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New views into an old craft: Nanotech methods provide new insight into the methods of ancient gilders.

PRECIOUS-METAL PLATING AND COATINGS on valuable objects have been under examination for centuries and under scientific analysis for decades, but the research has reached a new, intimate scale in Italy. Case in point is the research into thin metal plating on Roman coins, ancient and medieval artwork, and jewelry produced 1,200 to 2,000 years ago.

Their artistry and durability have aroused the curiosity of scientists at Italy's Institute for the Study of Material Nanostructures in Rome, which is part of Italy's National Research Council (Consiglio Nazionale delle Ricerche). Researchers at the institute are finding that state-of-the-art investigative technologies such as scanning electron microscopy are able to "see" beneath surfaces in depths measured in nanometers.

The investigations are the work of a team of scientists and chemists led by Gabriel Maria Ingo, senior scientist at the Instituto per lo Studio dei Materiali Nanostrutturati. In addition to SEMs, researchers used selected area X-ray photoelectron spectroscopy energy-dispersive spectroscopy, secondary ion mass spectrometry, and glow discharge optical emission spectrometry.

The results of the team's investigations were reported in a paper, "Ancient Mercury-Based Plating Methods," in the July 2013 Accounts of Chemical Research, published by the American Chemical Society.

Ingo's team examined coins from Carthage (circa 240 B.C.) and from the Roman Republic; a silver-coated fragment of a statue of a lion from the Roman Republic; and a gold- and silver-plated altar from the Holy Ambrogio basilica in Milan made around 850 A.D.

They looked at specimens coated with processes using mercury, a technique that may have originated in China and moved west along the Silk Road. One object of the study, a Roman copper coin coated in silver from the first century B.C., is one the oldest European artifacts coated by a mercury-based method. The mercury method lasted until the 19th century when it was phased out by electroplating.

Observations of the coin showed the presence of mercury and small grains and subgrains of silver with diameters ranging from 100 nm to 1 [micro]m. Areas lacking the silver coating contained copper corrosion products, including cuprite and atacamite.

The Holy Ambrogio altar is a rectangular wooden case measuring 250 x 150 x 130 centimeters. It is decorated with precious-metal enamels, filigrees, and gems such as emerald, topaz, amethyst, agate, and carnelian. The microchemical structure of the gold layers was revealed as well as the complex interface between painted-on gold layers and silver substrates.

The institute's work has led to a deeper understanding of the enameling and related techniques that used amalgams of gold and silver with mercury and several other elements.

Also revealed were the ways in which these objects' surfaces degraded over the past 1,200 to 2,000 years. The patinas, or precious metals in thinly plated or coated surfaces, suffered from exposure to oxides, sulfides, and chlorides stemming from the atmosphere, moisture, handling by humans, and long-term burial.

In the Holy Ambrogio altar, the team found "microcracks and a dense population of cuprite ([Cu.sup.2]O) microglobules that formed during the manufacturing of the panel." The researchers found other cracks as well, which they attributed to brittleness caused by the aging of the ancient silver.

The goal of the institute's thin-metal-coatings analyses is to better understand the ancient techniques of goldsmiths and silversmiths and the complex processes of degradation since then. The findings are considered vital for the preservation of Italy's cultural heritage, and intriguing in terms of rediscovered trade secrets.

JACK THORNTON, a technology consultant in Santa Fe. N.M., is a frequent contributor to Mechanical Engineering.
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Title Annotation:INPUT OUTPUT
Author:Thornton, Jack
Publication:Mechanical Engineering-CIME
Geographic Code:1USA
Date:Mar 1, 2014
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