Bluing liquid, also called laundry bluing (or just bluing) is a substance used in the rinse cycle to make white clothes appear whiter. Since its creation bluing liquid has found other uses including personal care and science projects.
Bluing liquid is basically two ingredients mixed together (though more ingredients may be added depending on the recipe): Prussian blue (ferric ferrocyanide) and water. The Prussian blue doesn't dissolve so bluing is actually a colloidal suspension where the Prussian blue remains suspended in the water.
Due to the fading of previous whitening processes and general dinginess accumulation, bluing is used to make white clothes look restored. Bluing liquid is also used to brighten light colored hair and as part of a recipe to make crystal gardens.
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By the 1900s bluing was widely used at home and in laundries. Before washing machines, the second rinse tub for clothes contained blue rinse.
Bluing liquid gives white clothes a blue-white hue. Blue-white is an intense looking white that looks more brilliant. Even though it makes clothes whiter, if bluing liquid is placed directly on the cloth it will stain permanently.
Bluing liquid is slightly toxic, but can become more toxic as Prussian blue can emit hydrogen cyanide when exposed to high temperatures, strong UV light or acid.