So I was thinking, and in real life a fire does produce smoke, which it is currently doing in Minecraft. But in real life even furnaces (and various other smelting machines) produce smoke. Why not make it so furnaces (and various other smelting machines) produce smoke, similarly to the effect waterfalls produce in the mod.
So the idea is basically to add similar smoke that fire has to all "light" (one-block smelting) machines, and another type of smoke to all "heavy" (multi-block-smelting) machines.
This type of smoke would:
1. Be bigger.
2. Go upwards for a speed of about 2 blocks per second, and, if it hits a block, spread to the sides like the particles of a morgul torch (but bigger) Until it finds an opening up (i.e. chimney or atleast a higher part of the roof). if it doesn't, it would gradually become thicker (like going downwards, from the roof)(I don't know if this is possible to code :/) until the smelting machines are done with their job or the whole room is filled with smoke.
3. last about 20-30 seconds. If more smoke is added, the timer resets until the smoke has stopped coming, preventing the smoke from fading away until the smoke has stopped coming (I said this 2 times -_-)
4. When walking in this smog, you would lose sight, like when staring into the sun, albeit not as much lose of sight as when actually staring into the sun. It would also be some sort of black instead of white.
5. It would fade away when it has traveled 50 blocks up or 20-30 seconds has gone by.
This might be very annoying, but it would encourage people to actually build chimneys so the smoke can get out, instead of staying inside and make your house into an orcish factory (even if it is one).
Constructive criticism welcome!