According to the world 10 announcement post, tar kilns now take 3.5 real life days to burn coal. I also noticed that you need to at least fill it up to half to light it now (not sure when that changed).
This feels like a change intended to delay metal production in the first couple days of the world. However, nothing was mentioned about Black Coal, which would allow you to smelt metals on day one. Has anyone been able to mine any Black Coal, or is it gone (at least from layer 1) to completely make it impossible to smelt metals at the start of the world?