Beautifulhag

Hello there, Can someone please tell me what , "fat liquor" is please? I saw it mentioned in a facebook post tanning fish skin. Also, do any of you know what proportion of oak gall tannin to weight of fish skin to be tanned is? I have use tea before and love the rich, brown colour it turned. I would like to try tanning with gall tannins to see if the natural skin colour remains. Thank you.