Making washing soda is a very simple process. You basically need to bake “baking soda”.
Tools: An oven, a pan, a heat-proof utensil and the baking soda.
Pour out the baking soda onto the oven-safe pan and spread it out to a thin layer. If you need a lot, use two pans.
It is important that the layer is thin because that way the heat can penetrate all of it more quickly and you won’t have to keep your oven on for so long.
Heat your oven to 200 – 230° C (400-450° F). Place the pan(s) of baking soda into your heated oven for about 1 – 1.5 hours. I stirred the baking soda around while it was in there.
The baking soda has turned to washing soda when it gets a slightly more grainy texture, less shiny, grey colour. Baking soda is fluffy, powdery, and slightly shiny.
Remove your pan of washing soda from the oven and allow it to cool completely. Then use it to make your laundry detergent or just store it in a clearly labelled container. I usually make a lot and use part of it immediately to make liquid laundry soap and part of it I store (it takes much less space as powder than liquid laundry detergent).