Blanching is the whitening of metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

, by various means, such as soaking in acid

Acid

An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

 or by coating with tin

Tin

Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...

. This term is commonly used in coin

Coin

A coin is a piece of hard material that is standardized in weight, is produced in large quantities in order to facilitate trade, and primarily can be used as a legal tender token for commerce in the designated country, region, or territory....

age, in which pieces are given a lustre

Lustre

Lustre or Luster may refer to:* Lustre , a description of the way light interacts with the surface of a crystal, rock or mineral* Lusterware, ceramics with lustre glazes* Lustre , a Free Software distributed file system...

 and brilliance before images are struck into the surface.

The ancient method of blanching involved putting the pieces, after heating, in a large vessel of water, and some ounces of aqua fortis

Aqua fortis

Aqua fortis, or "strong water," in alchemy, is a solution of nitric acid in water. Being highly corrosive, the solution was used in alchemy for dissolving silver and most other metals with the notable exception of gold, which can only be dissolved using aqua regia...

, but in different proportions for gold

Gold

Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

 and silver

Silver

Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

. This method is no longer used, partly due to its expense, and partly because it diminishes the weight of the metal.