Silver

Silver is the Moon in the alchemical scheme, and its symbol is a crescent. In Latin texts the metal is often simply called Luna, and the association is one of the most stable in the whole tradition, attested from late antiquity and unchanged through the medieval and early modern corpus.

The grounds for it are more obvious than for most of the seven. Silver is white and reflective; the Moon is white and reflective. Freshly polished silver is the brightest of all metals, reflecting more visible light than any other surface, and the mirrors of the ancient and medieval world, where they were not polished bronze, were silvered. The Moon is second to the Sun in the sky, and silver is second to gold in value. The correspondence is a straightforward reading of appearance and of rank, and it requires no hidden doctrine.

The vocabulary of the laboratory kept it. Lunar caustic is silver nitrate, so named because it is made from silver and because it burns tissue; it was a standard cautery and antiseptic into the twentieth century, and the name is still occasionally heard. Horn silver is silver chloride. The tincture of the Moon in alchemical medicine is a silver preparation.

Silver also has a real association with purity that is not merely symbolic. It is antimicrobial: silver ions disrupt bacterial cell function at very low concentrations. Water and wine kept in silver vessels genuinely spoil more slowly, and the practice of using silver for storage and for eating utensils, common in wealthy households across many cultures, had an effect its users could observe without being able to explain. Silver dressings and silver-impregnated wound coverings remain in clinical use. This is one of the few places where a traditional attribution of a property to a metal turns out to be literally correct.

For most of recorded history money meant silver. Gold was too valuable for ordinary transactions and copper too bulky; silver sat at the useful middle, and the great majority of coinage from the Lydian and Greek issues onward was struck in it.

The traces are in the language. The pound sterling began as a weight of silver. The French argent and the Spanish plata mean both the metal and money outright. The shekel and the drachma were weights of silver before they were coins.

Athenian power in the fifth century BCE rested substantially on the silver mines at Laurion, which paid for the fleet that fought at Salamis. Roman denarii were silver, and their progressive debasement through the third century CE is one of the clearest quantitative records of imperial finance available to historians.

The largest single event in the history of the metal was the discovery in 1545 of the mountain of silver at Potosi, in what is now Bolivia, together with the Mexican mines. The quantities were unprecedented. The ore was extracted by forced labour under conditions that killed on a very large scale, and the mercury amalgamation process used to refine it poisoned both workers and land. That silver flowed to Europe and, in enormous volume, onward to China, which had put its tax system on a silver basis. The resulting inflation across Europe, and the binding together of the Chinese and European economies through a single metal, is one of the foundations of the modern world economy.

Silver's monetary role ended during the nineteenth and twentieth centuries as states moved first to gold and then to fiat currency. The United States demonetised silver in 1873, an act its opponents called the Crime of '73, and the argument over silver coinage dominated American politics for a generation afterwards.

Silver is the best electrical conductor of any element, and the best thermal conductor. It is also the most reflective. All three superlatives belong to it outright, and in each case copper or aluminium is used instead purely on grounds of cost.

Where performance matters more than price, silver is used directly: in high-frequency conductors, in the contacts of relays and switches, in brazing alloys, and in the reflective layer of high-quality mirrors and telescope optics. Silver paste printed onto solar cells collects the current from every panel in the world, and photovoltaics now consume a substantial fraction of annual production.

Its defining chemical property is that its halides darken in light. Silver chloride, bromide and iodide decompose to metallic silver where photons strike them, and the amount of silver deposited records the amount of light received. That is photography. From the daguerreotype of 1839 through the whole era of film, the image was silver, and the entire visual record of the nineteenth and twentieth centuries, every photograph, every cinema print and every X-ray plate, was made of this one metal. Digital sensors ended that industry within about a decade, and the collapse in photographic demand is the largest change in the silver market in modern times.

Silver tarnishes, which gold does not. The black film is silver sulfide, formed from traces of hydrogen sulfide in the air, and its rate is a fair proxy for local air pollution. It is a surface effect only, and polishing removes it along with a little of the metal underneath.