Amorphous HTMW (AES and ASW) are produced from a molten glass stream which is aerosolised by a jet of high pressure air or by letting the stream impinge onto spinning wheels. The droplets are drawn into fibres; the mass of both fibres and remaining droplets cool very rapidly so that no crystalline phases may form.
When amorphous HTMW are installed and used in high temperature applications such as industrial furnaces, at least one face may be exposed to conditions causing the fibres to partially devitrify. Depending on the chemical composition of the glassy fibre and the time and temperature to which the materials are exposed, different stable crystalline phases may form.
In after-use HTMW crystalline silica crystals are embedded in a matrix composed of other crystals and glasses. Experimental results on the biological activity of after-use HTMW have not demonstrated any hazardous activity that could be related to any form of silica they may contain.