Bluing is a laundry additive that makes whites appear whiter – an optical illusion that has been employed since at least the 17th century and can still be found as little blue specs in many modern laundry detergent powders.
The earliest recipes for bluing were derived from indigo or smalt (ground glass containing cobalt.) The colouring was often mixed with starch and various additives that might include alum, gum arabic, or isinglass (powdered fish bladder) and sold as a powder or formed into lumps. An early synthetic blue discovered in Berlin in 1704 dubbed ‘Prussian blue’ was substituted for the indigo or smalt by the 1720s until a synthetic ultramarine was discovered in 1826 that was made commercially available by the 1850s.
Since the mid 19th century bluing was most often made into one ounce cubes from ultramarine and baking soda and kept in a little drawstring muslin bag that was dipped and swirled into the final rinse water on washday. As laundry was traditionally done on Mondays, the blue colour leant its name to one of the hardest work days of the week and became the source of the phrase ‘Blue Monday’.