The ink that marks voters' fingers comes from alchemists' 'lunar caustic'
Silver nitrate stains skin so stubbornly that election officials use it to mark fingers and stop people voting twice. Alchemists called it lunar caustic, because they linked silver with the moon. The same salt has coloured medieval stained glass, protected newborns from blindness, and kept cut flowers fresh.
Chemically it is AgNO3, a colourless solid and the cheapest silver salt, which makes it the usual starting point for other silver compounds. In the 13th century Albertus Magnus noted that nitric acid dissolves silver while leaving gold behind, and dissolving silver in the acid then evaporating the liquid is still how the nitrate is made. It melts at 198 °C, starts breaking down around 250 °C into metallic silver and red-brown nitrogen dioxide, and decomposes fully by 440 °C.
Silver ions snap up chloride, bromide and iodide to form insoluble precipitates, a reaction chemists use to detect halides. Steamship crews once used it to check whether boiler water had been fouled by seawater, and it still tests cargo for leaks. Mixing the salt with sodium or potassium halides inside gelatin creates the light-sensitive silver halide of photographic film. Drop copper into a solution and hair-fine silver crystals grow while the liquid turns blue.
Artists found it early. From the 14th century, glaziers painted a 'silver stain' onto clear glass to produce shades from pale lemon to deep gold, a technique almost unchanged today. In medicine, Credé introduced in 1881 the practice of putting a dilute 2 percent solution into newborns' eyes to prevent gonorrhoeal infection that could cause blindness; antibiotics have since replaced it. Sticks of fused nitrate cauterise tissue and seal small blood vessels in the nose. In a placebo-controlled trial of 70 patients, nine days of treatment cleared all warts in 43 percent, against 11 percent on placebo.
Its antimicrobial uses faded once antibiotics arrived, helped by fears of argyria. It is corrosive, irritates skin and eyes, reacts explosively with ethanol, and its cancer risk has not been thoroughly studied. Researchers keep refining it: a new bone-staining method using a warmed 1 mol/L solution cuts staining time from 55 minutes to 10.
Source: Silver nitrate
