Stir bars using rare earth magnets have significantly increased magnetic strength compared to those using AlNiCo magnets. They are almost totally resistant to demagnetisation. Stir bars using rare earth magnets are identified by a chemically inert, carbon black spot. N. B. The high magnetic coupling power of rare earth stir bars may cause abrasion of the PTFE coating against the base of the container and where stirring arrays are used, as in multi-well plates, lateral interaction between stir bars may be observed.