The blobs are made from waste solder, basically every bit of solder in my workshop is either on a PCB or ends up on the blob. No fresh solder was harmed during the making of the blob.
I use a couple (a few?) 250g rolls of solder a year, so it probably took the waste from 20+ rolls to make a 2kg blob.
I suppose it's a bit of a recycling thing too, no solder goes out in my trash although occasionally dead components do.
Maybe I didn't give the brass thing enough of a go, I didn't like the way it was springy so tiny splatters of solder flung off all over the place, and I don't think my brass had flux on it, it was some type of oil that felt slippery and left the tip needing to be re-tinned each time. It's still sitting in the workshop somewhere. Maybe with enough use the oil would burn off and the springyness would pack down?