Our featured artifact is currently on display in the Environment exhibit.

Man-made rock--the answer to tough environmental challenges?

Can you tell which rock is natural and which is man-made?

Obsidian, the naturally occurring mineral (on the left), was formed from rapidly cooling lava. This black, glassy rock, common to many volcanic areas, formed a million years ago after a volcanic eruption here in the Jemez Mountains.

The man-made rock (on the right) was made by heating gravel, cobbles, and soil with electricity. It was formed in a test of a technique (called vitrification) to isolate and immobilize radioactive contaminants in soil. If it proves feasible, the technique could be used in Los Alamos and other areas where it is too expensive or too difficult to remove or clean contaminated soil.

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Rodder Microbalance: Weighing Invisible Material with an Invisible Balance

Jerry Rodder, the inventor of this weighing device, became interested in microbalances while he was a chemistry student in the 1950s. Rodder figured out how to make quartz microbalances more precise and rugged while he was stationed in the Army. Without official leave, Rodder went to talk about his idea with his former professor, who remarked that it was "much too complicated and would never work."

 

But Rodder was convinced, rather than discouraged. Initial models were extremely fragile, but over time Rodder was able to develop a relatively sturdy, reliable, extremely accurate, and fast tool for measuring very small changes in mass (or weight). After nearly five years of development, Rodder completed construction of his first microbalance in about a month. In 1957, he founded Microtech Services Company and sold microbalances to universities and research organizations around the world.

 

This microbalance, built around 1970, is one of only three like it designed and built by Rodder. Because it was used in experiments under vacuum, the balance was housed in a glass chamber rather than the usual aluminum housing. Like a ship in a bottle, the beam with all the fine quartz fibers was constructed first and then placed in the Pyrex housing. Rodder, like a spider spinning a web, then fused the beam in place with the torsion fibers, working quickly so they were not weakened by excess heat. The beam was tied down to prevent it from moving during the glass-blowing operation. "It was a scary sight to see the balance rotating in the lathe," said Rodder. Finally, the end caps were attached and sealed to complete the housing. To ensure that this balance arrived at the Laboratory safely, Rodder flew to Los Alamos with it on his lap.

 

Scientists used this balance to measure the effects of heat on tiny samples of graphite containing traces of the element cesium. (Graphite was used to slow down the chain reaction in the reactors they were studying; cesium is a by-product of the fission reaction.) The sample, which hung below the base of the balance, was put under vacuum, inserted into a small laboratory furnace, and heated slowly over 1-2 weeks to a temperature of 800°C (1472°F). The experiments showed how much cesium vaporized at various temperatures over time.

The mass (weight) loss of a sample was measured electronically, by tracking the change in voltage in an electronic sensing unit (not on display). As the sample was heated, it emitted gas and became lighter, causing the magnet on the other end of the beam to move down inside the coil. As the magnet's movement changed the current, the voltage also changed. A strip chart (like a seismograph or lie detector chart) recorded the changes in voltage, which translated to changes in mass. The two platinum-coated quartz light deflectors were used to keep the beam steady. If the beam was in balance, they blocked light from reaching two sensors; if not, the sensors signaled an electromagnet to adjust the balance.

 

This balance is similar to less sensitive and more fragile models used here and elsewhere in the 1940s for the Manhattan Project. Glenn Seaborg, co-discoverer of plutonium, eloquently described this work as "invisible material was being weighed with an invisible balance." (Quotation from The Making of the Atomic Bomb.) Rodder remarked, "There was a large volume describing how to construct the balances used for the Manhattan Project. If I had read it, I would never have built my first balance."