At my friendly local recycling place there’s a big sign above the glass bin that says “Absolutely no ceramic”. I asked the guy who worked there why that was and he said “even a tiny bit of a dish or coffee cup can mix with the melted glass and make the whole batch brittle and unusable”. How could that happen? I thought that ceramic was clay-like and basically unmeltable. Could it have something to do with lead?
Also on the subject of recycling, As a young and morally ambiguous youth I used to put rocks and junk iron in aluminum cans to increase the weight and thus make more pocket money from recycling them. Do they hand-sort the scrap or maybe use an electromagnet? Would this junk just end up unmelted in the bottom of the ingot?