The metallurgical nature of solid pure iron can be studied from the experiment as described here. A bar of pure iron (e.g., 25 mm in diameter) is sectioned to form a thin disk in the shape of a quarter. A face of the disk is now polished on polishing wheels, starting first with a coarse grit polish and proceeding in steps with ever finer grits until the face has the appearance of a shiny mirror. The shiny disk is now immersed for around 20 to 30 seconds in a mixture of 2 % to 5 % nitric acid (HNO3) with methyl alcohol, a mixture often called nital (‘nit’ for the acid and ‘al’ for the alcohol). This process of etching causes the shiny surface to become a dull color. If the sample is now viewed in an optical microscope at a magnification of 100 ×, it is found to have the appearance as shown in Fig 1.
The individual regions, such as those numbered 1 to 5, are called iron grains, and the boundaries between them, such as that between grains 4 and 5 highlighted with an arrow, are called grain boundaries. The average size of the grains is quite small. In the figure at the 100× magnification, a length of 200 microns (1 micron is 0.001 mm and is also known as micrometer) is shown by the arrow so labeled. The average grain diameter for this sample has been measured to be 125 microns. Although a small number, this grain size is much larger than the grain size of most commercial irons. For a comparison, the thickness of aluminum foil and the diameter of a hair are both approximately 50 microns.