There are several different terms for measuring hardness in water, you may have heard of a degree Clark or a grain and also the use of the modern metric ppm (parts per million) system. The traditional British method was once degree Clark but more recently we have adopted PPM.
So where did all these terms come from? Back in the early 1800’s Chemist Mr Thomas Clark was looking into how hardness formed inside his boiler and where it originated from. Hard water was a nuisance even back then! He filled a bucket full of water and then heated it, allowing the water to evaporate. The left over residue (calcium and magnesium that forms limescale) was stuck to the side of the bucket. So he weighed the bucket before evaporation and then again after, the difference between the two became the weight scale formed by the procedure of boiling.
The increase in weight was minimal, only a few grains and so the Clark Method for measuring water hardness was created. Although here in Britain we used the term Degrees Clark, over in America they still use the term Grain.
When it comes to expressing hardness in milligrams per litre or the modern PPM, it is much easier. Take a measured litre of water and evaporate it, just as Mr Clark did, measure the left over residue milligrams in it and you have your milligrams per 1 litre of water, your PPM. If you have 400 milligrams left your water hardness would be 400PPM (aggressively hard).