There’s a lot of bad advice floating around about seasoning cast iron. You’ll hear people say to use butter, crank the oven ...
When seasoning cast-iron pieces, go for neutral oils with high smoke points, such as vegetable, canola, or grapeseed oil.
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Can You Really Use Soap On Cast Iron?

No doubt many grandmothers will want to scold us, but the truth is, soap isn't going to destroy your cast iron pans.