Mad About Science: Sharp Things

By Brenden Bobby
Reader Columnist

Sharp tools are one of the most important things human beings have ever developed. Where would humankind be were it not for the invention of the spear, the kitchen knife or Legos left on the floor after dark?

You have probably never thought much about the amazing idea of cutting things. In so doing, you are literally severing the chemical and atomic bonds that hold an object together, dividing matter so as to make it smaller and easier to handle. So how does it work?

Cutting objects follows a very basic principle in physics: Force applied to a wedge is amplified at its finest point. The finer the edge of the wedge, the more effective is the force multiplier. However, if that edge becomes too fine, you risk breaking the wedge at its weakest structural point.

“But Brenden! What about paper cuts?”

A worked shard of obsidian. Courtesy photo.

That is an excellent question, Strawman! Paper has a very fine edge that can prove effective at cutting skin, but only at certain angles. The force applied to a piece of paper must be precisely directional, with minimal downward force to avoid compromising the structural integrity of the paper, yet given appropriate lateral force to facilitate the paper’s edge in slicing through the chemical bonds of your epidermis. If you were to strengthen the structural integrity of the paper — perhaps by encasing it in epoxy or laminate — but retaining its fine edge, you could create a very potent cutting implement.

Throughout history, our default sharps have often been made of minerals, both natural and artificial. Obsidian — sometimes called volcanic glass — has been used by humans to cut things for tens of thousands of years. Obsidian is formed in the Earth’s mantle, where tremendous amounts of heat energy and pressure are applied to silicate (sand) to create a sturdy mineral structure that can be flaked away and sharpened to produce a cutting edge that can rival some 21st century surgeons’ scalpels. It’s no wonder Jon Snow needed so much of this stuff to kill the White Walkers — assuming anyone still gets that antiquated reference.

Around 5,000 years ago, humans started developing cutting tools made from bronze, primarily spear tips used in warfare. Bronze was cheap, malleable and easy to produce by smelting tin and copper together at around 1,700 degrees Fahrenheit. This was difficult to achieve with a basic wood fire, so blacksmiths of the time depended on charcoal in enclosed spaces such as forges or bloomeries to reach heat levels high enough to melt these metals. Charcoal is practically pure carbon, made after evaporating the water still locked away in wood, as well as the other chemicals like nitrogen after it has been cut. Creating charcoal is an involved process we’ll talk about in-depth another time.

The Iron Age began sometime between 1200 and 500 BCE, and ushered in a new era of metals. Iron has a much higher melting point than bronze, requiring 2,800 degrees Fahrenheit to become liquid. While bronze could be shifted into a liquid form and poured into a mold to mass produce things like spear tips, axe heads and even early swords, ancient blacksmiths lacked the energy sources to liquefy ferrous iron. Instead, they heated iron into a softer state and hammered it into desired shapes. This was a laborious task, but it produced a markedly superior product to mass-produced bronze.

Bronze implements are easily damaged by applied force — especially when striking armor — whereas iron is capable of delivering multiple blows before breaking. The difference would be akin to bringing a slingshot to a dodgeball game.

It wasn’t long into the Iron Age when metallurgists discovered that adding carbon to hot iron strengthened it and produced a vastly superior metal: steel. Adding carbon to iron lowers the melting point to around 2,500 degrees, requiring less energy to shape and mold it into a cutting implement — which, importantly, offered a longer window for blacksmiths to work the steel before it cooled. Swords and knives made from steel would be sharpened with a grinder, that is, a coarse stone wheel that would spin to deliver energy via angular momentum to scrape away excess metal and create an edge. It would then be buffed by a similar device to create the reflective metallic sheen we appreciate from steel objects today.

In the past 150 years, we’ve discovered new ways to create very high temperatures, primarily with the aid of fossil fuels, vacuum chambers and advanced insulation that can trap and retain immense levels of heat, which allows us to mass produce things like knives by melting steel into molds.

Metals aren’t the only structures capable of making very sharp objects. Ceramics, which you would normally find at your local pottery class, can also be formed into very sharp edges and then glazed with an enamel to create a structurally strong, razor-sharp cutting implement.

If you’re looking for the sharpest object ever created, look no further than the tungsten needle. Specialized tungsten needles have been developed using a combination of heat, pressure and magnetic alignment to create a point so fine it can separate and manipulate individual atoms.

I wouldn’t recommend carving the turkey with a tungsten needle, though. That would take forever.

Happy Thanksgiving! Stay curious, 7B.