Tikalon Blog by Dev Gualtieri

Amber and Glass Flow

May 27, 2013

The better science fiction stories start with a relatively believable premise based on some scientific fact. In the days of Mary Shelley's Frankenstein novel, continuing up to the time of the best known Frankenstein film (Frankenstein, 1931, James Whale, Director),[1] there was the idea of electricity as a vital force . It was by electricity that Victor Frankenstein was able to bring his monster to life. This idea still lingers in occult circles.

At the time of its writing in 1990, Michael Crichton's novel , Jurassic Park , had the believable plot device that dinosaur DNA could be extracted from mosquitoes preserved in amber , and that dinosaurs could then be cloned . This plotline, mixed with today's emphasis on making money from scientific knowledge, made for a great movie .

Although I wrote about fossil evidence of giant fleas sucking dinosaur blood in a previous article (Dino Farts and Fleas, May 16, 2012), even if dinosaur blood-sucking mosquitoes were well preserved in amber, the possibility of extracting dinosaur DNA is now thought to be impossible. The "half-life " of DNA is now estimated to be only about 500 years , so 150 million year old dinosaur DNA is now lost forever.[2-3]

Amber is fossilized tree resin . The resin consists of terpenes and trienes , which crosslink over the years into a hard polymer mass. One type of amber, Dominican amber , dates from Oligocene to Miocene , so it's about 25 million years old.

A specimen of Dominican amber .

(Photograph courtesy of Gregory McKenna, Texas Tech University)

Although some specimens of amber may be crystal clear , amber isn't crystalline , it's a glass . The distinguishing property of a glass is its glass transition . Above the glass transition temperature , glass becomes a viscous liquid , and it can be manipulated, as by a glass blower .

One persistent legend involving window glass is that it continues to flow after its initial solidification. The evidence being provided is that newer window glass is flat, but window glass from cathedrals and the American colonial period are thicker at the bottom than the top.

Modern glass is produced by the float glass process in which the molten glass is formed on the surface of a pool of molten tin , so it's uniform in cross-section throughout. Windows from centuries ago were made from cut sections of glass produced by a spinning process. Spinning caused a thicker cross-section at the edge of the spun circle . Tradesmen would naturally mount the window panes with the thicker part downwards, since the thick edge makes a better base.

14th century Gothic rose window.

If window glass did flow at a rapid rate, the fine detail in this window would have dulled after six centuries.

(Via Wikimedia Commons.)

There's abundant anecdotal evidence that flow of glass at room temperature doesn't happen. Arrowheads made from obsidian (volcanic glass ) still have an edge after thousands of years. Glass artifacts have been discovered in Egyptian tombs in a state that appears to be what the artist intended. Furthermore, antique telescopes have no apparent distortion introduced by flow of the glass of their lenses .[4] Edgar D. Zanotto published a 1998 paper "Do cathedral glasses flow?" debunking the cathedral glass legend.[5]

Gregory B. McKenna of Texas Tech University presented the following calculation relating to the time scale for glass flow at room temperature .[6]

• Silica glass has a viscosity of 10^15.82 Pa -s at 1,000°C .

• Using the reported activation energy , we can calculate that the viscosity at room temperature would be much greater than 10³⁰ Pa-s.

• The Maxwell model allows an estimate of the relaxation time , τ, of a material as τ ≈ η₀/G, where η₀ is the initial viscosity and G is the shear modulus .

• Since G ≈ 28 GPa for window glass, then t ≈ 10¹² years.

McKenna, a professor of chemical engineering at Texas Tech University is coauthor of a paper that extends experimental evidence on glass flow to a 20 million year time scale using measurements on amber.[7-9] Although amber is a polymeric glass, not an inorganic glass as in windows, glass dynamics follow the same laws.[9] Joining him as authors are Texas Tech graduate student , Jing Zhao, and Sindee L. Simon , who is chair of the department.

Grgory McKenna and Jing Zhao of Texas Tech University

Jing Zhao and Gregory McKenna of Texas Tech University, two authors of the amber flow study.

(Photograph courtesy of Gregory McKenna.)

The study was undertaken as a result of experiments that Zhao had done on polyvinyl acetate for her qualifying examination for admittance to the Ph.D. program. The polyvinyl acetate results were unexpected, since they didn't show a divergence in the relaxation time.[8] At that point, they decided to look at an older glass to extend the time scale of the investigation.

The Texas Tech team did a series of calorimetric and stress relaxation experiments on Dominican amber. Their measurement at various temperatures of the amber's relaxation time, the time it takes for the molecules to rearrange themselves, showed no divergence. This means that it wasn't acting as a fluid .[7,9] Internal rearrangement of the atoms in the amber specimens was found to have resulted in an increased material density of just 2.1% in 20 million years.[7,8]

Going for another order of magnitude in time scale, McKenna has obtained a piece of Triassic amber that's 220 million years old. This was provided by Eugenio Ragazzi, a professor in the Department of Pharmacology , the University of Padua , Italy.[8,10] This research was funded by the National Science Foundation under a grant from the Division of Materials Research .[8]

Amber and Glass Flow

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