The 1702 London Mint Assay
Pure silver or pure gold is rather soft and not practical for coins as it would wear down quickly. In order to make these metals more durable they are combined (alloyed) with other metals. In the Seventeenth and Eighteenth centuries each country (or sometimes each mint or city) used its own ratio of precious metal to alloy in minting silver and gold coins. Therefore the weight of a coin alone would not give you its true value, for one needed to know the percent of alloy (sometimes called impurities) in the silver or gold. The purity is known as its fineness and is measured in millesimals.
Due to Britain's international trade, silver and gold coins from various countries were regularly exchanged for British coins, both at home and abroad. To make an accurate determination of the relative value of the foreign coins an assay had to be performed. This entailed melting down unclipped full weight samples of each coin then extracting or adding alloy until the silver or gold was at a predetermined fineness. Once a uniform fineness had been achieved each sample would be weighed.
Government assays were undertaken at various times as in 1626, 1651, 1696, 1702, 1704 and 1717. However the most influential assay was in 1702 when the director of the London mint, Sir Isaac Newton, undertook a general assay of 44 foreign silver and 12 foreign gold coins in relation to the British standard. Newton undertook this project to assist British trade but it became quite important for colonial America as well, since several of the coins were used in the colonies. Even more significantly two years after the assay Queen Anne issued a proclamation establishing rates for coins in the colonies, which relied heavily on the assay for equivalencies. The proclamation and its effects are discussed in another of the Explanatory Essays on the "Comparative value of money in Britain and the colonies."
Gold was rather scarce in the American colonies so the major impact of the assay was in the results for the silver coinage. For silver the standard was British Sterling. Sterling refers to the alloy then in use for British silver coins which was 925 parts pure silver to 75 parts copper resulting in a 92.5% silver content expressed as .925 fine silver.
After refining each coin to the sterling standard for the fineness of the silver, each sample was weighed on the troy scale. This is an international standard for weighing precious metals first used at the medieval fairs in Troyes. The Troy scale is:
For example, the Spanish colonial eight reales "silver dollar" was authorized to be produced at 422.9 grains of .935 fine silver. An "unworn" sample would be melted down. As the fineness of this Spanish silver was greater than the sterling standard some alloy would be added to bring it down to .925 fine. If the fineness was below the sterling standard then alloy would be removed to bring it up to .925 fine. At this point the sample could be weighed. Although not part of the official 1702 assay Newton did assay the Spanish colonial eight reales in 1703 (see McCusker, p. 8). From Newton's records we know the actual sterling weight of his sample was 17 dwt. 9 gr. 3 mi (that is, 417.15 grains sterling).
According to Mossman's study Newton performed the following calculations to obtain a monetary value in British pence for a coin: The sterling weight in troy ounces was converted to grains, in this case 417.15 grains. The number of grains was multiplied by 62 (which was the pence value per troy ounce of sterling) the product (25,863.3) was divided by the number of grains per troy ounce sterling, 480 to get the sterling value of the coin (53.881875d) which was reported by Newton as 53.88d. Newton reported pence values out to two places and did not usually round off. For daily use his figure would need to be rounded to the nearest pence which would put the trading value of the Spanish "silver dollar" at 54d or 4s6d.
Below are some silver coin types assayed by Newton in 1702 with their equivalent sterling weight and value:
In all Newton tested fifty six different silver or gold coins including over a dozen different German reichsthalers.
1The calculation comes out to 65.584375d, reported by Newton as 65.59d
2The Spanish Netherlands, the calculation come out to 52.848541d, reported by Newton as 52.91d
3The calculation comes out to 55.29625d, reported by Newton as 55.3d
4The calculation comes out to 55.548125d, reported by Newton as 55.55d
5This is the debased Spanish new plate coin made in Spain. In 1702 the Spanish American eight reales passed in Spain for ten new plate reales. The calculation comes out to 43.109375d, reported by Newton as 43.11d.
For further information see the edition of Newton's reports in, William A. Shaw, Select Tracts and Documents Illustrative of English Monetary History 1626-1730, reprint of the 1896 edition, New York: Augustus Kelley Publishers, 1967, pp.133-179 with the July 7, 1702 report on pp. 136-149; also see Mossman, Appendix 3, pp. 275-278 and his chapters 1-2, especially his excellent charts including table 5 on pp. 61-62, table 7 on p. 68 and tables 8-9 on pp. 73-75. In these tables he gives the original weight and fineness of the coins as well as the grain weight in pure silver (not sterling alloy) followed by the sterling value. Also see John McCusker, Money and Exchange in Europe and America, 1600-1775: A Handbook, Chapel Hill: University of North Carolina (for the Institute of Early American History and Culture, Williamsburg, Virginia), 1978, pp. 1-12.