PPP: Mastering the Mix
Low-Tech MixingIndividual artists and small studios can simply mix their materials dry in a wheelbarrow or large flat-bottomed vessel. Make sure all materials are dry-blended thoroughly. Add enough water to thoroughly wet all of the clay particles, mix with a hoe or shovel, wedge, bag and age for several hours to several weeks, depending on the level of plasticity (workability) required. Materials can also be mixed in buckets to slip consistency. After thoroughly mixing with a mechanical drill-type mixer, you can let the clay body sit for a while (again, the aging period will depend on the desired plasticity) and then lay it out on plaster slabs until it is ready for wedging. The wet clay can also be hung in canvas or cotton bags to allow the excess water to drip out. Other variations on these methods can also be used, as long as they enable all of the clay particles to become thoroughly wetted. Although these mixing methods are effective and inexpensive, they are also very labor-intensive. They yield a small output with a large amount of physical work.
Higher-Tech MethodsAt the next level are studio-sized clay mixers and pug mills. Clay mixers are used to first blend the dry materials and then blend the water into the mix. After mixing, the clay is put through a pug mill that slices, dices, de-airs and finally recombines the mixture in a homogeneous, dense and air-free pug. Mixers and pug mills are available in various designs and capacities. Some mixing machines use bakery-style dough mixers. Several brands of mixers, such as Jiffy Mixers and Hanson plunge mixers, attach to hand-held power drills and can be useful for mixing small batches of materials with high viscosities, such as glazes, engobes and casting slips. These mixers are available in different shaft lengths with various diameters of mixing cages. Another method is to use an electric or air-powered variable-speed mixer, such as a Lightnin mixer, which combines a powerful in-line motor with a mixing shaft and prop. These devices have many mounting options, including a clamp assembly that enables the entire unit to be mounted on the edge of a stout barrel with the mixer at a certain angle to benefit the mixing action. These units are expensive but offer excellent mixing and dispersing qualities. Blades are offered in many configurations and designs, from propeller types to high-shear blades. Some clay mixers, such as those supplied by Bluebird, use horizontally mounted mixing blades. The Soldner mixer has stationary blades and a revolving concrete tub. Both systems produce a well-mixed clay body. Other mixing systems, such as the Laguna Back-Saver, use a reversing steel-paddle blending system to mix up to 300 lbs of clay in a matter of minutes. For casting bodies, a blunger is used to mix the clay and other materials with the water and deflocculant. After mixing and testing for specific gravity, the slip is screened and ready for use. For pugging operations, most studios use inline pug mills, in which the motor is coupled to the shaft using reduction gearing. The clay is fed into a hopper and passes through a number of specifically spaced and angled blades called “flytes” that compact and cut the clay. The clay then moves through screens that further reduce the size of the clay, and then passes under a vacuum chamber, where it is de-aired and further mixed and homogenized. The clay exits the machine in a smaller diameter cylinder than the pug barrel to achieve the maximum pug compression. The resulting pugged clay is well mixed, blended and devoid of air. Studio machines such as the Venco and Bluebird are offered in various capacities of clay mixed per hour, as well as various motor sizes and other configurations, and their inline design produces excellent clay. A mechanical device keeps the small vacuum slot open into the barrel of the pug mill, ensuring that these machines de-air the clay quite well. However, the vacuum chamber that sits on top of the barrel needs to be cleaned out periodically as the machine is used. Another type of pugging machine, supplied by Peter Pugger, combines the features of a pug mill and a mixer in one unit. It can mix a batch, deair it and pug it out in a ready-to-use condition. When selecting a pug mill or a clay mixer, budget, space and safety considerations are important, as well as the power necessary to operate the machines. These machines relieve the tedium of hand processing and help eliminate repetitive motion injuries caused by excessive wedging. While they can be expensive, the time savings and health benefits they provide clearly outweigh the costs.
