@sudsmaster - Actually both the 1930's professional laundry manual, and my 1940's domestic laundry work book prefer sodium hexametaphosphate (polyphosphates) for dealing with hard water to other alkalis.
Failing phosphates the next choice would be borax then washing soda. Other agents include ammonia (not often used), slack lime, and soda ash.
Washing soda directions even going back to my laundry manuals from the 1800's is supposed to be diluted into solution with water, then added to the wash bath. Never added directly to the wash tub. Everyone agrees that washing soda used in excess is hard on textiles (not to mention hands if washing by that method), and excess use can lead to yellow spots or all over yellowing of laundry.
My 1930's commercial laundry manual was written by Norbert J. Berg, and is still in some ways considered a standard.
Advice for using phosphates or other alkalis for dealing with hard water is the same we have seen here in the group. Only use enough to make water "slippery". Too much water softening agent/alkali is just as bad as too hard water.
Soaps for laundry:
Most all commercial and domestic vintage laundry manuals in my collection written before 1940's all recommend soap over new fangled "detergents". The 1930 book by Mr. Berg gives an in depth chapter on various types of soaps: potassium,ammonium and or sodium hydroxide alkali based.
Various soaps had different purposes in the laundry, much having to do with what water temperature the wash bath would be, and how easily they rinsed in warm or cool water. It even speaks of my favourite "Savon de Marseille", which is vegetable oil based soap (makes a softer and more soluble soap that rinses easily at lower temps).
Before I mentioned "titer" when it comes to soaps. Again according to the book the solidifying point of the fatty acid used in making the soap determines it's titer.
Low-titer soaps are good for low temperatures, high titer soaps for high temperatures. The former are usually made from plam kernel, olive, cotonsee, coconut, maize, and soybean oils. Each of these oils will contain several different fatty acids, most all having low solidifying points.
High titer soaps are made from animal fats and or mixture of the same and vegetable oils. These soaps are for use in temperatures ranging from 160F to 212F.
While it is possible to use a high-titer soap in low temperature washes, it does not produce much froth, clean very well and is hard to rinse out of fabrics unless the water is at proper temperature. These soaps are the normal ones found on supermarket shelves back in the day for housewives in beads, flakes, bars, and powder forms.
Use of these high-titer soaps (be they made at home or store bought) explains all that use of hot or boiling water for washing and perhaps the first rinse housewives and others went through back in the day. If this wasn't done the result was soap residue in textiles.
For use in the wash, I follow the old French custom (and using a vintage recipe still used today), to make "savon liquide" from bars of Savon de Marseille. I use a carpenter's plane to scrape off thin sheets of any soap one wants to make into solution. On this side of the pond and in the UK housewives and laundries would have made the same or soap jelly.
Liquidied or gel soap is much eaiser to dose (even in with the Miele or other automatic washing machines), just open the dispenser drawer and pour whatever amount you wish, followed by a kettle of hot water to flush it down into the tub. However once you have determined the correct dose it is a simple matter of sending it down just as with any liquid detergent as the machine fills.
