Oven hot enough and ready to use.

Fire getting started - not hot enough yet.

Fire just right - smoke burned off 2/3 of fireback.

Fire has been too hot.

You can use this phenomena to check if you are burning your fireplace too hot or too cool and to tell if your oven is hot enough to cook in.

A good brisk, efficient fire in a Rumford will burn off the smoke and creosote half to two thirds of the way up the fireback, indicating that the clean part of the fireback is at least 1200 degrees F. You don't really want all the smoke to burn off which would indicate that your are wasting heat up the chimney and may damage the throat. In an oven a good indication that it's hot enough and ready to cook in is when all the smoke and creosote has burned off the inside of the oven leaving it clean and new looking.

Ashley Eldridge, who teaches chimney sweeps about safety, points to an article published by the Chimney Safety Institute of America which he helped to write.