1. First, you must bake the baking soda. You can do this while pretzels are going through their first rise, or sooner.
2. Preheat the oven to 250F/120C/gas 1/2.
3. For one batch of pretzels, spread out 1/4 cup (70g) baking soda on aluminum pie pan, or small rimmed baking sheet covered in aluminum foil.
4. Bake the baking soda for 1 hour. It will loose weight as it bakes, but keeps the same volume, so you should have 1/4 c (60g) baked baking soda.
5. Allow it to cool completely, and then keep it in an airtight container until ready to use (if you see more than one batch of pretzels coming up, make a larger amount, as it keeps indefinitely. Sift it before using, as it cakes during storage).
6. Select a large stainless steel pot and fill it with 8c (2L) water. Make sure the is at least 2 inches deeper than and 3 inches wider than your pretzels diameter. You must use stainless steel, because aluminum and copper as well as nonstick surfaces may react with the baked baking soda.
7. Pour in the 1/4 c (60g) baked baking soda and bring to a simmer over medium heat. Once it dissolves, reduce heat to keep a gentle simmer.
8. Before baking the pretzels, dip in the solution and brush the tops with an eggwash of 1 egg yolk, beaten with 1tbsp of water. This gives them a glossy finish.