The first reported method of producing a metal foam, developed in 1948, involved heating a volatile metal with the metal to be foamed (the example given is a mercury-aluminium alloy). The process is shown schematically in Figure 1. During heating, the two metals are contained within a pressure vessel (1), and heated to a temperature above the vaporisation temperature of the more volatile component (2). The mercury is prevented from fully vaporising by the pressure within the vessel. Heating continues to the melting temperature of the metal to be foamed, when an aluminium melt is formed which is supersaturated with mercury gas (3). The entire molten mass is subsequently removed from the pressure vessel, and the mercury vaporises fully and expands within the molten metal, to produce a foam (4) which is allowed to cool and solidify.

Figure 1: Production of foam by the addition of a volatile phase (in this example mercury) to the metal to be foamed, melting the metal under pressure, and suddenly lowering the pressure to allow foaming.

Few details are known about this method, and it was not apparently a commercially success. The process involves rapid expansion of melts at high pressure and temperature, and all of the suggested volatile metals (mercury, magnesium, zinc and cadmium) are potentially hazardous. The foam structures consist of unsupported liquid metal, and are likely to be unstable and prone to collapse during cooling. There is very little scope for optimisation of the alloys used, and the speed of the foaming process means a uniform or reproducible cell structure is highly unlikely.