Tephras previously dated by the fission-track method used on glass shards have been redated using zircon crystals, which have greater track stability and consequently are expected to yield more reliable ages. The ages of the glass proved to be fortuitously correct in most cases, owing to two compensating errors–unrecognized annealing and inaccurate thermal neutron dose determination. Ages of glass shards should not be regarded as reliable until the glass used has been checked for annealing of the fission tracks. If annealing has taken place, a correction can be applied, but the method is very time consuming. The shards from the Borchers Ash, Kansas, suggested by some geologists as a glass age standard, are totally unsatisfactory because of proven annealing.