but I have tried baking soda in frying and it seems to help make things crispier.

Mother and Grandmother made cracklings from pork fat, just slow baked to render fat and added salt after fot flavor.

https://www.armandhammer.com/en/articles/baking-soda-meat-tenderizerBaking powder crisps the outer surface.''

Velveting is a simple process of using baking soda or a mixture of egg whites, cornstarch and oil to marinate and tenderize economical cuts of steak, chicken and other meats before cooking. Velveting meat softens fibers to reduce toughness and enhance juiciness for maximum flavor and tenderness.

You can velvet meat and make it melt-in-your-mouth tender by quickly tossing chunks of beef or chicken in about ¾ tsp of baking soda for about 15 minutes before your next stir fry, then rinse and pat the meat dry before putting in the pan. You can also add ¼ tsp baking soda per pound of ground meat before browning or shaping into patties or meatballs.
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https://www.seriouseats.com/how-to-get-crispier-chicken-turkey-poultry-skin-with-baking-powder

That trick is a sprinkling of baking powder, and it'll get you the crispiest, crackliest bites of fatty, salty skin imaginable, whether you're cooking just one thigh, a plate of wings, or an entire bird.

Baking powder, it turns out, is good for quite a lot more than baking. The slightly alkaline mixture raises the skin's pH levels, which allows proteins to break down more efficiently, giving you crisper, more evenly browned results.* Simultaneously, it combines with the bird's natural juices, forming carbon dioxide gas that leaves you with a layer of tiny bubbles. It's these bubbles that increase the skin's surface area, allowing it to develop a crunchy texture once cooked.

*It's worth noting here that while baking soda will produce a similar texture, it also adds an unpleasant metallic flavor to the skin, so I wouldn't advise substituting one for the other.

To reap those benefits, simply combine one part baking powder with three to four parts kosher salt (about a teaspoon of baking powder per tablespoon of kosher salt will work), add some black pepper to taste, then sprinkle it evenly over the surface of the skin. Then—and this is key—let it rest, uncovered, in the refrigerator for 12 to 24 hours. This resting period doesn't just give the baking powder time to form all those little bubbles; it also lets the salt do its thing, dry-brining the meat for more intensely flavored, better-seasoned results.
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I imagine the pork bellies' surface work the same way.