The quality of appearance and flavor of a cup of tea is affected by the water used in its preparation. The Chinese writer, Lu Yu, recommended spring water as the best. Today, most people rely on their domestic piped water supply and the quality, natural mineral content, and additional constituents such as fluoride and chlorine vary from region to region. Some tea blending companies create special blends for particular areas so that the local water will bring out the best from the leaf.
If tea is brewed using distilled, soft water or permanently hard water (containing calcium sulfate CaSo4), the infusion is bright and clear. If temporary hard water (containing calcium carbonate CaCo3) is used, the tea can be dull and flat and, if left to stand for only a short time, acquires a fine film of scum on the top. This is due to oxidation of tea solubles caused by the presence of the calcium and bicarbonate ions in the water. To prevent the formation of scum, avoid using temporary hand water or pass it through a water-softening filter prior to boiling.
The addition of acid to tea helps to eliminate the bicarbonate ions, so scum does not form if lemon is added. Adding sugar to tea also reduces the development of surface scum but the use of sugar is detrimental to the flavor of the tea and is not usually recommended. The addition of milk to tea made with hard water may help by lowering the temperature of the tea and therefore reducing the oxidation process that causes the scum, but experiments have shown that milk actually increases the amount of film on the surface, although reduced fat milks produce only half the amount of scum produced by full-fat homogenized milks.