Definitions for "Hard glass"
(borosilicate glass) A heat resistant glass made under the brand names "Pyrex", "Kimax", and "Duran". Boric oxide and alumina is added to the glass mix, reducing the COE to 33, and making it react far less to thermal changes. This makes it usable in the kitchen oven. Most glass figurines, ships, and sculptural pieces, are made of hard glass because of it's tolerance to some parts cooling, while other areas are being worked in a flame. It is called hard glass because it retains it's shape when heated, and does not flow or soften as readily as soft glass.
A roving product that is not very soluble in acetone or styrene which results in the tendency of the individual bundles to retain their integrity (hardness) and not filamentize in the matrix resin. Hard glass is often stiff and brashy. The size is generally less than 40% soluble in acetone (See Soft Glass).
See borosilicate. Borosilicate glass and others that have a high melting point."Soft" glasses are lead or soda-lime glass.