Let me tell you a little personal anecdote. About twenty-five years ago I was asked by a local library to give some lectures to the public. Since I had always been interested in increasing the public’s interest in science, the opportunity seemed tailor-made and I accepted. I was even given a small honorarium for the effort.
I decided at the time that I should use the money to buy some kind of memento which would represent my first venture into educating people outside the classroom. But what to buy? It had to be something meaningful, and something with a chemical connection. Gold was the most precious metal and would have been an obvious choice, but the honorarium would not have bought very much. But at the time silver prices were skyrocketing. Why not buy a piece of silver, which had great chemical connections, and which could even turn out to be a worthy investment? So I bought ten ounces.
A natural silver nugget. Photo credit: Jurii
Silver is more than just an object of beauty. It’s the stuff of memories. Prior to digital photography, all picture taking relied on compounds of silver. The reduction of silver ions to metallic silver by the action of light is at the heart of the chemistry of photography. Silver is in fact so light-sensitive that a crystal of silver chloride could detect the light of a candle on the moon! That’s what they say anyway. I haven’t tried it.
But I certainly have tried cleaning silver. The metal of course tarnishes. That’s because it reacts with small amounts of hydrogen sulfide in the air to form black silver sulfide. What’s the answer?
You can remove it with a generous amount of elbow grease by rubbing with a fine abrasive. Or you can dip silver objects into a solution of thiourea and sulphuric acid to dissolve away the silver sulphide. Or you can make use of some very clever chemistry. Just wrap the silver to be cleaned in aluminum foil and drop it into a glass bowl filled with boiling water to which a spoonful of washing soda has been added. Within minutes the tarnish disappears.
There’s some remarkable chemistry going on here. The silver sulphide is converted back to metallic silver as the aluminum strips the sulphur away, forming aluminum sulphide. This method has the added advantage of not removing any silver at all. However the silver that gets redeposited may not shine as brightly as the original surface. So this method, while by far the easiest, may not be suitable for grandma’s fine jewelry.
The challenge of silver tarnish was not what was on the mind of the Loomis gang, a bunch of horse thieves in 19th-century New York State. They wanted to eliminate the white markings on the stolen animal by applying a solution of silver nitrate and exposing the horses to light. The stolen stallions would never be recognized!
A pure silver crystal. Photo credit: Alchemist-hp
The problem, though, was that the animals would still recognize their stables and owners. The gang’s plans were done in when one of their dyed horses ran straight back to its stall. This picture-perfect crime quickly faded into obscurity, but could Sir Arthur Conan Doyle have heard about it? In one of his Sherlock Holmes short stories, Silver Blaze, a stolen horse is disguised by colouring his white spots. Doyle does not mention how the spots were coloured but as a physician, he knew about silver nitrate. He indicates as much in the first of the Sherlock Holmes short stories, A Scandal In Bohemia, where the lead character remarks: “If a gentleman walks into my rooms smelling of iodoform, with a black mark of nitrate of silver upon his right forefinger, and a bulge on the right side of his top-hat to show where he has secreted his stethoscope, I must be dull, indeed, if I do not pronounce him to be an active member of the medical profession. Maybe Conan Doyle stole an idea from the horse thieves.
My little ingot of silver hasn’t tarnished since I always kept it tightly wrapped in plastic. While it did not turn out to be a good investment — probably worth less now than when I bought it — it certainly has some valuable memories attached to it.
Joe Schwarcz, popularly known as “Dr. Joe”, is Director of the McGill Office for Science and Society. He is well known for his informative and entertaining public lectures on topics ranging from the chemistry of love to the science of aging. He has received numerous awards for teaching chemistry and for interpreting science for the public and is the only non-American ever to win the American Chemical Society’s prestigious Grady-Stack Award for demystifying chemistry. Dr. Joe also writes a newspaper column entitled “The Right Chemistry” and has authored a number of books including best-sellers, Radar, Hula Hoops and Playful Pigs, The Genie in the Bottle, The Right Chemistry, An Apple a Day, Is That a Fact?, and Monkeys, Myths, and Molecules.