Preheat the oven to 300°F/150°C.
Line a half size baking sheet with parchment. I like to use two pieces of parchment and secure it with clips – it contains the baking soda much better than just one piece. You can also use aluminum foil if you prefer.
Weigh the lined pan on a kitchen scale and make a note ofthe weight. Zero the scale and add the baking soda, weighing out 16 ounces, or about 450 grams.
Spread the baking soda over the parchment evenly. Place it in the oven on the middle rack and bake until the total weight is reduced by 30%, stirring every 30 minutes (This could take 1 to 2 hours). To determine how much weight has baked off, place the baking sheet with baking soda on the scale to weigh it, subtract the weight of the pan and parchment – the leftover weight is the baking soda. If you are baking 16 ounces/450 g of baking soda, the final baking soda weight should be around 300 g.
Allow the baking soda to cool completely on the pan before using it or storing it, around 1 hour. Pour it into a container and seal or cover it so that it’s airtight. Do not touch the baked baking soda with your hands, as it is now a caustic substance. If you do come in contact with it, wash thoroughly with soap and water. Also be careful to not inhale near the baking soda or touch your face or eyes when handling it.
Store in a cool dry place – it will store indefinitely.