A catchy Bollywood tune starts with the phrase “While this world is made of pital I am made of pure gold”. ‘Pital’ is Hindi for brass, the yellow metal you often see in door knobs, jars to hold water, or even at times in decorative materials placed on shelves in your homes. The lady in the song is teasing that while brass is all pervading, it can’t really substitute for authentic gold.
Is brass on the periodic table?
On your smartphone, search for “the periodic table of elements”. A monstrous creature should appear, with rows of boxes with letters and probably little numbers. Each of these boxes represents an element – hydrogen (H), iron (Fe), silver (Ag), and so on. You will also find a number written inside each box: this is the number of protons and electrons in one atom of that element.
Protons, neutrons and electrons are the fundamental particles that make every element. For example, hydrogen (H) has one proton, carbon (C) has six, and iron (Fe) has 26! The size of an atom is broadly larger if it contains more protons, so an iron atom is much larger than the carbon atom, which in turn is larger than a hydrogen atom, and so on.
The periodic table is a table of all the elements in the universe; indeed, the periodic table is one of the most amazing achievements of many, many people, the culmination of several painstaking discoveries.
Now, go through the table and look for brass. If you see Br, that’s bromine, a rather poisonous liquid, not what you are looking for. You will soon realise brass is not even there.
What is brass?
Brass is not supposed to be in the periodic table because it is not an element. It is an alloy, a mixture of elements. Brass is an alloy of copper and zinc in some percentages. Both copper and zinc are elemental metals with their own properties, but if you mix them, like you mix masala in a bowl, you can get new ‘flavours’ of metals. In fact, you can continue to experiment and with enough tries, you might even create a new recipe that surprises everyone!
For example, consider iron – present in the weighing standards the vegetable vendor uses. Add a pinch of carbon (C) and mix, and you will get steel, which you might have seen Sunny Deol advertise as the “highest form” of strength. But then steel can rust, so add a teaspoon of chromium and you get stainless steel, which does not rust. No wonder then that in our houses, we use stainless steel spoons, which last for many years.
Why, empty your wallet and put the money coins on the table, and you will find a host of alloys. The one or two rupee greyish coin is stainless steel. The new ten-rupee coin has a copper-tin (bronze) mix on the outer, yellow rim; and the five-rupee yellow coin is the all pervading brass, where our story began.
Why is brass useful?
You might wonder why we use brass for so many things and not just copper. Why mix a bit of zinc in it? This is because copper atoms are quite large. The copper metal’s internal structure is really an array of copper atoms arranged in a neat sequence, like the rows of trolley cars arranged in an airport or in the supermarket.
But as we know from our experiences, and at times mishaps, it is relatively easy to push a complete row of trolley cars. Because of their neat arrangement, the cars don’t cut across each other’s paths. This is the reason copper is ductile: it can smoothly change shape under pressure, allowing us to make the copper wires that are ubiquitous in electricity transmission cables.
As it happens, zinc atoms are slightly larger than copper. If you go back to the periodic table, you will find zinc (Zn) sitting just to the right of copper (Cu). It has one more proton and so it is a bit bigger. So a few zinc atoms, in place of copper atoms, are like the misshapen trolley cars that are present in between the otherwise neatly arranged well-shaped cars.
Add zinc and it can wonderfully change some of its properties. These few misshapen trolley cars obstruct the flow of the rest of the ‘copper’ trolley cars. This is not unlike a traffic jam, where a few misaligned SUVs can completely frustrate the regular traffic. Now, it becomes very hard to move a series of trolley cars anywhere under pressure because they are all jammed into each other. If we try to move one, we soon find that we need to apply enough force to move all of them.
While such traffic jams can really be a damper on a vacation drive, physics-wise it is great: the metal has become stronger. It won’t bend easily under pressure, by falling on the floor, if you jump or sit on it, etc. The same physics happens in steel, where you jam iron atoms with the much smaller carbon atoms, and prevent smooth movement.
Remember the beauty of alloys
This is why it is no wonder (or maybe it is a wonder), when we want to make utensils or durable objects like door knobs, where there is a lot of rough handling, we tend to use a material that is a good survivor, and go for brass. As it happens, pure gold, as an elemental metal, is often quite soft and can deform easily under pressure.
So the next time you are grooving to a popular Bollywood number that compares brass unfavourably to gold, remember the beauty of alloys, of jammed trolleys, and that simple yellow metal.
Adhip Agarwala is an assistant professor of physics at IIT Kanpur.
