a clear phosphor colloidal solution (<#taggant> ink) applied over a <#stamp>, or to its paper, or mixed with stamp printers’ ink. Tagging glows bluish-green or reddish when exposed to short wave ultraviolet light. Reddish tagging was only used for <#amstamp=air mail stamps> from 1963 to 1978. Beginning with the 1978 31¢ Wright Brothers (<#scott=Scott> C91-C92), bluish-green tagging was used for all subsequent air mail stamps. Tagging has a brief afterglow that is used by <#acm=automatic cancelling machines> to find, face, and <#cancel> an envelope’s stamp.