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How to Escape From the Iron Age?
(solar.lowtechmagazine.com)
This is a most excellent place for technology news and articles.
There's usually an interface material when using resisitive heat. And there's heat loss from heating the interface material before the heat getting to the actual material that needs to be heated.
Inductive heating can be applied directly without heating the interface material.
Though this is probably more applicable to cooking vs industrial kilns and furnaces.