Bluing, laundry blue, or washing blue is a household product used to improve the appearance of textiles, especially white fabrics. Used during laundering, it adds a trace of blue dye (often synthetic ultramarine, sometimes Prussian blue) to the fabric.
White fabrics acquire a slight color cast after use (usually grey or yellow), because they can never be cleaned perfectly. Since blue and yellow are complementary colors in the subtractive color model of color perception, adding a trace of blue color to the slightly off-white color of these fabrics makes them appear whiter. Laundry detergents may also use fluorescing agents to similar effect. Many white fabrics are "blued" during manufacturing. Bluing is not permanent and rinses out over time leaving "dingy" or "yellowed" whites. A commercial bluing product allows the consumer to add the bluing back into the fabric to restore whiteness.
On the same principle, bluing is sometimes used by white-haired people in a blue rinse.
Bluing has other miscellaneous household uses, including as an ingredient in rock crystal "gardens" (whereby a porous item is placed in a salt solution, the solution then precipitating out as crystals), and to improve the appearance of swimming-pool water. In Australia it was used as a folk remedy to relieve the itching of mosquito and sand fly bites.
Laundry bluing is made of a very fine blue iron powder suspended in water (a "colloidal suspension").
Blue colorings have been added to rinse water for centuries, first in the form of powder blue or smalt, or using small lumps of indigo and starch, called stone blue. After the invention of synthetic ultramarine and Prussian blue it was manufactured by many companies, including "Mrs. Stewart's Bluing" [1] in the USA, Reckitt's Crown Blue [2] in Hull and Dolly Blue in Cumbria.[3] It was popular until the mid-20th century in the UK and USA, and is still widely used in the USA, India and Pakistan. In many places, it has been replaced by bleach for its primary purpose.
Bluing is usually sold in liquid form, but may also be a solid. Solid bluing is sometimes used by hoodoo doctors to provide the blue color needed for 'mojo hands' without having to use the toxic compound copper sulfate. Bluing was also used by some Native American Tribes to mark their arrows showing tribe ownership.
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