Following on from my post about smoke-control-areas I thought I’d put down a few thoughts about firewood species and the importance of reducing the moisture content as low as practically possible.
Wet wood doesn’t burn!
All the nonsense you’ve ever read about clearing woodland in temperate Europe by setting it alight is just that, nonsense. Trying to burn standing timber or even under-storey wood is, as the late Oliver Rackham points out, “like trying to burn wet asbestos”. ( History of the Countryside p72 Dent 1986). Historical references to burning woodland normally refer to charcoal burning, a widespread industry that was both local and semi industrial in scale. The widespread notion that Britain was largely untamed wildwood well into historical times doesn’t bear any close examination- there is plenty of evidence that by the Roman occupation, Britain was only about 15-20% woodland, about the same as modern rural France.
Woodland soils in lowland Britain are generally more fertile than tropical rain forests and probably supported small clearings of crops for short periods with men clearing small wood with grazing animals and stone tools. The great clearances were probably Neolithic to Bronze Age but even now there is very little evidence as how they managed it, the labour involved and how long it took. We know from their legacy of stone monuments and track ways that they were very well organised and must have worked co-operatively in large numbers. Undoubtedly, it is one of the greatest agricultural achievements ever.
Coppicing for firewood
What those men did find however was that wood grows back quickly from cut stumps and may have been an early version of coppicing where areas are left to regrow so that successive crops of wood can be grown over periods ranging from 5-10 years. Anyone who has tried to cut a tree with a diameter of over 25cm and then reduce it to logs will realise its a lot of work. Growing smaller diameter wood 100-150mm in a coppice rotation is so much easier not only to saw to length but also split, especially as its straight grained unlike its counterpart, the tree branch, which is knotty, twisted and impossible to split. Coppice stools maintain the life of the tree indefinitely and are probably some of the oldest living things in our ancient woodland.
Britain has few but key hardwood firewood-species trees that make excellent firewood depending on their density and speed of burn-
The Firewood Rhyme – Anon
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.
Firewood- Moisture content and efficiency
There’s some truth bedded in the poem but the important thing about ALL firewood-species is that they are dry. Any firewood over 20% moisture ( including ash despite the poem) will not burn efficiently as the combustion process is slowed down from around 70% to 30% efficiency by first driving out the moisture as the wood heats and that heat being lost as steam and tar.
The length of burn is governed by the woods density, so as the poem reminds us, oak, hawthorn, beech, holly, yew, cherry, pear, apple and ash are comparatively dense woods and burn relatively slowly. To these we could add hornbeam, box, elder, crab apple, rowan, blackthorn, false acacia and even old ivy. To make a good balanced fire its worth adding faster burning firewood like birch, hazel, chestnut, sycamore, alder or poplar if dried well. All firewood has the same calorific value by dry weight but some woods give up that energy quicker than others.
Firewood supply in the UK is not only very variable in quality and moisture content but its eco-credentials are compromised by transport costs and delivery distance. Some supply kiln dried firewood others just seasoned firewood, some offer local tipped delivery others nationwide delivery in bulk bags or nets.
Clearly the best supply in terms of embodied energy is either going out foraging yourself or one that cuts and stores firewood locally to where you live and delivers locally in economic loads. Much of the firewood in our very competitive market is imported from Baltic states like Lithuania and Latvia where wood is plentiful and labour is cheaper. It may seem like a good deal but after you factor in the real cost of supply with packaging, double or triple handling, ferry fuel costs and distance, the poor old log you thought so cheap now has an embodied energy cost maybe 5-10 times that of a local supplier.
If you’re unsure of your supplier check out https://woodsure.co.uk a quality assurance scheme for wood based fuels and https://www.forestry.gov.uk/gib for British grown wood.
For the wood-fired pizza or bakery oven, the usual delivered split firewood size is normally a bit on the large size at 100-150mm and 250mm long so it pays to split this down further to 50-75mm to get a good fire going quickly and stacking the wood in alternate directions with the largest logs at the bottom. The stack should be filled with kindling/paper and set alight so that it burns top down producing a much hotter and steadier fire to heat up the mass of the oven. Larger logs can be added later and warmed away from the fire whilst the mass of the oven is being heated. Those logs will combust of their own accord and can added to the fire as necessary to keep a bright flame going.
Once the oven reaches around 300C, the flame will deepen to a deep yellow orange, any soot will burn away, smoke will clear and heat intensify making it perfect for cooking.
Main photo credit-https://commons.wikimedia.org/wiki/File:Firewood_stacked_up.jpg#file