A: WD-40 is often misunderstood. People use it as a lubricant and penetrating oil, and assume it’s the same thing as oil. While it does lubricate fine mechanisms temporarily, and frees them up if something’s stuck, its real purpose, suggested by the initials “WD”, is water displacement. For the most part, it’s kerosene, I think, and it does not actually contain much in the way of oil, or anything that would provide a persistent lasting film on the iron. So after you sprayed your mold, heating it pretty much removed all the WD-40. That’s why the metal then stuck.

You can remove the metal by just prying and scraping it off. WD-40 might even help in this. But when you pour ingots, try preparing the mold with an actual oil. Motor oil, even used motor oil, works fine. So do other oils, such as peanut oil, etc. Heat the mold till the oil smokes—a good bit hotter than the temperature at which WD-40 will appear to burn and smoke, since it’s much more volatile. The other common pretreatment for a mold is to coat it with soot. You can get that with most torch flames by turning off all the air or oxygen, so the flame is very yellow. The metal is held just beyond the visible yellow flame, so that soot is deposited in the cavity. I usually do this to a brand-new mold, then add some oil to the result, and heat it until it starts to really smoke quite a bit. This leaves a sort of cured surface similar to a well broken-in iron wok.

Once initially cured like this, the mold will only need a little more oil applied every few ingots. Machined molds, with smoother surfaces, take a lot less preparation than rougher cast iron molds, which initially need a lot of soot and oil to clog up the rough pores enough so the metal can easily separate from it.