2.1 What is Pumice?

Pumice is a very porous form of vitrified volcanic rock, usually of very light colon Its true density, i.e. the density of the powdered material, amounts to between 2 and 3 kg/ dm³ and its bulk density, i.e. the density of the loosely piled material, amounts to between 0.3 and 0.8 kg/dm³. In other words, pumice is very light. It has roughly the consistency of a mixture of gravel and sand, with light, porous individual granules that normally either float on water or sink only slowly. Pumice particles are either round or angular and measure up to 65 mm in diameter. Only particles in the 1 -16-mm size range should be used to obtain good building material.

Figure 5: A volcanic eruption

In addition to light-colored pumice, there are also various dark-colored forms referred to as lava, tuff, etc. They, too, can be used as building material, but the light-colored pumice processes better, as described in Chapter 2.4.

Pumice has the following chemical composition:

Pumice originates during volcanic eruptions, when molten endogenous rock is mixed with gases before being spewed out (Fig. 5). The light, spongy particles are hurled up and carried off by the wind. As they cool and fall back to earth, the particles accumulate to form pumice rock or boulders. Sometimes the molten rock is too heavy to be ejected, in which case it flows out and collects at the foot of the volcano as a compact, fairly homogeneous, usually somewhat less porous rock formation. Most such lava deposits can be cut up into natural stone blocks for direct use in construction work.