What Is Washing Soda? (And How to Make It From Baking Soda)

Step by step

1. 1

   Pick a pan that is not aluminum

   Use a glass or stainless steel baking dish. What comes out of the oven is washing soda, and the washing soda label says not to use it on aluminum.

2. 2

   Weigh your baking soda

   Set the empty dish on a kitchen scale and zero it out. Pour in 16 ounces (1 pound) of baking soda and write that number down. You will need it in Step 5.

3. 3

   Spread it thin

   Shake the dish or use a spoon to spread the baking soda into a thin, even layer. A thick pile may not finish in the time given, so keep it flat with no deep spots or mounds.

1. 4

   Bake at 350°F and stir once

   Bake it at 350°F for 30 minutes. Take it out with oven mitts, stir it, and put it back for up to 30 minutes more. Do not try to judge it by eye. The scale in the next step tells you when it is done.

2. 5

   Weigh it again

   Let the dish cool enough to handle, then set it back on the scale. Your 16 ounces of baking soda should now weigh about 10 ounces. If it still weighs noticeably more than that, spread it out again and give it more time in the oven.

   What it looks like when it worked: the scale reads about 10 ounces, about a third less than you started with.

3. 6

   Cool it, jar it and label it

   Let the powder cool all the way. Put your gloves on, because from here on you are handling washing soda. Spoon it into a dry jar with a tight lid and write on it in big letters: “WASHING SODA. NOT FOR COOKING.”

   Store the jar away from food and away from vinegar and other acids. The safety sheet for sodium carbonate says to keep the container tightly sealed, store it away from foodstuffs and never store it with acids.

When it will not work, and what to do instead

Sometimes the homemade batch does not come out right, or it is not the tool you need. Here is what to do.

• The weight barely changed. The layer was too thick or it needed more time. Spread it thinner, bake it longer and weigh it again.
• You only have an aluminum pan. Do not use it for this. Wait until you have a glass or stainless dish, or buy the box.
• You want to bake with it. Do not. The washing soda box says “Do not ingest” and that it is not a food product. Keep the jar out of the pantry.
• You were hoping to clear a slow drain. Arm & Hammer’s directions and the box label do not include drain cleaning. Start with a plunger, and see Does baking soda and vinegar unclog a drain?.
• You were hoping it would disinfect. It will not. The box makes no germ claim. See Clean, sanitize or disinfect: what the label really means.

Why it works

Here is what the oven is doing. Heat splits baking soda into three things: washing soda, water and carbon dioxide gas. The water and the gas float off into the air, and the washing soda stays in the dish.

According to safety data quoted by PubChem, that change starts above about 122°F and is finished by about 518°F. Your oven at 350°F sits in between. That is why a thin layer and a little patience matter.

The weight check works because of what left the dish. Think of a wet sponge that weighs less once it dries out. Here the baking soda gives off gas as well as water, so it ends up about a third lighter.

Arm & Hammer says soda ash, the form with no water, is more concentrated than the box. Your oven batch has had its water baked off, so treat it with at least the same respect.