Nextel® 440 and Nextel® 312 are two similar materials - compositions of ceramic specially developed for the production of continuous filament yarns which are sufficiently flexible to manufacture conventional textile articles such as fabrics and sewing threads whilst retaining much of the high temperature and chemical resistance typical of ceramics. Nextel® 440 has somewhat better tensile properties and a higher upper working temperature but is less flexible than Nextel® 312 and is available from Goodfellow as a yarn.

The fibres are coated with finishes during manufacture which can be removed, if desired, by "heat cleaning" at about 550C. If the best possible properties are required, further heat treatment at about 1215C should be used. More details of recommended treatment methods are available on request.