Thanks for the nudge, David. I was following this thread and might have been stimulated to reply anyway, but here goes.
1. Nothing to do with glass, but Anne's comment made me cringe as have many others in the past, on TV, in the press etc. "Slag heap" is the problem. These heaps (I lived almost within sight and certainly within smell of several) were colliery waste, stone and low grade coal and had nothing whatever to do with slag. In Durham they were always called pit heaps - I don't know what other areas called them. I can't believe that anyone who knew what they were would call them "slag heaps", but I'm probably fighting a losing battle. Thank you, Anne, for the chance to get that off my chest!
2. "Slag glass". I THINK that I only heard this horrible name in recent times (like cranberry) but I've been so brain-washed by newly invented words that I could well be wrong. I presume that it means the same as "malachite" and I'm pretty sure that I learned that one in my youth. As we didn't make the stuff in my time it would only be when looking at bits dug out of old furnace bottoms that old-timers would have used the word.
3. Although I have no first-hand knowledge of the composition of malachite/slag glass I would be astonished if it contained any slag. Why would it? The old glassmakers (those even older than me!) used readily available waste such as horse manure, coal dust and coke dust (all produced or used on the premises) but the nearest blast furnace would have been many miles away. I don't know the composition of blast furnace slag but I would guess that the only glass colouring agent present in significant amounts would be iron, and lots of it. Looking (from memory) at malachite/slag glass I can't see a requirement for any iron at all.
4. Never having had a bit of blast furnace slag in my hands I don't know what it looks like, but it might well have a swirly, marbled sort of appearance. If so, could "slag glass" refer to appearance? However, please don't anyone start a new old wives' tale running from nothing more than that casual guess!!