Blooming, a popular finishing technique for karat gold jewelry from 1870 to 1890, was first documented in 1853. The means for creating a bloom finish involved dipping a karat gold item into a boiling mixture of hydrochloric acid (muriatic acid), saltpeter (potassium nitrate), salt and water. This process burned off any alloys on the surface resulting in an alloy-free, thin layer of soft, pure gold on the item. This thin gold skin was dotted by an infinite number of microscopic pits or holes which caused the matte sheen. The effect is reminiscent of the soft skin found on a peach. Dipping the item into the boiling mixture was referred to as colouring, and the resulting sheen was termed bloom.
Art Nouveau Bloomed Gold Locket, c. 1890.