Nextel® 312 and Nextel® 440 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 higher upper working temperature but is less flexible than Nextel® 312. Nextel® 312 is available from Goodfellow as a yarn, a narrow fabric and sewing thread sizes. Sewing threads are made from individual yarns, known as "singles", wrapped around the outside with rayon and then twisted together in two stages to give a balanced, non-snarl, construction. The rayon comprises about 20% of the initial weight of the thread and will "burn" out by about 300C if/when the sewn article is subsequently heated.

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 920C should be used. More details of the recommended methods of treatment are available on request.