|Monthly Tech-Tip |
In ceramics, drying performance is very important to optimizing production. More plastic clays shrink more and crack more, but they are also better to work with.
Key phrases linking here: drying performance - Learn more
Refers to the ability of a clay to dry without cracking (excluding process factors). Lab results for drying factor (DFAC test) and drying shrinkage (SHAB test) along with observations of the material's performance in actual use can give a well-rounded picture of its drying performance. Of course, the expected performance is always related to the type of ware being made and the economic and convenience costs associated with drying it the necessary way to avoid drying cracks.
Clay having a lower drying shrinkage normally dries better (without cracking). However, it can also dry poorly if lacking excessively in dry strength. Likewise, clays with higher drying shrinkage normally dry more poorly. But good dry strength can help them dry better than you might expect (also because stronger clays have higher plasticity and thus dry slower). Adding an aggregate or fiber to a plastic body can greatly improve its drying performance (because of shrinkage reduction, greater permeability to water passage and micro-crack termination by the larger particles). In fact, bodies that would be very difficult to dry on their own can be superior driers if they contain lots of grog.
Drying performance tests can be done in simple ways. Typical ones normally accelerate the drying of one section of a sample while slowing down water release in another section. This sets up a situation where the rigid section resists the shrinkage of the undried section. Differences in the type of failure provide opportunities to rate one clay against another.
Drying performance is best viewed as the coupling of a clay body of known capability with a process tailored to it. When cracking occurs there are generally many measures that can be taken to improve the drying process. As an interesting example, a large studio reported a "handle failure rate" of less than 1% - by workers and students despite greatly increasing the speed of their process. This was possible because the focus was put on evenness of drying.
Half of these Plainsman Polar Ice mugs cracked. But I know exactly why it happened! After throwing them I put them on a slowly rotating wheelhead in front of a fan to stiffen them enough so I could attach the handles quickly. Of course, I forgot them and they got quite stiff on the lip (while the bottom was still wet). I quickly attached the handles and then covered them with cloth and plastic and let them sit for two days to let them even out. Notwithstanding that, that early gradient sealed their destiny. The lesson: At no time in the drying process should any part of a piece be significantly ahead of another part.
The lid of my firing kiln seems to be just the right environment for even drying, even of freshly thrown pieces. By the time this mug really got under way here the kiln was at 1000F and the lid was getting pretty hot. The bottom was the warmest and the top coolest, the exact opposite of how drying normally becomes uneven (the top drying first). This principle could be employed to make a heated drying chamber. The interior space could be kept at high humidity and a draft of air through it could remove humid air and the needed rate.
These are made from a 50:50 mix of bentonite and ball clay! The drying shrinkage is 14%, more than double that of normal pottery clay. These should have cracked into many small pieces. Yet notice that the handle joins with the walls are flawless, not even a hairline crack (admittedly the base has cracked a little). Remember that the better the mixing and wedging, the smaller the piece, the thinner the walls, the more even wall thickness, the better the joins, the fewer the sharp contour changes, the more even the water content is throughout the piece (during the entire drying cycle) and the damper the climate the more successful drying will be. What did it take to dry these in our arid climate? One month under cloth and plastic, changing the cloth every couple of days. Implementing these same principles on a normal clay body will assure drying success.
The foot ring on these leather-hard mugs has already been trimmed. At the stiff-leather-hard stage an engobe was applied to the inside. This rewets the bodies of the mugs, almost to the same point as freshly-thrown. But the handles did not get rewetted. To re-dry these mugs to the point of being able to turn them over will take 4-6 more hours. But by that time the handles will be too dry. To prevent that I wax them after trimming (leaving a just the inside handle-curves bare). That slows their drying down enough to keep them even with the body of the mug. This method works well enough that none of my mugs need covering during drying, even in our desert climate. Keeping all parts of a piece at the same water content throughout the process, that is a key to successful drying.
