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Specific Gravity

Specific Gravity.  The measurement of density by a ratio of a weight of a substance's mass to an equal volume of water. Specific gravity is an excellent test for the genuineness

of metallic numismatic items; it does have shortcomings, however, as it cannot determine the amount of alloy in a composition. If the alloy is nearly pure the resulting figure will be near that of the established specific gravity of pure metals. It also cannot reveal if a piece is cast or struck, the specific gravity will be the same.

 

History of specific gravity.  The first specific gravity measurements of coins were conducted by R. Boyle in 1675. He employed a hydrometer float for weighing above and below water.

Here are some metals used in numismatics and their specific gravity:

Specific

Metal                          Gravity

Heavy-Weight Metals

Platinum . . . . . . . . . . .     21.45

Gold 24 karat (pure) . . . .19.32

Gold 22 karat. . . . . . . . .  17.60

Gold (coin, .900). . . . . . . 17.32

Gold 20 karat. . . . . . . . .  16.50

Gold 18 karat. . . . . . . . .  15.55

Gold 14 karat. . . . . . . . .  13.60

Gold 12 karat. . . . . . . . .  12.75

Gold 10 karat. . . . . . . . .  11.70

Gold 9 karat . . . . . . . . .   11.50

Gold 6 karat . . . . . . . . .   10.90

Lead . . . . . . . . . . . . .       11.35

Silver (pure). . . . . . . . .    10.49

Silver (sterling, .925). . .  10.36

Silver (coin, .900). . . . . . 10.31

Silver (coin, .800). . . . . . 10.30

Silver (quaternary, .500). . 9.58

Mid-Range Metals

Copper (pure). . . . . . . . .    8.96

Nickel-silver (12-16 Ni) . . 8.95

Copper (bronze .900) . . . . .8.94

Copper-nickel (10-30 Ni) . 8.94

Nickel (pure). . . . . . . . .      8.90

Bronze (commercial). . . . . 8.80

Red brass. . . . . . . . . . .       8.70

Yellow brass (oroide). . . . .8.47

Iron (pure). . . . . . . . . .       7.87

Aluminum bronze. . . . . . . 7.80

Tin (pure) . . . . . . . . . .       7.30

White metal, Pewter. . . . . 7.28

Zinc (pure). . . . . . . . . .      7.13

Light-weight Metals

Aluminum . . . . . . . . . . .    2.70

Magnesium. . . . . . . . . . .   1.74

References:                                                                                                                                      

C34   {1954} Singer, 3:68, (reference 59).

NE42 {1982} Doty, p 309-310.

excerpted with permission from

An Encyclopedia of Coin and Medal Technology

For Artists, Makers, Collectors and Curators

COMPILED AND WRITTEN BY D. WAYNE JOHNSON

Roger W. Burdette, Editor


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