Heat until soap is nearly dissolved, stirring occasionally.
Turn off heat and add Borax and washing soda. Stir gently to dissolve.
1/2 cup washing soda, 1 cup Borax
Add remaining 1 gallon of water. Stir to combine.
Cover pot and allow to sit overnight.
Notes
Store the soap in a large container with a lid. The glass container you see here is a one-gallon container and holds only about 2/3 of the recipe; the remainder is in another sealed container. I’ve opted for this container because the lid is easy to lift off.
A two-gallon bucket works well, as do a number of upcycled containers.
Keep a scoop handy for transferring soap to the washing machine. Use 1/8-1/4 cup per load.