Actually, Lionello, I had been struggling with the translation of a definition in one of my dictionaries. This was version six or so. Sorry I couldn’t make it any clearer. Looking for a good description, I came across this website that describes aluminium casting in Dutch and English. It uses ‘slak’ alongside English ‘slag’ rather than ‘slug’ so I’m beginning to believe that ‘slak’ and ‘slug’ may not be entirely the same. Although Van Dale explains the etymology of ‘slak’ as ‘pieces of metal, flying off by beating’, the common meaning is that of ‘residue from melting’, for which I think ‘scoria’ would be a better word. OTOH we can now add English ‘slag’ to the list, about which Etymonline has this to say.
In this context I would also like to mention Dutch ‘slaag’ meaning ‘a beating’ and ‘slag’ meaning ‘stroke’.
And while we’re at it, under ‘slug’ I found this on Etymonline:
meaning “strong drink” first recorded 1756, perhaps from slang fire a slug “take a drink,” though it also may be related to Ir. slog “swallow.”
This immediately brought Dutch ‘slok’ to mind. It means ‘a mouth full to swallow’ and by extension ‘a strong drink’. Again, apparently not related. It’s from Dutch ‘slokken’ (to swallow).
The Dutch cognate of NHG ‘schlau’ is ‘sluw’ for which my dictionaries don’t give an etymology but it appears to be related to a group of words including Du ‘sluipen’ and ‘sluiken’ (to sneak).
A fruitful combination of sounds, it seems.