The chamber can be constructed of steel or even wood with an inch or two of simple insulation. The higher-temperature dryers require a more thought-out construction with much more insulation. Companies that are not producing a large volume of ware have found the use of so-called "bakery racks" quite sufficient. A simple dryer two, three or four bakery racks long can provide both a controlled atmosphere storage area before drying (immediately after making) and the drying chamber. In some cases two such dryers may be required, especially if the product quickly gains hydroscopic moisture from the air after drying. Two dryers may also be required if the drying takes longer than 24 hours so that production is not affected. The dryer can be constructed as an insulated wooded box with an internal plenum on both sides. The face of the plenums is covered with perforated metal having the correct perforations for the particular job at hand The size and spacing of the holes is calculated by the designers for the given ware specifications. The blower and heater should be mounted on top of the chamber with ductwork moving the air in and out of the box. The ductwork should have two openings-one to exhaust the wet air and the other to bring in the fresh air-with dampers that are controlled by a motor that receives its signal from the humidity control in the control box. The heater (if it is electric) is generally controlled by an SCR (silicon control rectifier) control, which, in turn, is controlled by a programmable controller connected to a thermocouple in the air line. Control System The other dryer controls are relatively simple. Once the ware placement has been determined and the air velocity set, the remaining variables are the temperature and relative humidity. A programmable temperature control enables the temperature to be gradually changed over time according to the known ability of the loosely held water to be driven off. A relative humidity sensor, controlled by another programmable controller, can be used to manipulate damper openings. The program is capable of gradual changes once the loosely held water is driven off. These controls generally work best if at least two seasonal programs are used. One setting group can be used in the winter, when the temperatures are lower, while another setting group is used during the summer. For the dryer to perform most efficiently, tests should be run on the ware in the same configuration in which the ware will be fired in actual production. These tests should, among other things, determine when the shrinkage is finished and what the maximum safe rate of drying is to get to that point. These tests are usually performed in the finished dryer, and the control settings are manipulated to achieve the best results. Since the compositions used by most potters are basically similar, the suggested initial control settings are often completely satisfactory. The difficulty comes with attempting to completely dry difficult shapes such as very tall, or very thick, or very complicated shapes within 24 hours from the time they were thrown, pressed or cast. Sometimes, good drying simply requires more time, perhaps 36 or even 48 hours. However, compared to typical non-controlled times that extend up to weeks, it is a certain improvement. The test of a good dryer is its ability to dry all products regardless of mix or shape without defects, and do so in less than two days. Optimizing Ware Quality While the mixes and materials used to make ceramics vary widely, the principles of drying remain the same. Drying is an important step and can often mean the difference between success and failure in producing ceramic pieces. Optimizing ware quality requires careful consideration of the drying method. A good ceramic dryer can fit nearly all normal situations and typically requires only a change in the final programming.