"Pure" basalt can easily be melted and formed as a glass. While basalt has the advantage of being a single component raw material for glass production, often times it is not suitable for forming fibers because the basalt melts are too viscous. In order to more easily fiberize basalt, modification to reduce viscosity may be necessary. A viscosity of approximately 10 poise (log n=1.0) at 2600.degree. F. is recommended in the literature as an "optimum" high temperature viscosity for forming mineral wool fibers. The viscosity of basalt at reasonable forming temperatures may be too high for producing satisfactory mineral wool (Log n=1.0 at 3235.degree. F.).
Viscosity modification, however, often affect the rate of crystallization, solidus temperature or softening point of the resulting fibers. The fibers should crystallize sufficiently rapid so as to be "fire resistant." Also, the fibers should not soften to the point where the ceiling tiles or boards will fall out of their suspension system during a fire.
These tiles are low density composites consisting primarily of mineral wool, clay, organic binders and organic fibers. During a fire, a number of complex reactions takes place in the composite which include organic binder and fiber burn-out, low temperature bonding between inorganics and high temperature deformation and crystallization of the mineral wool.