Temporary hardness is determined by the measure of bicarbonates. The hardness that bicarbonate ions impart is temporary because it is easily precipitated and is removed when water is boiled or treated with certain acids.
A bicarbonate measure greater than 100 ppm is undesirable because of its contribution to the alkalinity of water, which in turn, imparts harsh and bitter flavor to beer. Highly alkaline water creates an imbalance in pH affecting adequate mash conversion and, if used for sparging, will extract undesirable harsh grainy flavors.
Permanent hardness is determined by the measure of calcium and magnesium ions, the calcium being more significant. It is that portion of total hardness remaining after the water has been boiled.
Generally, permanent hardness and the calcium ion lower the pH of water. A certain amount of permanent hardness is desirable in the homebrewing of all-grain beers-where enzyme conversion works best at a mash pH of 5.2 - 5.6.