a <#fugink=fugitive ink> made with synthetic organic pigments produced as derivatives of nitrobenzene. Aniline ink was used in the production of stamp ink from before the <#washfrank=Washington-Franklin> era until its importation from Germany became impossible during World War I. When inferior quality aniline ink was used, bleeding of pigment resulted in stamps known as <#pink=pink backs>. Aniline ink was also used to <#surcharge> some <#revenue> stamps.