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Chemical Bonds

How the world is held together

Our ability to understand how materials behave has been central to the story of human progress over the years. One of the best ways to begin to understand this important subject is to read a book written more than 40 years ago.


Very few science books are worth reading 40 years after they were written. Only a few classics—like Richard Feynman’s books on physics or perhaps some of Richard Dawkins’ books—are so well-written and clear that they still make for good reading. James Edward Gordon’s book, The New Science of Strong Materials: Or Why You Don't Fall Through the Floor, belongs in this unique category. Despite the word “new” in the title, was written back in 1968.

I have always wondered why some materials are strong and some are weak, why some crack under pressure and others flow, and what materials can be used for building bridges and tall buildings. There are so many amazing things about the properties of different materials. Why is it when you look around for tall things they are mostly made of iron or wood? What is special about metals? Why is iron with various impurities such a unique material? How does nature build strong things like wood? Why is a diamond hard but not strong? What is the magic of composite materials?

A few years ago, I toured the Hagia Sophia, a 1,500-year-old cathedral (now a museum) in Istanbul considered to be one of the greatest building projects of all time. Looking up at the enormous central dome and smaller half domes, I marveled at the genius of the ancient engineers and architects who figured out how to span pumice bricks across an area 200 feet by 100 feet, and 240 feet high, with no supporting pillars or beams. Sophia’s builders understood enough about the science of materials and the physics of large structures to know that they needed to maintain a state of compression in critical regions of the building. But not everyone back then did, as Gordon wryly notes: “It is not surprising that the roofs of churches continued to fall upon the heads of their congregations with fair regularity throughout the ages of faith.”

I always thought that at some point in the evolution of chemistry, after we understood crystalline structure, we would begin to learn why some things are strong. Unfortunately, that never really happened. I went and bought some graduate texts on materials but they assumed I already knew a lot about concepts like dislocations and different type of materials. What I wanted was a recapitulation of the history of materials told in an interesting and approachable way. That is what The New Science of Strong Materials provides.

There are a lot of words used to describe materials, such as strong, brittle, tough, or ductile. What do these terms really mean and where do these properties come from?

Gordon starts by explaining how chemical bonds matter. It you push down on a material, the bonds vary in terms of how much they give elastically, which accounts for the stiffness of the material. Lots of materials, like rock, can take a huge amount of compressive force before breaking.

There is a big difference in how a material behaves when you push down on it (compression) versus applying force to pull it apart (tension). Rock, brick, or normal cement can take a lot of weight in compression but then will break apart very easily under tension due to cracking. Under compression, most materials deform by about 1% before they break. Under tension, only metals and composite materials can reach that level.

Amazingly, the science of cracks was not well understood until the 1960s. Gordon and his colleagues were key players in making the breakthrough in understanding. This was important because lots of ships used to break in two and bridges used to fall down because engineers weren’t able to compute stress levels. They didn’t understand that holes in a sheet of material—even small ones like a hatch on a ship or a bolt hole in an airplane superstructure—allow stress to build up in a dramatic way.

Cracks in glass come mostly from imperfections on the surface. If you can make the surface super smooth, it is very strong and can be stressed over 2% without cracking. Cracks in solid materials are different because they result from the way stress builds up in one location in the crystalline structure.

Gordon has a love of natural materials such as bones, teeth, insect cretin, and wood. He thinks we have a lot to learn from nature and he correctly predicted that there would be breakthroughs in composite materials based on the way nature defeats cracking by having “weak interfaces.” Fiberglass, which was already being used in the 1960s, was an early step in composite materials. Gordon explains how the combination of glue (resins) and fibers together makes for a strong and crack resistant material. Glues have always confused me. Even after Gordon’s explanation I need to study them a bit more.

The book closes with an explanation of metals, particularly iron in its various forms—steel, pig iron, wrought iron, and all the various alloys of steel that can be made to withstand heat or corrosion. The key to why metals don’t crack is that the various layers of the crystal structure slide over each other, a characteristic called ductility.

Iron and many other metals in their pure form are too ductile. To make a strong material, you have to avoid ductility (flowing under pressure) and brittleness (cracking). Iron with impurities (particularly carbon at about 3% by weight, but 15% by the proportion of atoms) strikes this balance very well. Iron ore has some carbon in it so the history of smelting is figuring out how to achieve the high temperatures needed to get the carbon level just right. Iron is a key material for civilization and its price came down by a factor of over 10 during the 1800s as it fueled the industrial revolution.

I admit this book is not for everyone, but if you’re fascinated about how our world is held together, it is a very cool book that I recommend. If you’re interested in the science of engineering and the physics involved in building the world’s greatest structures, I recently wrote about a fascinating course offered by the Teaching Company, called Understanding the World’s Greatest Structures. For the curious, both are certainly worth the time.