Blue uniforms issued to Union soldiers during the American Civil War today possess brown sewing thread. The aims of this research were to verify the original color of the sewing thread, identify the dye(s) used, and hypothesize the mechanism of color change. The original thread color was verified through examination of an unpublished Quartermaster manual and Army Quartermaster thread contracts. The number of possible major commercial blue dyes was narrowed through a search of the mid-nineteenth century dye literature. Thread samples were taken from fifteen pairs of Civil War trousers. Organic compounds in the thread were searched for with infrared, infrared multiple internal reflectance, and ultraviolet-visible spectroscopy. SEM x-ray fluorescence and wet chemical analysis were used to identify inorganic ions. Civil War thread and modern dyed controls were subjected to the same analytical scheme that maximized data from the small sample sizes. The hypothesis for color change was based on historical data that defined thread exposure from processing, use, and maintenance. The subsequent laboratory simulated aging followed these historically verified conditions and included light exposure using a carbon-arc fadeometer, and accelerated aging of samples in a forced air oven at 148[degree]C. The three questions posed by this research have been answered. The original flax thread was blue. Contracts showed that 73.6% of the flax thread ordered was blue. Three blue dyes were in commercial use on cellulosics during the mid-nineteenth century – indigo, Prussian blue, and logwood. Laboratory analysis provided characteristic responses for all three. The nineteenth century thread lacked the characteristic visible absorption peaks of indigo. The ferrous cyanide ion of Prussian blue was also absent. Both indigo and Prussian blue control thread color did not change on simulated aging. The logwood dyed controls changed to brown within ten hours of both light and heat treatment. Iron was present both on the logwood dyed controls and nineteenth century thread. In conclusion, the dye used on the nineteenth century sewing thread was logwood mordanted with iron or iron plus alum. The hypothesized color change mechanism was by dye degradation through the action of light and aging in a normal atmosphere.