Having spent the last two entries looking fairly deeply into the science behind fires, I thought I would start to head in a more practical direction, although I will still be linking in the science. For my birthday, knowing how much I enjoy camping and outdoor activities, my parents bought me a copy of The Wood Fire Handbook by Vincent Thurkettle and it is from this book that I will be taking most of my information for this entry. Firstly, I will be looking at the trees that are most commonly found in British woodlands, and how they are rated as firewood, then I will go into the science of what makes a good firewood and the factors that affect the flammability of the wood. Identifying the different trees and plants in the woods can be an engaging distraction for the children, I have done it with the Cubs I used to lead and for weeks afterwards they delighted in pointing out trees and telling me what they were.
“When embracing the wood fires lifestyle, some knowledge of trees is good –
it shows respect and helps to develop an understanding of the fuel you have chosen to use,
which is one of the greatest resources we have and one of the few that is
truly sustainable” – Vincent Thurkettle
There are many rhymes and traditional tales that give advice on fire woods, however the one we read during one of our sessions was this one by Lady Celia Congreve.
Beech-wood fires burn bright and clear
If the logs are kept a year;
Store your beech for Christmastide
With new-cut holly laid beside;
Chestnut’s only good, they say,
If for years ’tis stored away;
Birch and fir-wood burn too fast
Blaze too bright and do not last;
Flames from larch will shoot up high,
Dangerously the sparks will fly;
But ash-wood green and ash-wood brown
Are fit for a Queen with a golden crown.
Oaken logs, if dry and old,
Keep away the winter’s cold;
Poplar gives a bitter smoke,
Fills your eyes and makes you choke;
Elm-wood burns like churchyard mould,
E’en the very flames are cold;
It is by the Irish said;
Hawthorn bakes the sweetest bread,
Apple-wood will scent the room,
Pear-wood smells like flowers in bloom;
But ash-wood wet and ash-wood dry
A King may warm his slippers by.
This poem is full of good advice, including which woods are best when seasoned, which woods are dangerous to burn which give lots of heat and which smell nice, but how accurate is it? The first thing I notice looking at the poem is the fact that it sings the praises of Ash wood in every verse. Ash wood produces very few sparks when burned and produces little smoke which could contribute to its popularity. Another contributing factor may be that even when freshly felled, Ash has a low moisture content meaning it can be burned while green, however an American survey only ranked it 8th when properly seasoned. Oak, when seasoned for at least a year to remove the moisture, burns very well due to its density, as will most hardwoods. Beech burns brightly, makes good embers, and as it produces very little smoke is ideal for cooking on. Beech is traditionally used in wood fired bread ovens. Fruit woods, from apple, pear and cherry trees are fairly dense, burn well, and produce pleasantly scented smoke making them ideal for burning when all you want is a fire to sit and talk around.
Birch wood dries quickly, although care must be taken as it can rot if it gets damp with the bark still on. Birch bark also is extremely useful as it peels apart into fine, papery, sheets very easily which make an excellent tinder. If you are burning logs of birch with the bark still on it can produce thick, sooty smoke due to the high oil content, however if you put logs with bark on in the embers to smoulder away without flame then it will produce a pleasantly fragrant smoke in a similar manner to an incense stick. Willow and Horse Chestnut, being softer woods, do not burn well, giving off sparks and unpleasantly scented smoke. Conifers, such as fir, pine and cedar are all best burned in stoves, the resin pockets spread throughout the wood boil in the heat of a fire causing the wood to pop and give off sparks. Conifers take longer to dry than other trees, due to the structure of the wood retaining moisture better. Cedar is considered to be the best coniferous fire wood, it is dense, burns well and does not spit as much as others. Pine is a reasonably good fire wood, however it sparks a lot and gives off a sooty smoke. Fir trees do not burn well, they are much softer and more resinous, however they are very common so the wood is available cheaply.
Greenheart, Laburnum, Sassafras, Wenge, Yew, and Cherry Laurel among others should be avoided at all costs as they give off poisonous fumes when burned, and may be harmful to touch. For a full list, go to http://www.wood-database.com/wood-articles/wood-allergies-and-toxicity/.
So now we know some of the best and worst woods to burn, what makes this so? Firstly, Moisture content has a big effect on the flammability of wood. This is because before the wood can burn, the moisture in the wood needs to be boiled off. This is what causes the lighter grey smoke that appears when green wood is placed on a fire, as the steam from the boiling wood mingles with the smoke from the rest of the fire. This can be prevented by seasoning the wood, or storing it in a dry area to allow the water to evaporate. This is called seasoning as the common advice is to buy fire wood in the winter and leave it to dry over the spring and summer seasons for use the following winter. Harder woods like oak may take over a year to season. Wood cut in the winter is considered to be the best as during the winter less sap is flowing through the tree, meaning there is less moisture in the wood. The density of the wood effects the burn time, with denser woods burning for longer, however less dense woods can be easier to ignite so are often used for kindling, with logs of dense woods being placed on later. The ambient temperature can also effect firewood, trying to light a cold piece of wood will be more difficult than lighting a log that has been lying next to the fire for a while as colder wood requires more energy to heat it to the point of ignition.
This has been a very basic look at the different fire woods, if you want to learn more I would strongly recommend reading The Wood Fire Handbook or go online and look at some of the sites that specialise in this sort of thing. Despite this, I hope you have found my efforts helpful, and as always questions are appreciated!!