If you do any home metal casting, you’re probably going to pour some ingots, sooner or later, if only because you need something to do with the leftovers after your main pour is done. Ingots, of a size chosen to suit your crucible, make charging a furnace quick and easy compared to most forms of raw scrap. Steel muffin tins, which are cheap, ubiquitous, and available in a range of sizes handy for small furnaces, are a common choice for ready-made ingot molds.
Of course, melted metal is hard to identify without a label—did that one come from a screen door or a piston head? The alloys they’re made of won’t be the same, and you might not want to mix them in later pours. Sure, you could just write on them with a Sharpie, but ink labels are susceptible to abrasion, solvent exposure, and sunlight, and who knows how long it’ll be before you’ll want to remelt them?
Enter this brainstorm from bulletcasting forum member Centaur 1, who took a set of steel letter punches (like this one for $8 from Harbor Freight) to the bottom of a muffin tin to make a quick, cheap, effective self-labeling mold for his remelted bullet ingots. He’s using the trick for lead casting, of course, but there’s no reason it shouldn’t work just as well for zinc-based pot metals and/or aluminum.