Not necessarily "blast furnace waste". I used to be a welder. Slag is whatever brittle shell bits are formed when molten metal oxidizes. Before graduating to welding, my job on one construction site was to follow the welders around with a small pointy 'slag hammer' with a springy handle, tapping off the slag. Had to be done to ensure all welds were smooth - and slag could not be left on prior to painting.
Minder had a lot "Gordon Bennett" and "leave it out" and "do what?" and "do me a favour" - without ever resorting to swearing and was all the better for it. Lock, Stock and Two Smoking Barrels was just Minder with swearing and a dildo and a bit more violence.