When buying firewood it should be properly seasoned. The best way to get seasoned wood is to buy this year’s wood for next year! Wood yards or sellers may tell you that even though the wood was split this year it will be just fine.
Seasoned wood is darker on the outside, it’s bone white on the inside. You should be looking for grey, or darkened, brittle wood that has a lot of cracks in the inner rings. Wood that is seasoned looks grey, or dark due to the fact that it has been sitting in the sun, or has been stored in doors, drying, and collecting dust.
Also testing the wood with a moisture meter is recommended.
Unseasoned wood will produce less heat than the equivalent seasoned and will also produce tar deposits in the chimney, with the possible result of chimney blockage or chimney fires.
For more information read the Stove Industry Alliance advice leaflet here: Using-Dry-Logs