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fineness of precious metals

Precious metals are almost always actually used in the form of alloys, a mixture of the metal with other metals. Traditionally, the precious metal content of an alloy was described, not by a fraction or percentage, but by a mass. This mass was the mass of the precious metal part of a standardized mass of the alloy. So, for example, for silver in Britain the standard mass was 12 ounces, each of 20 pennyweights, or 240 pennyweights in all. A silver alloy's fineness might be described as “11 ounces, 10 pennyweights,” which in fact is the fineness of new sterling.

The carat or karat is almost universally used to describe the fineness of gold. Pure gold is 24 karat. 18 karat gold is 18/24 or 75% gold.

Describing the Fineness of Silver, mainly pre-20ᵗʰ century
Britain	Pure silver had a fineness of 12 ounces. An ounce could be subdivided into 20 pennyweights; there were 240 pennies in a pound sterling. Sterling silver, or coin silver, has a fineness of 11 ounces, 2 pennyweights, and is thus 222/240, or 925 parts in a thousand, or 92.5% silver. In 1697 its use by silversmiths was forbidden, in favor of Britannia silver. Britannia silver, or New sterling, introduced by Act of Parliament in 1697, for plates, urns, etc. has a fineness of 11 ounces, 10 pennyweights, and is thus 230/240, or 95.83% silver. The hallmark showed a seated woman, Britannia, hence the name of the alloy (or perhaps the other way around). When use of sterling silver for wrought silver was permitted again, beginning in 1720, production of pieces in Britannia silver declined, but never ceased. Since 1999 the millenial hallmark has been 9584. Silver bullion coins currently issued by the United Kingdom are Britannia silver. Britannia metal is not a silver alloy.
China	Pure silver was 100 touch (also used for gold).
Sweden	Pure silver was 16 lods, each of 18 grains. Thus 15 lod, 5 grain silver would be 275/288 or 95.49% silver.
Turkey	Pure silver was 100 carats, each of 4 grains.

Sources: Waterston, de Carle.

sources

Secondly, that the purest silver containes twelve ounces silver in each pound Troy weight: And that Edward the first, King of England, keeping the Feast of Christs Nativitie at Barwick, in the yeere 1300, did upon Saint Stevens day decrie the value of base silver moneys, and after did altogether forbid the use of them, and shortly after commanded sterling money to be coyned, so called of the Easterlings, who first coyned silver money of that Standard, which is of eleven ounces two penny weight.

Fynes Moryson.
An Itinerary containing his ten yeeres travell through the twelve dominions of Germany, Bohmerland, Sweitzerland, Netherland, Denmarke, Poland, Italy, Turky, France, England, Scotland & Ireland.
Glasgow: James MacLehose and Sons, 1907.
Part I, Book III, Chapter 6.
Page 134 of this edition. The first edition was published in 1617, but the travels took place between 1591 and 1603.