Smithsonian Science » materials science

Fungi still visible in wood charcoal centuries after burning

John Barrat — Sat, 13 Mar 2010 23:23:05 +0000

Scientists from the Smithsonian Museum Conservation Institute, the University of Valencia in Spain and the University of Minnesota, recently made an important observation regarding charcoals from wood fires excavated at ancient archaeological sites. They verified that the remains of fungi that had been living in the wood before it was burned could clearly be seen with a scanning electron microscope. Scientists have long known that the carbonization process preserves the anatomical structure of a wood well enough to identify its tree species. Now, new evidence shows that even the fine hyphae, mycelia and other structures of fungi living in the wood often survive the burning process and are clearly evident in wood charcoal. They are preserved well enough that scientists can sometimes distinguish between different fungi groups.

Charcoal analysis, a science known as anthracology, is an area of archaeology that can reveal important information about the wood used by ancient civilizations and, in turn, the users themselves, says Melvin Wachowiak, a senior furniture conservator at the Smithsonian. Knowing what type of fungus infested a wood before it was burned can reveal where the wood came from and why and how it was gathered, say for firewood, a funeral pyre, building a house or other purpose. Dry wood gathered from a forest floor for cooking often has a different fungi content than fresh wood cut and dried for fires or construction. Archaeologists can use this information to better understand wood management strategies of long vanished civilizations and the environment in which they may have lived.

Images: The scanning electron microscope image above shows living brown-rot fungi growing on a fragment of wood. The image at left shows the same fragment with its fungi still visible after the wood has been turned to charcoal. (All images by Magdalena Moskal-del Hoyo)

To confirm their findings Wachowiak and Magdalena Moskal-del Hoyo of the Department of Prehistory and Archaeology at the University of Valencia, burned hardwood and conifer wood samples infested with three primary wood fungi—brown rot, white rot and soft rot—in a laboratory, turning them into charcoal. The fungi were identified prior to burning by plant pathologist Robert Blanchette of the University of Minnesota, a co-author of the study. Then, using a scanning electron microscope, Wachowiak and Moskal-del Hoyo examined the charcoal for visual evidence of the carbonized remains of each fungus. Evidence of all three fungi was visible in the carbonized samples.

Image right: Wood mircostructure and its accompanying white-rot fungi can clearly be seen in this scanning electron microscope image of a charcoal fragment.

The scientists then compared these samples with charcoal taken from ancient archaeological sites: two Neolithic settlements in eastern Hungary, one of the Körös culture and one of the Tisza culture of Polgár-Csőszhalom, and wood from the Bronze Age necropolis of Kokótow in what is now Kraków, Poland. In their research, the scientists were careful to verify that the fungus structures they observed in the ancient charcoal samples had attacked the wood before it was burned and not after.

Other scientists have observed this phenomenon before, Moskal-del Hoyo points out. But this is the first time it has been verified “in a systematic manner,” she says, by using known fungi-infested wood samples, burning them, and then verifying that the same ultrastructure of the wood and fungi are still visible.

A paper on this research “Preservation of Fungi in Archaeological Charcoal” was published recently in the Journal of Archaeological Science. –John Barrat

Conservators challenged by contemporary artworks at the Hirshhorn Museum

John Barrat — Mon, 25 Jan 2010 16:10:26 +0000

Digital Stradivari: computer models of violins reveal master luthier’s techniques

John Barrat — Mon, 02 Nov 2009 16:00:34 +0000

With the help of a high-resolution CT scanner, researchers at the Smithsonian Institution recently took a giant leap forward to a better understanding of the engineering and craftsmanship that Antonio Stradivari (1644-1737), master violin maker of Cremona, Italy, built into each of his renowned instruments. The CT, or computed tomography, scanner is a medical imaging device normally used in hospitals to create 3-D X-ray images of the human body.

Image right and below left: These two images show measurements of wood thickness on the top board of two different violins used in the study. Red indicates a thickness of 4 millimeters or higher, green a thickness of 2 millimeters or less. Yellow is a mid point between the two thicknesses.

In a pilot study that used seven Stradivari violins made between 1670 and 1709, the researchers scanned each violin with a CT scanner then used the data to create digital, 3-D images of each violin. Using the scanner they recorded exact digital measurements of the dimensions of each instrument; recorded the volume of material used to build each instrument; recorded the volume of air inside the body of each violin; and measured variations in the thickness of the thin layer of wood that makes up the top board and back board in each instrument. A number of the violins used in the study are from the collection of the Division of Musical Instruments of the Smithsonian’s National Museum of American History. The project was a collaboratin between the American History Museum and the Smithsonian’s National Museum of Natural History.

Many intricate and previously unseen details were revealed in the digital images, such as repair patches in a violin’s interior, the exact yet subtle slope of each back and front board, and the location of ivory and ebony inlays. Most importantly, when the data for each violin was compared, the researchers could see how the manufacture of the violins changed over time.

