Baking – How to tell if a metal can is safe to bake in

The best approach is to use most of it to make a starter and feed the mother with flour and water for the next bread.

Once you have the mother, you can feed it with as much flour as you need.

Let's say you have 1 cup of mother at 100% hydration. You should use 3/4 cups of it to make a starter for the bread (add flour and water to form a starter your formula suggests). Then, feed the remaining 1/4 cup with 3/8 of both water and flour to make it 1 cup again.

Then, when your starter is active enough (it's better to rest overnight), you can make the bread the same way you do with commercial yeast. Just be sure to calculate the flour and water in the starter for baker percentages.

The numbers are symbolic, it's better to use weight and not volume for correct calculations of baker's percentages.

Reading through the transcript on that episode of Good Eats it seems that he intended for the recipe to be done with the ice cream type rock salt:

I also like plain old rock salt. Not only is it good for de-snowing, de-icing your, your front stoop, it's good for, of course, making ice cream, and believe it or not, for baking stuff.

That being said, I would personally feel fine using the rock salt since it doesn't come in direct contact with the edible portion of the shrimp. But if I were buying bulk salt specifically to salt bake I'd err on the side of caution and purchase pretzel salt which has very coarse grains and is safe for consumption.