Actually, we burnt then for a couple of years without problems (wife used to work for a lumber co).
2X4', 2X6's, and 2X8's, all about 12"long, wood room was clean, no bark or dirt.
I found that you had to stack a few on top of one another to form a larger "split", and have a few of these little stacks fairly close together. If you keep the fuel load fairly tightly packed together, and not too much at a time it works okay.
Keep an eye on the chimney though, it gets dirty quicker!