Freeze-gelation, is a form of sol-gel processing of ceramics that enables a ceramic object to be fabricated in complex shapes, without the need for high-temperature sintering. The process is similar to freeze-casting. The process is simple, but the science is, as of 2005, not well understood. The most common process involves the mixing of a silica solution with a filler powder. For example, if we were making a component out of alumina, aluminium oxide, then we would still use a silica sol, but alumina filler powder. The relative amounts used differ, normally between 3 and 4 times more filler than sol is added by weight. A wetting agent is added, such that the filler powder disperses properly in the sol, which is mostly water. This makes the mixture doughy and stiff. The mixture is, however, highly thixotropic, so that when vibrated it turns liquid. The stiff dough is placed in a mold and the mold vibrated to liquefy the mixture, filling the mold and releasing any trapped air. The filled mold is then frozen. On freezing, silica precipitates from the sol, forming a gel. This gel holds the filler powder together in something approximating a sintering greenform. The component is then dried in a furnace, leaving the component. The advantages of freeze-geleation over sintering are essentially cost-based. It doesn't require high pressure equipment or powerful furnaces (drying temperatures are only just above water's boiling point), yet it creates a useful product which takes the shape of the mold very accurately. History In terms of being simply a process by which powder can be made into a monolith, freeze casting could be as old as the earth. A material called laminar opaline silica or LOS is believed to be formed by the freeze casting of volcanic ash, some soils containing the required sols to make the gel. Artificially it is also an old process, having been known and studied for 100 years or more, but never brought to significant industrial application. Lottermoser, a German, wrote a paper on 'das Ausfrieren von Hydrosolen' (the Freezing of Hydrosols) in 1908. Through the 20th century various people have patented techniques using freeze-gelation, most being centred on the use of ceramics as refractory materials. A furnace lining brick, or an investment casting mold, can be easily fabricated using this method. Recently there was a flurry of interest in freeze-casting at the University of Bath, UK,