Volcanic Glass D.B. Heat-foaming

Heat-foaming of volcanic glass particles in the pyroclastic deposit

Fig. Microphotographs on heating stage (Particle size: 0.2mm, Heating rate: 1000°C/min).

weathered surface layer in which Na⁺ is exchanged by H⁺

crystallite or impurity groove-like crack
1) 25°C 2) 500℃ 3) 900°C 4) 950°C 5)1000°C 6) end of foaming

Fig. Schematic model of foaming mechanism by heating.

1) Natural particle is heated.
2) At about 500°C, some cracks appear on the surface by shrinkage of the surface layer according to volatilization of H2O molecules formed from H+ ions.
3) At about 900°C, the inner part is softened earlier than the surface layer, because of volatilization of H+ ions from the surface layer and the gradual increase of Na+ ions density from surface to inside. At the same time, bubble generates from the nucleus that is composed of an alien substance such as crystallites.
4) At about 950°C, bubble expands to the direction of fin-like edges, because the surface layer is harder than the inner part.
5) At about 1000°C, this hard layer becomes surface. The shape is formed spherical by the hard layer.
6) At the end of foaming, small groove-like patterns are formed on the surface by expansion of the sphere.

1)	Natural particle is heated.
2)	At about 500°C, some cracks appear on the surface by shrinkage of the surface layer according to volatilization of H2O molecules formed from H+ ions.
3)	At about 900°C, the inner part is softened earlier than the surface layer, because of volatilization of H+ ions from the surface layer and the gradual increase of Na+ ions density from surface to inside. At the same time, bubble generates from the nucleus that is composed of an alien substance such as crystallites.
4)	At about 950°C, bubble expands to the direction of fin-like edges, because the surface layer is harder than the inner part.
5)	At about 1000°C, this hard layer becomes surface. The shape is formed spherical by the hard layer.
6)	At the end of foaming, small groove-like patterns are formed on the surface by expansion of the sphere.