I Truly Don't Get This 'Coin'...

Thanks for the link, Chris. Interesting. But I will not be collecting these lumps. I can generate them all day long with a butane torch.

When picturing the type of person who would be interested in rude aes, a vision of a numismatic hipster comes to mind

I suppose it all really depends on the provenance of the 'ingot'. It isn't actually slag as you say. Slag would be the material that floats to the top of the crucible and gets poured or skimmed off. This piece does have an old number glued to the surface which museums often did back in the 19th century (even on coins !). If I had a good reason to believe that this item came from an excavation of a small hill in North Western Anatolia (Hisarlik) circa 1880 ...... I would jump on it faster than any coin I can think of and probably be willing to pay over a thousand clams as well !
Many many things are just so much junk to the average person ...... It really requires knowing what something is to put a value on it.
About ten years ago I read a small article in the local paper. Someone at the Museum was poking around the basement and they found two crates dated to the 1890's buried in a corner. They appeared to be filled with so much ancient junk (pottery shards and unpainted grey earthenware and 'stuff') All very unimpressive stuff. The article mentioned that the only 'clue' was the label on the crate. "Purchased from Frank Calvert" along with the precise date of purchase.
I almost lost my mind ...... Frank Calvert had owned the hill at Hisarlik which he later sold to Henry Schliemann. Calvert had excavated the site for years with a few locals as help and collected all the artifacts he found. When Schliemann arrived he hired hundreds of locals and ripped off the top and center of the site which he dumped onto the plain below.
It wasn't until the year before he died (1890) that Schliemann realized what he had done. In antiquity Alexander the Great had wanted to 'rebuild' Troy and his engineers had leveled the entire site. What Schliemann had now done was to obliterate what remained of Troy's great Citadel which lay just a few feet below the now flat hilltop. Schliemann had found much unimpressive unglazed pottery which he call 'Grey Minyan'. Most of it he discarded as unimportant.

So here I am researching all things Troy ........ and sitting in a basement less than a mile from my home is probably one of the largest 'authentic' collections of Trojan artifacts in the world ....... Grey Minyan Ware.

I would pay dearly for just one shard

This is a bit embarrassing but ..... I actually like the look of that.
Is it because I am a metdet? possible. Or is it because I am so far gone I will embrace any chunk of metal?
Either way, I do actually find that quite an 'attractive' chunk of rubbish.
Explanations on a postcard please

If you dropped that on a cinder/gravel path, you would never find it again.

I would need proof that this piece actually qualified to be used as 'money'. I understand what 'aes rude' is, but what is the proof that this lot is actually a piece of 'aes rude'?

I would have a problem in how I would include it in my collection of ancients.
I would anticipate some difficulty in putting it in a 2x2, then into an album pocket.
Perhaps I could flatten it out a little.

The reference would have to included in your collection, in hard copy.
I keep a separate file of purchase records for most of my more valuable coins.