Chemistry Pic of the Week: On Point

“I am ready and the words are beginning to well up and come crawling down my pencil and drip on the paper. And I am filled with excitement as though this were a real birth.”

-John Steinbeck, in a letter to his editor, Pascal Covici.

pencils2 - Copy.jpg

I’ve been away from the blog during the holidays and have returned with this picture of my desk. What you see is mostly some pretty standard writing pencils, a caseless pencil for drawing, and a carpenter’s pencil, which I like for making thick lines on the roll of butcher paper, which you can see I keep on my desk as a kind of moveable blotter. I sharpened up a bunch of pencils before I started working, as I tend to do, and got to thinking about them.

Pencils are, much like humans, most interesting for what’s inside of them, but their outsides are pretty interesting, too. There is a good chance that the pencil lying in a drawer nearby as you read this is made of incense cedar. If you’re not sure, pick it up, give it a quick sharpen, and smell it. It should smell coniferous, resinous, and vaguely like pine cones. To me, and perhaps to you, it might smell like the start of a project, second-grade pencil fights, and procrastinatory sharpening. In the increasingly dated 1998 rom-com You’ve Got Mail, Tom Hanks’s character writes to Meg Ryan’s character,

“Don’t you love New York in the fall? It makes me want to buy school supplies. I would send you a bouquet of newly sharpened pencils if I knew your name and address.”

To a chemist, smell is always interesting, because it represents one of one of the two most reliable senses we have available for direct chemical sampling and sensing. We are even trying to replicate it and improve it in various attempts at “electronic noses.” I recommend against it, but before the advent of reliable chemical tests and the concept of the atom, alchemists and early chemists tasted everything. The word “acid” comes from the Latin, acere, meaning “sour.” This hold true in other languages significant in alchemy and early chemistry, like Arabic, where the word hamdhiyat, meaning “sour things” still means acid to this day.  In cedars and pines, the smells we most vividly associate with them are with a class of compounds called terpenes.

If you’re linguistically astute, you might notice the similarity to the word “turpentine,” which is the distillation of pine resins. Terpenes are found abundantly in plants, and are responsible for all kinds of scents that we normally associate with them. The most obvious and famous terpene is probably pinene, that fresh pine scent that we put in everything from high-end perfumes to cheap car-deoderizers (which are of course, shaped like little pine trees). But the category includes limonene (in lemons) and humulene, which can be found in plants ranging from oranges, to hops, to various spices, and even tobacco and cannabis.

The reason incense cedar is picked for pencils is not its smell, however, but how easily the wood sharpens, and how easy the trees are to grow and farm. It’s the inside of the pencil, as I said earlier, that carries something far more interesting and potent: Graphite. The word itself is interesting, literally meaning “writing mineral” or “writing stone.” It was, initially called plumbago, because of its color, shine, and softness, it was thought to be a kind of lead ore, at first. The Latin word for lead, plumbum, explaining the chemical symbol Pb, and why people who fix water pipes are called plumbers–an artifact of the unfortunate tendency the Romans had to use lead for pipes and drinking vessels. It was also useful for writing, and making soft marks on surfaces, much like old lead styluses used since the Ancient Romans.

Soon enough, it was realized that this black rock that flaked onto surfaces had nothing to do with lead. Trying to untangle precisely when this happened and who did it turned out to be surprisingly difficult. Looking at old documents, it was confused with a lead ore known as galena for a long time, and it’s exact discovery is probably obscured to me both by the amount of time I want to spend researching this short post, and my inability to read German. In any case, it appears to have been Carl Wilhelm Scheele who discovered that graphite was not a form of lead, though it would be someone else to give it the name, “graphite.” Scheele’s life as a chemist was very interesting, and is worth picking up on in a later post.

