Also, impregnation of most that are not volatile to water contact[clarification needed] can be achieved by mixing the metal into a Thixotropic slurry, coating and squeezing out the excess to achieve the desired density. Metal foams are commonly made by injecting a gas or mixing a foaming agent into molten metal. [8] Under certain circumstances metallic melts can be foamed by creating gas bubbles in the liquid. Normally, gas bubbles formed in a metallic melt tend to quickly rise to its surface due to the high buoyancy forces in the high-density liquid. This rise can be hampered by increasing the viscosity of the molten metal, either by adding fine ceramic powders or alloying elements to form stabilizing particles in the melt or by other means. Metallic melts can be foamed in one of three ways:
by injecting gas into the liquid metal from an external source;
by causing an in-situ gas formation in the liquid by admixing gas-releasing blowing agents to the molten metal;
by causing the precipitation of gas which was previously dissolved in the liquid.
In order to stabilize the molten metal bubbles, high temperature foaming agents (nano- or micrometer- sized solid particles) are required. The size of the pores, or cells, is usually 1 to 8 mm. When foaming or blowing agents are used, they are frequently mixed to the metal in the solid state at a powder form. This is the so-called "powder route" of foaming and it is probably the most established (from an industrial standpoint). After metal (e.g. aluminium) powders and foaming agent (e.g.TiH2) have been mixed, they are compressed into a compact, solid precursor, which can be available in the form of a billet, a sheet or a wire. Production of precursors can be done by a combination of materials forming processes, such as powder pressing,[9] extrusion (direct[10] or conform[11]), flat rolling.[12]