Sooner or later sprue plates end up with lead around the underside of the hole, often because a sprue has been cut too soon and molten metal spread across the underside of the plate.
One method to remove this entails a 50:50 mix of hydrogen peroxide and white vinegar. It works, but sometimes takes a while. Then put a sniff of synthetic two-stroke oil on the clean surface to minimise recurrence.
Another annoyance from sloppy pouring is lead in the slot of the sprue plate hold down screw. I fix that one by pouring more molten lead on it, forming a larger blob which is then removed by tapping on the handle joint.
Which brings us to an alternative for cleaning the sprue plate. I did one this morning, removing it from the mould and dropping it in the lead pot while also adding some wax to flux the melt. The wax makes the steel easy to wipe down afterwards and prevents slag adhering to it. The result after wiping with steel wool is a nice clean plate with a lovely blue finish.
This is actually an extension of something else I have been doing for a while - bluing small parts and screws in the lead pot. Usually I do this while the melt is coming up to temperature, with some molten wax running around and burning on the surface at the start of the flux. I just put in the part, then pull it out when I like the color. Color goes from straw, through a beautiful blue, and eventually a darker black. I'm not too worried about softening things, since lead pot temperatures are about what is used to turn hardened brittle steel into springs - I've been down that path too.
You are only as good as your library.
