Notes From the Jeweler's Bench: the Bloom Technique
Have you noticed that gold jewelry and articles from the late 19th century often gleam with a particularly rich, velvety matte finish? They have that lustrous patina created by time to be certain, but there is something else at play as well.
Victorian lorgnette in 14k gold with a bloom finish, Lang collection
In the 1850's it was discovered that submerging alloyed gold jewelry into a boiling mixture of hydrochloric acid, potassium nitrate, water and salt resulted in the surface dissolution of the alloyed metals, revealing a thin layer of soft pure gold. The acid bath not only dissolved the traces of silver, copper, nickel and zinc on the surface, but left behind microscopic pitting that created a lovely matte sheen on the gold.
Art Nouveau brooch in 14k gold with bloom finish, Lang collection
The process came to be called colouring, and the resulting finish to be called a bloom. It was a very popular way to finish jewelry in England and America, particularly in the 1870s to 1890s, which saw an increasing demand and ensuing rise in production of lower karat jewelry. The bloom allowed even a 14k piece to glow like it was made of pure 24k gold.
late 19th century brooch with gold prospector motif, in 14k with bloom finish, Lang collection