No ceramic allowed in glass recycling?

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?

I’m not any kinda super-expert but I did used to do ceramics…

When clay gets very hot it “vitrifies” or becomes glass-like (this is what makes it waterproof – low fired clay is porous) I believe this is due to the quartz? and feldspar? that make up clay body.

If you heated it to a very high heat (and I suspect that the heat needed to melt commercial-grade glass is very high indeed) it would definately melt.

Ceramics that are able to withstand super-high heats (space shuttle tiles,etc.) are specially formulated not to melt.

I think it’s for the same reason you don’t put aluminum cans in a glass recycling bin – 'cus it isn’t glass. < shrug >

Just a WAG, but I think when you melt glass for recycling most impurities either burn up or melt and/or float to the top, where they can be skimmed off. My guess is ceramic does none of these things and that’s why it’s such a nuisance.

I’m not sure I believe the “make the whole thing brittle” routine. Sounds like a scary story to convince you not to do it.

As for adding heavier objects to aluminum – I used to work in a steel foundry that used scrap steel and iron as raw material. The scrap pile was hardly homogeneous – lots of other stuff was visible. But when it went into the carbon arc furnace most of it burned off or became slag, which was skimmed off.

They had a metallurgist on hand who sampled every batch and added nickel or chromium, etc., as needed to form the correct alloy (very few metals are used in a pure form – aluminum certainly is not). I never saw them throw a batch away. (I’m not sure how they would – it was a BIG furnace!) Just a guess, but they may have also had special chemicals on hand to draw out certain noxious metals that would spoil the mix.

“non sunt multiplicanda entia praeter necessitatem”
– William of Ockham

Heres my theory, and it isn’t so much based on material sciences. Basically David’s answer is the most correct. “Cause it ain’t glass”. Realize there are lots of dumbasses out there and most haven’t a clue about different materials. The dumbest of goobers can usually understand the difference between a metal (iron, steel, aluminum) and a glass. Many likely don’t know that a ceramic is much diferent than a glass. They think “I drink out of it, its brittle and broke when dropped it so it must be glass”. So the most common item corrupting glass would be ceramic. I also imagine its a little more labor intensive to sift out considering it doesn’t liquify. The “I ruins the entire batch” idea is horse shit.

Also, please don’t forget that most high-fired ceramic ware has glaze - and that is a type of glass. It’s just that it’s a type of glass they probably don’t want mixed in with regular glass, and the properties of it could completely screw up a batch of recycling. A lot of glazes (like raku, though it’s not high-fired) contain substances that are toxic. Probably not what you want showing up in the brown glass around your ICB Root Beer.

“I’m surprised that you’ve never been told before, that you’re lovely, that you’re perfect, and that somebody wants you.” - Semisonic, f.n.p