Clark's Degree of Hardness
Water hardness measured in grains per gallon.
Origin
Devised by Dr Thomas Clark, Professor of Chemistry at Marischal College, Aberdeen, and patented in 1841 alongside his water softening process.
The catch
One Clark degree is one grain of calcium carbonate per imperial gallon. Clark built his scale from a medieval weight and a gallon that was barely seventeen years old when he patented it, and both have since left everyday use, so the scale now reads as a fossil twice over.
Soap lathers freely. Typical of upland reservoir supplies in Scotland and Wales.
Little scaling. Much of northern England and the south west.
Noticeable kettle scale over time.
Persistent scaling. The Midlands and much of central England.
Heavy scaling, poor lathering. London and the chalk south east.
Severe scaling. Parts of East Anglia and the Thames valley.
Clark invented not only the scale but the remedy: adding slaked lime to precipitate out the calcium carbonate, a process still used in municipal water treatment. His scale mattered because hard water cost money, scaling boilers and wasting soap on an industrial scale, and a city considering a new water source needed a number to argue about. Modern British reporting uses milligrams of calcium carbonate per litre, where one Clark degree is about 14.3 mg/l.