That's an unusual and interesting question. Really... Most people don't care about this kind of stuff until their first night in a lean-to <img src="images/graemlins/laugh.gif" alt="" /> Then, they soon find out that sleep deprivation is a big problem in survival <img src="images/graemlins/laugh.gif" alt="" />

To answer your question, it's all a matter of dispersion...

A good, thick bed of coals can ignite and will keep the combustion of a large log going. Yes, even just one log, provided you can concentrate the heat properly. A few large rocks around your fire, very close to your bed of coals, will avoid the dispersion of coals as well as heat (watch out, any rock containing water can explode if heated). Ideally, you want to shape the bed of coals with the rocks so it makes a nice, even and thick bed for your next log.

Fireplaces act pretty much the same way as the large rocks in concentrating heat and coals... that's why your father had no problem keeping the fire going in it.

I can keep a fire going for a long while using the rocks method, without needing to take care of it at all for hours... That's a good thing for those long cold nights when you want to sleep a little, and not always take care of the fire. It also burns a little more slowly, so you save on wood. Even better, when the fire is almost dead, the large rocks will keep on radiating heat for a while (depending on their size, etc.)... and when you wake up because of the cold, there are always hot coals left to start the fire again. Just blow on the coals a bit and throw a log over them.

Hope this helps,

David