Arsenical bronze is an alloy in which arsenic, as opposed to or in addition to tin or other constituent metals, is combined with copper to make bronze. I was watching “The Curse of Oak Island”[1] and Gary Drayton[2] found a trade token while metal detecting. It turned out to be made of arsenical bronze.
The use of arsenic with copper, either as the secondary constituent or with another component such as tin, results in a stronger final product and better casting behavior. It is still not entirely clear to what extent arsenic was deliberately added to copper and how much its use arose simply from its presence in copper ores that were then treated by smelting to produce the metal.
Using these various ores, there are four possible methods that may have been used to produce arsenical bronze alloys.[3] These are:
- The direct addition of arsenic-bearing metals or ores such as realgar to molten copper. This method, although possible, lacks evidence.
- The reduction of antimony-bearing copper arsenates or fahlore to produce an alloy high in arsenic and antimony. This is entirely practicable.
- The reduction of roasted copper sulfarsenides such as tennantite and enargite. This method would result in the production of toxic fumes of arsenous oxide and the loss of much of the arsenic present in the ores.
- The co-smelting of oxidic and sulphidic ores such as malachite and arsenopyrite together. This method has been demonstrated to work well, with little in the way of dangerous fumes given off during it, because of the reactions together among the different minerals.
Although arsenical bronze occurs in the archaeological record across the globe, the earliest artifacts so far known, dating from the 5th millennium BC, have been found on the Iranian plateau. Artifacts made of arsenical bronze cover the complete spectrum of metal objects, from axes to ornaments. The method of manufacture involved heating the metal in crucibles and casting it into molds made of stone or clay.
After solidifying, it would be polished or, in the case of axes and other tools, work hardened by beating the working edge with a hammer, thinning out the metal, and increasing its strength. Finished objects could also be engraved or decorated as appropriate. Sulfide deposits frequently are a mix of different metal sulfides, such as copper, zinc, silver, lead, arsenic, and other metals. The metals could theoretically be separated out, but the alloys resulting were typically much stronger than the metals individually.
While arsenic was most likely originally mixed with copper as a result of the ores already containing it, its use probably continued for a number of reasons.
- It acts as a deoxidizer, reacting with oxygen in the hot metal to form arsenous oxides which vaporize from the liquid metal. If a great deal of oxygen is dissolved in liquid copper when the metal cools the copper oxide separates out at grain boundaries and greatly reduces the ductility of the resulting object. However, its use can lead to a greater risk of porous castings, owing to the solution of hydrogen in the molten metal and its subsequent loss as a bubble (although any bubbles could be forge-welded and still leave the mass of the metal ready to be work-hardened).
- The alloy is capable of greater work-hardening than is the case with pure copper so it performs better when used for cutting or chopping. An increase in work-hardening capability arises with an increasing percentage of arsenic, and the bronze can be work-hardened over a wide range of temperatures without fear of embrittlement. Its improved properties over pure copper can be seen with as little as 0.5 to 2 wt% As, giving a 10-to-30% improvement in hardness and tensile strength.
- In the correct percentages, it can contribute a silvery sheen to the article being manufactured. There is evidence of arsenical bronze daggers from the Caucasus and other artifacts from different locations having an arsenic-rich surface layer which may well have been produced deliberately by ancient craftsmen,[9] and Mexican bells were made of copper with sufficient arsenic to color them silver.
Arsenic is an element with a vaporization point of 615 °C, such that arsenical oxide will be lost from the melt before or during casting, and fumes from fire setting for mining and ore processing have long been known to attack the eyes, lungs, and skin. Chronic arsenic poisoning leads to peripheral neuropathy, which can cause weakness in the legs and feet. It has been speculated that this lay behind the legend of lame smiths, such as the Greek god Hephaestus. A well-preserved mummy of a man who lived around 3,200 BCE found in the Ötztal Alps showed high levels of both copper particles and arsenic in its hair. This, along with Ötzi’s copper axe blade, which is 99.7% pure copper, has led scientists to speculate that he was involved in copper smelting.
Arsenical bronze has seen little use in the modern period. It appears that the closest equivalent goes by the name of arsenical copper, which is defined as copper with under 0.5 wt% As, below the accepted percentage in archaeological artifacts. The presence of 0.5 wt% arsenic in copper lowers the electrical conductivity to 34% of that of pure copper,
and even as little as 0.05 wt% decreases it by 15%. Therefore, there is no demand for copper-containing arsenic in electric wires, etc., one of the major modern uses of copper and steam engine boilers is no longer made from it, leading to no modern use.
Footnotes
- The Curse of Oak Island is a multi-season reality television series that chronicles an eclectic team of treasure hunters and their search for legendary treasure on Oak Island, off the shore of Nova Scotia, Canada. It is an American television production that premiered in Canada on the History network on January 6, 2014. The show features what is known as the Oak Island mystery, showing efforts to search for historical artifacts and treasures. The tenth season premiered on November 15, 2022. [Back]
- Gary Drayton was born May 30, 1961, in Grimsby, Lincolnshire, England, UK. Some call him the Metal Detecting Ninja, and some call him Mr. Bobby Dazzler. Either way, it’s fair to say Gary Drayton is a key member of Oak Island’s ‘Fellowship of the Dig’, sharing Marty and Rick Lagina’s dream of uncovering the long-buried secrets of this fabled slice of Nova Scotia. Gary has certainly come a long way since he first startled metal detecting on British riverbanks. He’s lived in the United States for years, establishing himself as an authority among treasure hunters (and – even more importantly – introducing Americans to the phrase ‘bobby dazzler’). [Back]
Further Reading
Sources
Taylor & Francis Online
Wikipedia
Kidadl
Parts Community
