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Ancient gold workers were crooks as well as craftsmen 

How artisans centuries ago achieved sophisticated gilding, such as on the St. Ambrogio golden altar from 825 AD, is now coming to light.
Metal workers from the time of ancient Rome were experts at shiny finishes.

As long as humans have lusted after shiny objects made of solid gold and silver, there have been scammers passing off plate as the real thing. They just used to be better at it.

Metal workers 2,000 years ago perfected precious metal plating techniques that modern methods can't touch, new research shows. The artisans covered knobby sculptures and flat coins with exquisitely thin layers of gold and silver. But the shiny finish wasn't just for show. Forgers and crooks likely coated cheap metal or wood in a gilded skin and sold the objects for a profit. Rather than flaking off or turning green, these master fakes stood the ages.

In Ancient Cartage, copper was cooled slowly, so that shinier metals like tin, arsenic or antimony, that were mixed into the metal as impurities, would move to the surface. When the object was cooled, it was polished.

More popular in Europe in the first centuries A.D. was a mercury layering technique that made its way west from China. Sometimes, liquid mercury would be the glue that held thinly beaten gold sheets in place, on a lesser metal. Other times, gold or silver dust would be mixed into a mercury amalgam, painted over a metal, and then heated. The mercury would evaporate, leaving behind a fine mist of gold or silver.

The workers "produced objects of an artistic quality that could not be bettered in ancient times and has not yet been reached in modern ones," a team of authors from the Institute of Nanostructured Materials in Rome, Italy, write in the Accounts of Chemical Research A this month.

The team analyzed a copper Denarius coin from the Roman Empire, dating back to the first century B.C. Once carefully plated in silver, the layer has now worn off, and the patina from the oxidizing copper below has grown over most of the surface. But traces of the handiwork remain in small amounts of mercury that hinted at how the silver was pasted on. This superior plating technique would ensure the tell-tale copper didn't show through as the coins were passed from hand to hand.

The altar at the Basilica of St. Ambrogio, dating back to 825 A.D. is a wooden case with gold and silver plated panels. The researchers found mercury mixed in with the plating, which suggested the liquid metal was used as glue.

The team also analyzed the ceramic back leg of a statue of leaping lion, which was wrapped in a thin copper layer and finished off with a extra-fine layer of silver that was just a single micrometer — one thousandth of a millimeter — thick. The statue left looking like it was solid silver, the authors write, and could have allowed the designers to make a tidy chunk in profits.

Gabrial Ingo, Giuseppe Guida, Emma Angelini, Gabriella Di Carlo, Alessio Mezzi and Guiseppina Padeletti are the authors of "Ancient Mercury-Based Plating Methods: Combined Use of Surface Analytical Techniques for the Study of Manufacturing Process and Degradation Phenomena," published in the Accounts of Chemical Research.

Nidhi Subbaraman writes about science and technology. Follow her on Facebook, Twitter and Google+.