The tradition of burning a Yule log has largely fizzled out in most parts of the world. While holiday cards often feature cute, picturesque birch rounds in the hearth, old-time Yule logs in 6th and 7th century Europe were monster tree trunks that were meant to burn all day, and in certain cultures for twelve continuous days, without being entirely used up.
Apparently, if you didn’t have a leftover bit of this log remaining after the marathon burn, you were doomed to misfortune in the upcoming year. The remnant piece of charred wood was tucked away in the ceiling and was used to light the following year’s Yule log. I assume it was extinguished before being squirreled away in the rafters or some really bad luck would ensue.
While a birch log is charming, it doesn’t compare with many other hardwoods in terms of heat value and how long it will burn. Heat value from wood and other fuels is measured in British thermal units (BTUs), one BTU being the energy required to heat a pound of water one degree Fahrenheit. If you look at firewood BTU-value charts you’ll see that few of them agree exactly. This is to be expected, as the heat value of a given species varies according to the conditions in which it grew.