Graphite itself is, of course, carbon, in layers of graphene. It’s this structure of sheets lying on top of each other that makes graphite what it is. Contrary to popular belief, graphite and diamond are not the same material. They are both different arrangements of carbon. While that may make them seem to be the same thing, let me remind you that the lyrics to your favorite song and the words in a toilet installation manual could technically be the same words, arranged differently. Graphene is one atom thick sheets of carbon, and while these sheets are very strong horizontally, they slip off of each other quite easily, making them suitable for drawing, and for lubrication. It’s this tendency for graphite to slip along the graphene planes like playing cards might slip over each other, that allows it to be deposited onto paper with simple friction. This same slipping tendency is also why it makes a good lubricant for situations where moving metal parts have to move against each other, and especially where in application where you might not want to use grease.

Graphite’s strong graphene sheets have made it useful for creating strong, lightweight composite materials, perfect for aeronautics. It found high tech uses not long after its discovery in England. Graphite was a military secret and asset in Elizabethan Britain, where it was used to lubricate the cannonballs that helped make the British Navy the military powerhouse that it was. But perhaps the most interesting secret use of graphite was in nuclear reactors.

Nuclear reactors have an undeserved reputation for being volatile. From the relative few nuclear incidents and accidents that have occurred and been documented, the general public has gotten the notion that nuclear reactors are one false step from catastrophe. The reality is, as any nuclear engineer will tell you, that they’re really most often one small incremental change from simply not producing any power. A nuclear chain reaction has to be carefully managed or it “fizzles” and stops happening. One of the ways that a nuclear reaction has to be managed is using something called a “moderator.” Moderators slow down neutrons so that they can more easily be captured by uranium atoms and those atoms can then fission producing more neutrons for the chain reaction. If the neutrons move too fast, they essentially whiz past the uranium atoms too fast for a nuclear reaction to happen. Graphite happens to be a great moderator, slowing neutrons down to the right speeds.

When the British started their own nuclear program post-World War II, they built a reactor at Windscale. The problem was that the Americans kept their cards close to the vest, and didn’t really want to share nuclear secrets with Britain, despite their alliances and the fact that numerous British scientists had worked on the Manhattan Project. The British knew graphite was an excellent moderator, but they did not know about an effect called “Wigner expansion” or “Wigner growth.” When neutrons bounced off of a carbon atom, they’d sometimes hit it hard enough to push it out of the crystal structure of the graphite. After a while, the graphite would start to expand due to this effect. If you knew about it, you could build for it. The British, initially, did not know about it, and they didn’t build for it.

There was no room for the graphite moderator to grow, and the graphite would form stresses and cracks. The scientists at the Windscale reactor would solve this problem with a process called annealing, which involved heating the graphite. Understaffed, in the middle of a flu epidemic, and engaged in a desperate nuclear arms race, the Windscale reactor was working furiously in October of 1957. The reactor fuel was metallic uranium, which is flammable in air, and was contained in metal tubes. They also had tube of flammable lithium and magnesium to produce tritium, an isotope of hydrogen necessary for hydrogen bombs. Flammable is actually an understatement. These materials were pyrophoric, meaning that they would burst into flame on contact with air. Air was used to cool the whole operation. To reiterate:

 

  • The staff was overworked.
  • The fuel was flammable.
  • The graphite was flammable.
  • The tritium precursors were flammable.
  • Both the precursors and the fuel would ignite on contact with air.
  • The reactor was air cooled, with industrial blowers pushing air through the piles.

In addition the controls were cumbersome and slow. In an era before computer microprocessors, much of the reactor control was manual. After an annealing cycle, temperatures began to rise uncontrollably in sections of the reactor, culminating in a full fledged and disastrous fire, which burned up radioactive materials and carried them in smoke. 200 square miles north of the reactor were contaminated, much of it dairy land. The official inquiry blamed the fire on the graphite annealing process. The reality, based on subsequent thinking, and espoused by nuclear engineers such as James Mahaffey, is that one of the canisters with flammable materials was very likely to have burst. It remains the worst nuclear accident in British history. One of the more interesting things about the Windscale fire was its location, only about 13 miles from the town of Seathwaite, in Cumbria, England. That’s where graphite was first discovered, and was used to mark sheep. Of course our atomic age mineral and then later, a space-age one, started life as a humble pencil.

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