No OSM pages that I can find refer to kilns.

TODO: What shall we map these as?

HB: kiln is fairly distinct. Make up something new!

I suggest "man_made=kiln"
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RP: man_made=kiln/oven/furnace looks good
maybe use product= for the substance extracted tag?
this is already somewhat accepted in osm - see 'works'
maybe old -> disused=yes
ruins -> historic=ruins