A New Way To Preserve Old Bones

The world’s museums are full of old bones, the remains of humans and animals dug up at various sites and stored for archaeological and paleontological studies. Those bones tend to be fragile, having lost minerals during their years underground. Conservators try to preserve the bones by treating them with vinyl or acrylic polymers to fill in cracks and hold pieces together. But such polymers aren’t all that compatible with bone, and after a few years the polymers start to yellow and pull away from the bone. Now a team of Italian scientists, borrowing ideas from dentists and mollusks, have come up with a new way to preserve these remains that is potentially more compatible with bone’s natural makeup (Langmuir 2014, DOI: 10.1021/la404085v).

The researchers, from the University of Florence, decided to use calcium carbonate instead of the normal polymers, because it’s known to bind well to bone. The scientists tested their method on fragments of human bone less than a square centimeter in size, obtained from a church in Milan and said to be relics of St. Clement from the late Middle Ages. The team cleaned and disinfected the fragments, then brushed on a suspension of calcium hydroxide nanoparticles in isopropyl alcohol. Calcium hydroxide reacts with carbon dioxide in the air to form calcium carbonate. They let the solvent evaporate, and then repeated the treatment up to 10 times, depending on the quality of the bone.

Under most reaction conditions, the resulting calcium carbonate would take the form of the mineral calcite. But because of traces of collagen in the bones, the reaction led to the formation of aragonite, a metastable form of the crystal. It took two weeks for complete carbonization.

Chemist Luigi Dei, who led the work, says the collagen acts as a template for the nucleation of the aragonite crystals, which don’t form under normal atmospheric pressure. Mollusks use collagen as a template to build the nacre that strengthens their shells. Dentists also treat the walls of cavities with calcium hydroxide when they’re filling teeth, which contain collagen, for both its antimicrobial and tooth-strengthening effects.

The needlelike shape of the aragonite crystals strengthens the bone considerably. Using various methods, the team measured the strength of the bones before and after treatment. They found that the fragments had been strengthened by 50 to 70%. When the researchers ran the tests three months later, they saw the same results. Dei says the material should remain stable for a very long time, because mollusk shells have been found with the nacre intact after several thousand years. But his team hasn’t yet performed long-term tests of the process.

He says the next step will be to work with the university’s museum of natural history to test the treatment on bones of various ages and in various conditions of decay. He’s hopeful he can convince conservators that this approach is a superior way to preserve bones.

C. Wayne Smith, director of the Archaeological Preservation Research Laboratory at Texas A&M University, says Dei’s approach seems plausible to him. He and many archaeologists prefer treatments that are reversible, which this approach is not. But waiting for the development of a reversible method is not always practical, he says. “More artifacts are lost every year because conservators choose to do nothing until a better process is invented,” Smith says.