Sophisticated MixingCompanies that produce high volumes of clay, such as clay suppliers and large-volume production potters, often use custom-designed mixing systems. One such system uses large blungers that mix the dry materials with water into a slip-like consistency. After mixing, the liquid is sent through a vibratory sieve that screens off the offending organic and carbonaceous materials that create loss on ignition during firing, and lets the clean materials pass through. The resulting clay mixture is stored and aged in larger tanks that use slowly revolving paddles to keep the mixture in suspension. After aging, the clay is pumped under high pressure into a filter press that de-waters the clay into cakes with the desired moisture content. These cakes are then fed into a large horizontal pre-pug to be mixed. A gate on the bottom of the pre-pug is opened, and the mixture slowly falls into the hopper of an over-under pug mill. These machines move the clay through a series of blades or augers to shred it into small, pellet-sized amounts before it enters a large vacuum chamber. After de-airing, the clay falls into a second set of blades or augers, where it is finally recompressed and extruded. Another type of sophisticated mixing system uses a horizontal pre-pug to dry blend the materials. Water is added, and the mixture is blended thoroughly. A gate opens on the bottom of the machine, and the mixture drops into an over-under pug mill with a high-horsepower motor that operates two shafts running in separate barrels on the machine. The first shaft and barrel uses augers or flytes to slice and dice the clay, and then forces the clay through a series of shredding plates. The shredded clay drops into a large de-airing chamber, which uses a heavy-duty vacuum pump to de-air the small bits of clay. At the bottom of the vacuum chamber is another shaft that uses augers or blades to further compact the clay and force it out through a smaller diameter nozzle. The resulting mix is highly de-aired and homogenous. These systems are quite costly and take up considerable space. However, the cost is typically justified for potters with large production runs since the clay can be made in any consistency—from soft for hand throwing or jiggering, to stiff for hydraulic pressing.
Choosing the Right MethodEach mixing system has its own benefits and drawbacks. Studio-sized equipment is usually affordable for individual artists and small studio potters, while the larger systems might not be. For some potters, buying pre-mixed clay might be a more cost-effective alternative than investing in the equipment necessary to mix their own. Regardless of the mixing method you choose, it is important that proper research, testing and analysis be done on the clay materials first. With the right ingredients and thorough mixing, the resulting clay body should perform well and produce wares of consistent and uniform quality.
Editor’s note: This article is not intended to endorse any specific products or suppliers. For a list of mixing equipment suppliers, please visit our online 2002-2003 Data Book & Buyers' Guide at http://www.ceramicindustry.com/FILES/HTML/BuyersGuide/0,2784,,00.html. An overview of how to select and blend clays to obtain the perfect clay body, written by Jonathan Kaplan, will appear in the March 2003 edition of Pottery Production Practices.
SIDEBAR: Pre-Mixed for QualityIf you prefer to buy pre-mixed clay rather than mixing your own, rest assured that most clay suppliers use an extremely sophisticated mixing system to ensure that the clay you receive is of the highest possible quality. For example, Mile Hi Ceramics, located in Denver, Colo., uses a very complex system for batching, mixing, and pugging its 27 stock bodies and more than 40 custom blends. The company stocks over 800 tons of dry clays and materials in its warehouse, and these dry materials are batched into mixers that hold 10,000 lbs of dry materials. The mixing system features two sides—one for dark clays and the other for porcelains and white bodies—and the clay is moved into the correct side of the mixer by a series of elevator buckets. Coarser materials are screened to remove impurities before being transported to the mixer.
Some of the mixed dry material is taken off and automatically bagged into 50 lb bags, while the rest is directed through a series of enclosed chutes to smaller “pre-pugs” that hold 1500 lbs of dry material. A precise amount of water, controlled by a computer, is also fed into the pre-pug. These machines mix the dry materials and water together by moving the clay from left to right in the trough.
After the materials are thoroughly blended, a gate is opened in the bottom of the trough, allowing the clay to drop into an SE-10 over under extruder/pugmill.* The extruder/pugmill is on a fixed track and can move left or right under each pre-pug, depending on the body color being mixed. The clay is then pugged and de-aired, and the resulting solid square extrusions are cut into 25-in. pieces and boxed, ready to be shipped to the customer.
For more information about Mile Hi Ceramics, contact the company at 77 Lipan St., Denver, CO 80223; (303) 825-4570; fax (303) 825-6278; e-mail email@example.com; or visit http://www.milehiceramics.com.
*The SE-10 is supplied by Western Claymachinery Sales Inc., Los Angeles, Calif.