I wouldn't put them above the furnace, that justs accelerates the problem. The drying process must be done slowly.
You could go to a wood working shop and get some sealer that is wax based to seal the end grain. Wood turners will rough turn green wood and then seal it in a variety of concoctions to slowly dry the turning to prevent cracking and minimize warping before doing the final turning.
If you have smaller blocks you could try placing the blocks in a brown paper bag filled with wood shavings or saw dust to slow the drying process.
Do a search for drying green wood, you'll find plenty of discussions in wood working forums.
Bottom line, air drying should be done slowly (not close to the furnace) and sealing the end grain minimizes the cracking.