as molds or as substrates for metal spray-forming. However, with suitable post-processing, they could fulfil many other applications, such as silicon chip mounts, or even engine blocks. Theory The science is not particularly well understood. It has been known for years that silica sols (also known as colloidal silica, silicic acid, polysilicic acid) will gel when exposed to temperatures around 0 °C (32 °F). The theoretical mechanism is quite simple: Colloidal silica is produced by the polymerisation of monosilicic acid, Si(OH)4, until the chains of polysilicic acid become so long they form silica particles with hydroxylated surfaces. On freezing of the sol, the silica particles are rejected away from the solidifying interface and forced into the interstices between the ice crystals. Here, they come into contact with each other, and link via the condensation of their surface hydroxyl groups into siloxane bonds. This, happening throughout the sol, forms a gel. In a filled sol, the ceramic powder is trapped within the gel, and forms a monolith. References Further reading A. Lottermoser, Uber das Ausfrieren von Hydrosolen, Chemische Berichte, 41, 1908, 532–540 J. Laurie, Freeze Casting: a Modified Sol-Gel Process, University of Bath, UK, Ph.D Thesis, 1995 M. Statham, Economic Manufacture of Freeze-Cast Ceramic Substrate Shapes for the Spray-Forming Process, Univ. Bath, UK, Ph.D Thesis, 1998 V. Petrenko & R. Whitworth, Physics of Ice, Oxford University Press, 2002. Casting (manufacturing)