“The use of the scanner has improved our access to research data which otherwise would be inaccessible,” says Bruno Frohlich of the Anthropology Department at the Smithsonian’s National Museum of Natural History. “Obviously, we cannot take these instruments apart and study how they were made. Yet the digital models made with the scanner are factual representatives of the original objects and allow us to study how the instruments’ general architecture and other features have changed over time.”

One discovery the team made was that the volume of wood made in the construction of the violin bodies varied by 41.6 percent from 1670-1709, yet, the volume of air inside the violin bodies varied by only 8.2 percent. “Stradivari tried to keep the air volume in his violins as constant as possible, even as the trend of construction over time moved in the direction of a thinner wood board,” Frohlich says. The thickness of wood used in the construction affects the weight of the instrument, the strength and possibly tone.

With the success of the pilot study, the researchers now plan to analyze and compare data collected from scans of 47 other old instruments made by Stradivari, Nicolo Amati, Joseph Guarneri and other luthiers. When compiled this data should tell an interesting story of how violin making has changed, or remained remarkably the same, in the last 350 years.

Image: This 3-D model reveals the shape and volume of the air mass located inside the body of a Stradivarius violin.

The research team included Bruno Frohlich, Gary Sturm of the Division of Music, Sports and Entertainment at the National Museum of American History; Janine Hinton, of the Department of Anthropology, National Museum of Natural History and Else Frohlich of the Department of Biomedical Engineering, College of Engineering, Boston University. Support for the project was provided by Siemens Medical Solutions in North Carolina and Materialise in Belgium and Ann Arbor in Michigan.

Problem plastics in museum collections have conservators on alert

John Barrat — Wed, 12 Aug 2009 16:25:32 +0000

While society at large frets about plastics that pile up and seem to last forever— grocery bags clogging landfills and soda bottles washing up on beaches—museums worry about the thousands upon thousands of artifacts in their collections made of plastic, objects that in many cases are crumbling and cracking and sometimes damaging other artifacts as they break down.

“Plastic is a 20th-century material—it is everywhere,” says Jia-sun Tsang, a conservator at the Smithsonain’s Museum Conservation Institute. But most plastics—cheap to produce and adaptable to almost infinite uses—were not created for the ages. “When they get to a museum, it’s our problem. We have to take care of them.”

Photo: Jia-sun Tsang studies a set of 49 plastic samples from a 1920s Lumarith salesman’s kit in the collection of the Smithsonian’s National Museum of American History. (Photo by Donald Hurlbert)

Typical plastic degradation was Tsang’s shorthand for the problem before her. The object of her concern, from the collections of the Smithsonian’s National Museum of American History, Behring Center, was a salesman’s sample kit for a brand of plastic sold under the name Lumarith in the 1920s.

Easily melted and molded into virtually any shape, the product was popular with makers of everything from fountain pens and toys to electrical appliances. But with the passage of years, the kind of plastic used for Lumarith, cellulose acetate, broke down, giving off a telltale vinegar smell. Several of the brightly colored samples in the kit were warped, cracked, and unpresentable.

Tsang and a team of Smithsonian scientists used a variety of noninvasive analytical techniques, including several forms of spectroscopy to pinpoint the molecular structure of the plastic and figure out precisely the cause of the samples’ degradation. Tsang determined that much of the problem was due to leaching of triphenyl phosphate, a fire-retardant compound often added to cellulose acetate to facilitate its softening and flow during the molding process.

Tsang’s study concluded that triphenyl phosphate in cellulose acetate was a particular risk factor for plastic degradation, and that artifacts containing the chemical should not be stored in close contact or in an enclosed space with other cellulose acetate plastics and metals. Case closed, but the larger problem was far from settled.

Photo: This scanning electron micrograph image of a Lumarith salesman’s sample shows fully formed corrosive crystals consisting of oxygen (orange), carbon (green) and phosphorus (purple), as well as a crack in the upper left.

A year-long survey at the American History Museum revealed that many plastics in its collections are from among the five types that museum conservators have identified as “malignant.” Not only are these problem plastics—cellulose nitrate, cellulose acetate, polyurethane, polyvinyl chloride and rubber—more prone to deterioration than other plastics but also their breakdown often produces gases that damage metal, paper or other plastics stored in their vicinity. In most cases, this harmful offgassing can’t be prevented, only slowed, which means degrading plastic artifacts must be isolated from other objects in museum collections.

Photo: Residue from a silicon breast implant stains a storage box at the National Museum of American History. (Photo courtesy National Museum of American History).

Tsang’s specialty is conserving modern paintings and other contemporary art, so a history museum might not seem like her usual haunt. However, before entering the field of fine-art conservation, she was a clinical chemist at the Medical College of Ohio. “Because of my background, I always like to get into the science,” she says. —Mike Lipske