Logs are available from various sources that are air or kiln dried and are sourced from hard and soft wood species or a mixture of the two.
While the type of tree the log comes from can have some impact on the calorific value, it is usually extremely small. Most of the variation in calorific value will come from the moisture content depending on the species. Ash has a particularly low moisture content when first felled. Hard and soft wood species with the same moisture content have very similar calorific values per weight. However, hard woods have a higher density and so the energy content per unit volume is greater. Some woods are considered to be better than others in terms of ease of ignition, heat output, flame length etc. In practice most hard woods are acceptable and preferable to soft woods.
The Firewood Poem
Beechwood fires are bright and clear
If the logs are kept a year
Chestnut's only good they say,
If for logs 'tis laid away.
Make a fire of Elder tree,
Death within your house will be;
But ash new or ash old,
Is fit for a queen with crown of gold
Birch and fir logs burn too fast
Blaze up bright and do not last,
it is by the Irish said
Hawthorn bakes the sweetest bread.
Elm wood burns like churchyard mould,
E'en the very flames are cold
But ash green or ash brown
Is fit for a queen with golden crown
Poplar gives a bitter smoke,
Fills your your eyes and makes you choke,
Apple wood will scent your room
Pear wood smells like flowers in bloom
Oaken logs, if dry and old
keep away the winter's cold
But ash wet or ash dry
a king shall warm his slippers by.
The Firewood Rhyme
Logs to Burn, Logs to burn, Logs to burn,
Logs to save the coal a turn,
Here's a word to make you wise,
When you hear the woodman's cries.
Never heed his usual tale,
That he has good logs for sale,
But read these lines and really learn,
The proper kind of logs to burn.
Oak logs will warm you well,
If they're old and dry.
Larch logs of pine will smell,
But the sparks will fly.
Beech logs for Christmas time,
Yew logs heat well.
"Scotch" logs it is a crime,
For anyone to sell.
Birch logs will burn too fast,
Chestnut scarce at all.
Hawthorn logs are good to last,
If you cut them in the fall.
Holly logs will burn like wax,
You should burn them green,
Elm logs like smouldering flax,
No flame to be seen.
Pear logs and apple logs,
They will scent your room,
Cherry logs across the dogs,
Smell like flowers in bloom.
But ash logs, all smooth and grey,
Burn them green or old;
Buy up all that come your way,
They're worth their weight in gold.