The “hardness” of a material describes its ability to withstand surface indentation and permanent, localised deformation caused by direct contact with another substance. In simpler terms, it tells us how likely a material is to be scratched or abraded by other materials. The hardness of a mineral is controlled by the strength of its atomic bonds, and the structure of its crystal lattice. Considering bismuth’s metallic properties and density, you may be surprised at where bismuth stands in comparison to the hardness of other materials.
One of the most common ways to measure hardness is the Mohs hardness scale. This is a relative scale, used in geology to compare different minerals’ resistances to being scratched. The scale ranges from 1 to 10, with the softest minerals, such as talc, at number 1, and the hardest minerals, like diamonds, at 10. The hardness of any mineral can be easily hypothesized by trying to scratch the mineral with different materials. If a mineral can be scratched with a fingernail, which has a hardness of 2.5, we know it is at the lower end of the scale. If a mineral can resist being scratched by a steel nail, which has a hardness of 6.5, we know it is on the higher end of the scale. This easy test is often used to help identify unknown minerals.
Bismuth a colorful metal, low on the Mohs scale of hardness.
The hardness of a mineral is also what determines if it leaves a residue behind. Some different minerals that all appear to be black, will leave different coloured streaks on a ceramic plate based on hardness and composition. A common example of this is the graphite used in pencils. Graphite has such weak bonds between its layers of carbon that it easily separates and leaves a streak of graphite on the paper. Alternatively, Diamond, which is also composed purely of carbon, has such a strong crystal lattice that it refuses to be pulled apart and cannot be easily scratched.
Bismuth, though it is a heavy metal and is incredibly dense and brittle, is actually a very soft metal and sits at about 2.5 on Mohs hardness scale. That means raw bismuth has about the same hardness as your fingernails. Bismuth is soft enough that it leaves a light streak behind when scratched on paper. A point of note when testing the hardness of bismuth is that most bismuth has a layer of bismuth oxide on its surface, a result of the metal reacting with the oxygen in the air. This colourful surface layer has a slightly higher hardness at about 4-5 on the Mohs hardness scale, but this layer is so thin that it is quite easy to scratch through this harder layer into the soft silvery bismuth beneath. It is important to note that this hardness rating describes only its ability to be scratched by other substances, and is not to be confused with malleability. Bismuth can indeed be scratched easily, but it is still not likely to be bent or stretched without snapping.