I think Mr. Edge could have been referring to the Bessemer Process that about the 1870's started to flood the market with cheap and almost slag-free steel. The presence or absence of slag would then be a first indicator about the authenticity of a particular object. Not definitive, but at least significative enough. Specially taken in the right context: this kind of steel was, for example, frequently used to make parts for armour in order to "complete" it, at the end of the 19th - beginning of 20th. Not necessarily with an evil intent, but after so much time it's sometimes hard to tell apart what's new-ish and what's not, so this kind of examination can help. Again, not definitive, but it's, let's say, "another tool in the box".
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