They need ten days to dry in our climate! This 14" plate has been thrown with a 1" thick base. The rim is a quarter of that. During the first few hours, the rim would dry quickly, leaving the base far behind. So as soon as it will support the weight of a cover-cloth I put it under plastic for several days. After that it does not detach from the plaster, it has to be cut off with a wire (there is a lot of clay here, it waterlogs the bat). The rim is stiff enough to support it for trimming but the base will still be quite soft. Thus, it is doubly important to trim it deep enough to create a cross-section of even thickness. Then, to try to even out the water content between base and rim I place it under layers of cloth and under plastic for several more days. Finally, out of the bag, it dries, with cloth still covering. Hopefully, the base will not bow upwards or crack. These are difficult, there is no getting around it!
The ideal drying chamber is a tunnel. Typical tunnels pass wheeled-ware-carts single file. Hot dry air enters where the ware exits. The moving air touches all surfaces and picks up humidity as it moves toward the entrance. The tunnel must be calibrated so that air reaching the entrance, is still very warm and laden with water it got from ware down the tunnel. When an equal volume of ware is passing constantly, manual calibration of cart movement, air volume and temperature is possible. But if flow is not constant multi-location monitoring and intervention is needed. ESP8266/ESP32 controllers are revolutionizing industrial control. As cheap as $5, these tiny battery-powered WIFI servers can display a web page, email or text you and communicate with an online dashboard to relay measurements. And they can respond to commands and actuate relays to control blowers, heaters and vents. Hiring a technician on Upwork.com to design a system is only a matter of a few thousand (even hundreds) of dollars. Shown here is an Amazon listing for a development kit of an 8266, sensor and cables (the humidity/temperature sensors are the key element, search for them specifically on Amazon). Shown also is a listing for a ready-made one, dozens of these will be found for the search "esp32 humidity temperature sensor controller". Greenhouse automation uses this same technology.
This DFAC test for drying performance compares a typical white stoneware body (left) and the same body with 10% added 50-80 mesh molochite grog. The character of the crack changes somewhat, but otherwise, there is no improvement. While the grog addition reduces drying shrinkage here by 0.5-0.75% it also cuts dry strength (as a result, the crack is jagged, not a clean line). The grog vents water to the surface better, notice the soluble salts do not concentrate as much. Notice another issue: The jagged edges of the disk, it is more difficult to cut a clean line in the plastic clay.
The heat lamp dries the out edge in minutes (this photo makes it appear hotter than it really is). The center section of the disk is protected by the glazed bowl and takes an hour or more to dry. This sets up stresses that cause the disk to crack. The nature and size of the cracks enable establishing a drying factor value for the clay.
The center portion was of this DFAC test disk was covered and so it lagged behind during drying, setting up stresses that caused the disk to crack. This test is such that most pottery clays will exhibit a crack. The severity of the crack becomes a way to compare drying performances. Notice the test also shows soluble salts concentrating around the outer perimeter, they migrated there from the center section because it was not exposed to the air.
Drying disks used for the DFAC test are 12cm in diameter and 5mm thick (wet). A crack pattern develops in almost all common pottery clays as they shrink during drying. This happens because the center portion is covered and stays soft while the perimeter dries hard. This sets up a tug-of-war with the later-drying inner section pulling at the outer rigid perimeter and forcing a crack (starting from the center). If the clay has high plasticity and dry strength it can pull so hard from the center that cracks appear at the outer dried edge to relieve the tension. Or, it can create cracks that run parallel to the outer edge but at the boundary between the inner and outer sections. The nature, number and width of the cracks are interpreted to produce a drying factor that can be recorded.
Fibers visible along a broken edge of a dried paper clay slab
It dry shrinks much more yet cracks less. How is that possible?
Various grogs available in North America
Large particle grogs are difficult to produce
Grog does not always have the intended effect
The DFAC Drying Factor test visually displays a plastic clay's response to very uneven drying. The test is extreme enough that almost all plastic clays will crack.
During drying, clay particles draw together and shrinkage occurs. During firing the matrix densifies and shrinkage continues. More vitreous bodies shrink more.
Drying Ceramics Without Cracks
Anything ceramic ware 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.
Clay Cracking During Drying
The best way to avoid drying cracks when making ceramics or pottery is to avoid doing the things that cause it. Do not just blame the clay, anything can technically be dried.
|By Tony Hansen|
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