Baked Baking Soda (Sodium Carbonate) Recipe
from https://www.seriouseats.com/

Baked baking soda is particularly useful in recipes where a stronger alkali is desired, like when making pretzels for a deeper brown color or ramen noodles for a chewy texture. Baking baking soda transforms it from sodium bicarbonate into sodium carbonate, a stronger alkaline salt, which is useful in situations where a more potent alkali is needed, like when making pretzels or ramen noodles, as it can provide a more pronounced flavor and texture compared to regular baking soda; essentially, "baking" it makes it act more like lye without the dangerous aspects of handling lye directly.

Warning: Do not let the baked baking soda come in contact with your skin, as it is a caustic substance. If you do make contact with the baked baking soda, flush the area thoroughly with water.

920g (about 32oz) baking soda [this is a large amount, do by what is in the recipe it is for]

Preheat oven to 350°F (175°C).

Line a rimmed baking sheet with foil. Note the weight of the foil-lined baking sheet using a digital scale. Spread baking soda in an even layer and place baking sheet in oven.

Bake, stirring the baking soda every half hour, until the weight of the baking soda is reduced to 613g (~21 oz), which is 2/3 of the starting weight (weigh the baking sheet with the baking soda, then deduct the recorded weight of the lined baking sheet to determine the weight of the baking soda itself). The amount of time this can take can vary quite a bit, anywhere from 2 to 5 hours.

Remove from the oven and, when cool, store in an airtight container; be careful not to handle the baked baking soda with bare skin.

Sealed in an airtight container, the baked baking soda will keep indefinitely.