method one:
Water may be softened by any of the following methods:
1. For each gallon of water, use 2 tablespoons of a solution made by dissolving 1 pound of washing-soda in 1 quart of boiling water. The solution should be bottled and kept on hand, as it is a useful cleansing agent, or detergent.
2. For each gallon of water use 1/4 tablespoon of lye dissolved in 1 cup of water.
3. For each gallon of water use 1 tablespoon of borax dissolved in 1 cup of water
method two:
If water is temporarily hard, however, it may be softened by being boiled, then allowed to stand until the lime settles. The top water is afterward drawn off. Boiling water to soften it is without doubt the best method if it softens the water sufficiently, since no harmful chemicals are left in the water to injure fabrics.
Either temporarily or permanently hard water may be softened by adding washing-soda to the water, then allowing it to stand in open kegs for several days before its use. The water should then be drawn from the top.
This section is from the book "A Manual Of Home-Making", by Martha Van Rensselaer