Here are some images from hide tanning workshops from Diderot’s Encyclopedia, 1769 that I found interesting as a leather worker and occasional hide tanner.
If you have done any hide tanning you’ll recognize the tools of the trade. Not much changes for the small-time home tanner.
I suspect this is some hot and smelly work and judging by the way they’re dressed it is a hot room. The only large traditional tannery I have visited was in Morocco and it had an odor on a hot summer day that hit you like a brick wall. I’m not sure what they’re doing with the fire at this stage but maybe adding some amount of smoke rather than heat.
Any thoughts or insights?
Preindustrial Craftsmanship 
Many natives here in Oregon smoke tanned their hides. Perhaps here in your picture too.
Yes, but they need a lot more time and smoke than could be provided there, even to just get a bit of color. I smoke my hides as well and it takes a lot of time and huge amounts of smoke. I’m still not sure what these guys are doing unless they are just heating them or singing them.
Mind you…I just purely guessing…but some “brain tan” and “fat tanning” methods “heat soak” the tanning agent into the green leather…