I’m a person that loves soapy science & testing. I always test “lather” same way for every bar of soap. After I lather up, I scrape all the suds off my hands & onto ledge/edge of stainless sink to let sit until soap bubbles have all “popped”. Here’s what I have noticed. Soap made with soy wax leaves the most “soap scum”, soap with lard also leaves a lotion like “sludge behind” but a bit less than the soy wax. However, ZNSC bars leave basically nothing behind but a very very faint soap ring. I didn’t snap any photos but I will on next “lather” test.