Metallurgy
of Constantinian "Bronzes"

    The bronze coinage of the Constantinian period from A.D. 307-348 should actually be thought of as a true silver denomination since these coins had carefully measured amounts of silver and quite often, a surface enrichment of silver.¹ At the very least, these coins should be called argentiferous bronze coins! The percentage of silver during this period fluctuated from 1-5% silver, and varied from mint to mint. Fractional coinage always had less silver, and the commemorative fractions issued by Constantine in 317 had almost no silver. These coins had only trace amounts of silver-- 0.3% or less. The argentiferous alloys were comprised of mainly copper (Cu), lead (Pb), tin (Sn) and silver (Ag). Other impurities might include iron (Fe), nickel (Ni), cobalt (Co), zinc (Zn), gold (Au), arsenic (As), antimony (Sb), and Indium (In).²From A.D. 307-317, the amount of silver was circa 2-5%, and generally eastern mints had higher percentages of silver than the western mints, until Constantine became sole ruler of the Empire. From A.D. 318-320, there was circa 4% silver in the coinage. From A.D. 321-330, the silver content was circa 2%. After A.D. 330, the silver content was circa 1%, which was maintained until A.D. 341, when the silver dropped to less than 0.5%. Coins with only a trace amount of silver can no longer be thought of as argentiferous.³

   Many argentiferous coins of this period exhibit silvering on their surface. Through experimentation, some possible methods of surface enrichment of silver have been theorized.⁴ For flans with more than 5% silver, cold hammering followed by annealing resulted in lead and silver being forced to the surface. A dilute acid bath would give the flan a silvery surface. For flans that contained less than 5% silver, a bath in molten silver chloride displaced silver and deposited it on the surface, which gave the flan a silvery wash. Hot working and blanching prior to hot striking also enriched the surface silver content. There are other ways a coin could have a silvery surface. Corrosion may also promote a silvery surface on a coin. Lead corrodes very easily, and as it is displaced, silver can be deposited on the surface. Various  methods in the cleaning process can also give a coin a silvery appearance, such as heating a coin or even washing a coin. Lead can  be removed by prolonged washing, leaving more silver on the surface.⁵A recent study of the same coins analyzed by Cope showed that the silvering often contained mercury. Experiments were performed with a silver-mercury amalgalm and various heating cycles. Copper sheets were coated with silver-mercury pastes and heated. The alloy that best withstood the 600 degree temperatures had 62% mercury and 38% silver.⁶