I would say it was used for metal working. Coke is produced in a forge by burning coal- I have forged iron with 'fresh' coal and also with a good bit of the coke that tends to build up in the fire- yes, until the coal burns down into at least some coke, you have a big, smokey, sulfuric mess. (A good smith will tend the fire in a way that encourages the buildup of coke.) The first coal fire of the morning is a "light and run" sort of deal...that is, at least if you had the kind of crap chimney I worked with. (Fresh coal is routinely added to the fire a little bit at a time to keep it burning once it gets going as well.) Coal, and better yet, coke, is the best material that the Romans likely had to forge weld- you need a VERY hot fire to do that- one that's easily achieved with coal and coke and not so easily achieved with wood and charcoal.
Just don't breathe the yellow smoke mmkay? There will be a lot of yellow smoke.
I would never personally recommend building a smithy with a thatched roof...the most fireproof materials available are recommended.