<Tr> <Td> Carbon - containing iron </Td> <Td> 140 </Td> <Td> 40 </Td> </Tr> <Tr> <Td> Pure, single - crystal iron </Td> <Td> 10 </Td> <Td> </Td> </Tr> <P> The mechanical properties of iron and its alloys can be evaluated using a variety of tests, including the Brinell test, Rockwell test and the Vickers hardness test . The data on iron is so consistent that it is often used to calibrate measurements or to compare tests . However, the mechanical properties of iron are significantly affected by the sample's purity: pure, single crystals of iron are actually softer than aluminium, and the purest industrially produced iron (99.99%) has a hardness of 20--30 Brinell . An increase in the carbon content will cause a significant increase in the hardness and tensile strength of iron . Maximum hardness of 65 R is achieved with a 0.6% carbon content, although the alloy has low tensile strength . Because of the softness of iron, it is much easier to work with than its heavier congeners ruthenium and osmium . </P> <P> Because of its significance for planetary cores, the physical properties of iron at high pressures and temperatures have also been studied extensively . The form of iron that is stable under standard conditions can be subjected to pressures up to ca . 15 GPa before transforming into a high - pressure form, as described in the next section . </P>

What is the purest form of iron called