Well seasoned firewood is lovely - it burns hot and cleanly, lights easily and doesn't cause creosote to build up in your chimney. Obviously we need to know how to tell if wood is well seasoned, if we are to get the best from our wood burning stoves.
When drying firewood we aim to get the moisture content down to around 18% by weight, or hopefully lower. In most climates this can be achieved by simply letting a stack of firewood dry in the open air for two years. The trouble comes when you don't know for certain the provenance of your firewood, for example when buying "seasoned logs", or simply when you need to know if your logs are ready to burn.
Here are some simple checks you can do to get an idea of how well seasoned your logs are, as well as a couple of "scientific" tests you can try to get an accurate measure
Firewood shrinks as it looses water which causes small cracks to form which are often visible in the end grain. Different woods check differently however, so this can't be relied upon totally.
Seasoned timber loses around half its weight between being cut and being ready to burn. When you are familiar with the type of wood you have you'll get to know when it feels ready. Unfortunately all different tree species have different timber densities, and in a mixed load of logs it can be hard to tell if you have a wet piece of heavy poplar, or a well seasoned piece of holly or oak. Experience can help here, but weight alone won't be sufficient.
Logs which have dried for a year or so noticeably change in appearance - bark often becomes loose and drops away, split surfaces gain a silvery patina (unlike the yellow of fresh cut logs). This won't tell you anything about the overall moisture content of the log, but can at least tell you if the logs were only very recently felled, or if they have been exposed to the elements for a while.
This is the quickest of the reliable methods - a moisture probe is a little handheld gadget that estimates the percentage of water in a log by measuring it's resistance to electric currents. They are reasonably accurate below moisture contents of 40%, but often don't work at all at higher water contents.
To get a good idea of how dry a whole pile is you will need to test a few different logs from different places in the pile. If you are able to also split some logs, you should take a reading from the very middle of some pieces. This will give you the best idea of the overall, rather than surface, water content.
A moisture probe is inexpensive, and the simplest way how to tell if wood is seasoned.
This is a trick I picked up when learning how to kiln dry lumber for construction, where accurate assessments of moisture content is vital. What you need to do is take a standard log and split it down so that you have two pieces, each of a representative size of the pieces in the pile. We'll call them piece A and piece B for ease.
The idea is to use one part as reference, which you dry in a kiln or oven and the other stays with your wood pile, seasoning with the rest of your firewood.
This piece we will use to work out how much water is in each piece of wood at present. We need to be able to weigh pieces of wood accurately, to the nearest gram or smaller is best. Split this wood down to small slivers, from the center of the log, which weigh a couple of hundred grams total. This portion of wood needs to be weighed exactly, and then put somewhere hot, such as on a rack above your stove, or in the oven on a low heat. We are aiming to drive off all the water from these slivers so we can find out the dry weight of the wood.
Lets say that the slivers of piece A originally weighed 226g, and that after drying they now weigh 138g. This means that the wood has lost 88g of water through evaporation. We can, therefore, work out the original moisture content of the piece of wood as:
88g/226g * 100 = 38.9% moisture content by weight.
We might surmise at this point that our wood isn't freshly cut, but still needs significantly longer to season before we can burn it well.
This piece is left in your firewood stack,and we can use it to check how well the whole pile is seasoning. It will be helpful to mark this log in some way - a dollop of white paint, or even tipex will do - so you know which piece you are looking for in a year's time. Weigh this log carefully and make a note of the weight (even on the log itself in tipex!).
Log B weighs 2547g. It has the same moisture content as piece A (they were split from the same log remember?), which is 38.9%. To find out how much water this log needs to lose to reach the desired moisture content calculate:
Current weight *(currentwater% - desiredwater%)/ 100
Say I want to reach 12% moisture content for this log, it would need to lose:
2547 * (38.9 -12.0)/100 = 685g!!!!
So the target weight for log B is 1861g. When this log reaches that weight you can be reasonably sure that all the other logs in your log pile are also well seasoned.
This may all seem a bit obsessive, but it is how to tell if wood is seasoned properly, and useful to know if you want to compare two different ways of drying your firewood, such as in the open versus in a woodshed.