The ceramics sequence
Industrial ceramics cover a wide range: floor and wall tiles, sanitary ware, bricks and roof tiles, tableware, electrical insulators, refractory bricks and technical ceramics. The products differ, but most follow the same chain: prepare a mixture, shape it, dry it, fire it and finish it. Water is removed in two stages, and heat does the final transformation, which is why drying and kilns receive so much attention.
Preparing and mixing the body
A ceramic body is a blend of clays and other minerals such as feldspar, silica and sometimes recycled material. Raw materials are crushed, ground and stored. Mills grind hard materials to the needed fineness, a typical case of size reduction using ball mills. Mixing then takes one of several routes depending on how the product will be shaped:
- Slip (slurry) preparation. Materials are blended with water in large tanks with agitators, often with a small amount of additive to keep the slurry fluid. The slip is screened, passed over magnetic separators to remove iron particles that could cause specks, and stored under gentle stirring.
- Spray drying. Used for tile making. Slip is pumped through nozzles into a hot-air tower, where droplets dry into free-flowing granules suitable for pressing.
- Plastic mixing. Clay and water are blended in pug mills, which mix and push the material through a vacuum chamber to remove trapped air, producing a uniform extruded column.
- Dry mixing. Used for some technical ceramics.
Control of moisture and particle size is crucial. Differences in the body show up later as warping, cracks or color variation.
Shaping
The shaping method matches the product:
- Dry pressing. Granules are pressed in a die by hydraulic presses, common for tiles and many technical parts.
- Extrusion. Plastic clay is forced through a die and cut, the method for bricks and pipes.
- Slip casting. Slip is poured into absorbent molds, usually made of plaster, which draw out water and leave a solid layer. Common for sanitary ware and some tableware. Pressure casting speeds this up with pressurized molds.
- Jiggering and throwing. Rotating molds and shaping tools used for plates and cups, along with hand-throwing in craft production.
- Isostatic pressing and injection molding. Used for technical ceramics.
Presses rely on hydraulic power, with pumps, cylinders and valves in common with other hydraulic systems. Press tooling wears and must be maintained for dimensional stability.
Drying
Shaped ware, called green ware, still contains water. If it is fired at once, the water turns to steam and can crack or burst the product. Drying has to remove water slowly and evenly, because the product shrinks as water leaves and uneven shrinkage causes cracks and warping.
Typical dryers include:
- Chamber dryers, in which carts or racks are loaded into a room where temperature and humidity are programmed.
- Tunnel dryers, in which ware moves continuously through zones of controlled air.
- Roller or vertical dryers, common in tile production.
Dryers use warm air from fans and burners, and waste heat from kilns is often reused for this purpose. This is the same family of equipment as industrial dryers used in other industries, with the added need to protect fragile ware from abrupt temperature changes.
Kilns and firing
Firing converts the shaped, dried body into a hard, durable ceramic. At high temperatures, which for common ceramics are generally in the range of roughly 900 to 1,300 degrees Celsius and higher for some technical or refractory products, the materials undergo chemical changes and partially melt together (a process called sintering or vitrification). Fired ceramics shrink further, so the firing schedule matters.
Common kiln types:
- Tunnel kilns. A long tunnel through which kiln cars carrying ware move slowly, passing through preheating, firing and cooling zones. They run continuously and efficiently, and are widely used for bricks, sanitary ware and tableware.
- Roller kilns. Ware sits on or between ceramic rollers and moves through a long kiln. Tile plants often use these for fast firing.
- Shuttle kilns. A batch kiln with a movable car that slides in and out. Flexible for varying product ranges.
- Intermittent (periodic) kilns. A chamber is loaded, fired, cooled and unloaded, for small runs, large pieces or special products. They are less energy-efficient but versatile.
- Electric kilns. Heated by resistance elements, used for small production, laboratories and technical ceramics. Many other kilns use gas burners.
A kiln is lined with insulating bricks and refractory materials. Good insulation reduces fuel use, and a poor lining loses heat and creates uneven temperatures. Related concepts appear in the discussion of refractory linings and in the wider topic of process heating.
Temperature control
Ceramics respond to temperature history, not just to peak temperature. The firing schedule defines heating rates, hold times and cooling rates. Some stages need slow heating, for example when remaining moisture or organic material leaves the body, and cooling must avoid thermal shock, especially around temperatures at which some minerals change their crystal structure. A programmed cooling rate that is too rapid can crack otherwise good ware.
Control systems usually combine thermocouples placed in the kiln, controllers that adjust burners or heating elements, and recorders for quality records. Thermocouples drift with age, so regular checks and replacement are part of good practice, a point that connects to sensor calibration drift and regular calibration. Atmosphere also matters: oxidizing or reducing conditions affect colors and chemistry, which is why burner combustion and air supply control receive attention. Combustion systems must have flame supervision and safety interlocks, installed and maintained by qualified personnel according to codes.
Glazing, finishing and handling
Many products are glazed, meaning coated with a glassy layer applied by spraying, dipping or waterfall application and then fired. Others are decorated, ground, polished or sorted by automatic inspection. Conveyors, robots and kiln-car handling systems move the ware between stages, with the aim of avoiding damage to fragile pieces.
Safety and upkeep
Kilns are hot, and ceramic dust can be harmful if breathed in over long periods, so dust extraction and respiratory protection programs are common. Kiln car mechanisms, burners and gas trains need regular inspection. Work on kilns and dryers should use lockout tagout and follow manufacturer instructions, with trained personnel only.
This is a general overview and not engineering advice. Temperatures and schedules vary widely by product and plant.