Laboratory identification of three major 19th century blue dyes - indigo, Prussian blue, and iron-mordanted logwood - has been investigated. Acid digestion of the cellulosic fiber was followed by separation into organic and inorganic phases. The organic phase was examined with visible spectroscopy showing a maximum peak range of 602 to 605 nm for indigo and for logwood a maximum peak of 518 nm. The inorganic phase produced positive identification for Prussian blue through use of wet chemical analysis for ferrocyanide ions. Laboratory dyed samples were artificially aged with light and heat to stimulate historic textile conditions. These artificially aged samples were analyzed as above and produced results similar to the nonaged dyed thread except for a loss of the visible spectroscopy peak of logwood. A flow chart for identifying these three blue dyes on small samples of cellulosic fibers is presented.