The acidity or alkalinity of the water used for natural dyeing (both in the mordant bath and the dye bath) will affect the colour. Soft water is best for practically all natural dyes with the exception of madder, weld, logwood and brazilwood. These dyes develop better in hard water (containing calcium and magnesium salts). Most natural dyers consider rainwater best (although in places it may be more contaminated than tap water), river water next best, and well or tap water the last choice as it often contains the largest amount of contaminants.
For dyes that prefer hard water, calcium carbonate can be added in the form of finely ground chalk, or an antacid (Tum’s, Rolaid) tablet. Also soda ash, household ammonia, or wood ash water can be added to push the pH up.
If local hard water needs to be made acidic, add vinegar, lemon juice or a few crystals of citric acid. Water that contains iron is difficult to use for natural dyeing as it will not be possible to achieve clear, pure colours. In this case colours will be “saddened” that is, muted and darker.
Neutral (pH7) water should be used for rinsing and washing naturally dyed fibres and fabrics, otherwise there may be unwanted colour changes. A set of pH strips is a good way to test the water.