Sentence:  Consequently, mounds of large ‘cakes' of <WSD>slag</WSD> are often found near the smelting sites of the Late Bronze Age, as for example at Ramsau in Austria (Doonan et al. 1996). 
Meaning of the word marked with <WSD> tags:  impurities formed and separated out when a metal is smelted from ore; vitrified cinders