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How to make washing soda
Gather your supplies.
You will need a large metal baking dish (though stone or glass will also work), a one-pound (454 grams) box of baking soda, and a wooden spoon.
Preheat your oven to 400 f (204 c).
Cover the bottom of the baking dish with a thin layer of baking soda. Baking a thinner layer that's about one-quarter inch (0.6 cm) thick will ensure that the process occurs uniformly.