Is robinia good firewood? – myths and facts
Robinia series · Firewood guide
One of the best hardwoods for the stove, the one that dries fastest – and yet the one you are better off not burning.
"Is robinia good firewood?" is one of the questions most often asked about this species – and the answers in circulation mix the best with the approximate. Let's be frank: yes, robinia (Robinia pseudoacacia) – also known as black locust, and often sold as acacia – is an excellent firewood, among the very best European hardwoods, and the one that needs the shortest wait before it is ready to burn.
Our trade is structural and outdoor timber – stakes, decking, shingles – not selling firewood by the stere. But it is the same tree: here is what the sawmill can tell you about how it behaves in the fire, figures and nuances included.
≈ 18 GJ per tonne
the calorific value of oven-dry robinia – and still about 14.4 GJ per tonne net calorific value once dried to 20 % moisture content, on a par with oak and beech
Why robinia heats so well
It all starts with density: 720 to 800 kg/m³, more than oak. But beware of the shortcut, because the ranking changes with the unit. Per tonne, the energy is almost the same for all timbers: oven-dry robinia yields about 18 GJ per tonne, and about 14.4 GJ remains at 20 % moisture content – the same values as oak or beech, within a few per cent. When dry, a tonne of wood is a tonne of wood. It is by volume that everything is decided: the denser a timber, the more combustible matter a given stere contains, and that is where robinia takes the lead. It lights easily, gets hot quickly, then leaves dense, long-lasting embers, prized for keeping the stove going overnight. Well seasoned, it gives off fine smoke that fouls flues very little.
An amusing detail of nomenclature: traditional French firewood classifications put robinia among the "medium-hard" hardwoods (group G2), whereas its density and yield place it at the level of group G1 – oak, beech, hornbeam. G2 on paper, G1 in the stove.
Its little-known asset: it contains little water
Firewood is not judged on its calorific value alone: it is judged on how long it takes to become ready to burn. On this score, robinia starts with a lead that few people know about. It comes out of the forest far less waterlogged than other hardwoods: the US Forest Service's Wood Handbook gives about 40 % moisture content for freshly felled robinia heartwood, whereas oak sits between 60 and 80 % and beech at around 55 to 70 %. For the same volume, there is therefore far less water to evaporate before reaching the 20 % of wood that is ready to burn.
Two things explain this lead. Robinia heartwood is dense and saturated with extractives – the flavonoids that also give it its durability – and it takes up almost the whole cross-section of the tree, the waterlogged sapwood forming only a narrow ring. And the wood moves little as it dries: about 10 % volumetric shrinkage, against 13 to 17 % for oak and beech, which limits drying splits. Felled when the sap is down, between September and March, as we do for our structural timber, it starts even lower.
The practical consequence: under identical storage conditions, split robinia is ready before oak split on the same day. And once dry, it keeps for a very long time without losing any of its qualities – a stere of robinia under cover stays good from one season to the next.
Seasoning: 18 to 24 months, and no folklore
Allow 18 to 24 months of seasoning for split wood, stored split, under cover and ventilated, to get below 20 % moisture content – the target below which wood burns cleanly. The good news: despite its density, robinia dries faster than oak – it contains less water to begin with, as explained above – and it then keeps for a very long time without losing its qualities.
The wood that burns fresh. You often read that black locust "burns green", and this is presented as an American myth. It isn't one. In Hungary and Romania, where robinia is the everyday firewood par excellence, people burn it the same year they buy it – that is even a good part of its reputation there: no need to build a woodpile two years ahead. Its heartwood starts from such a low moisture content that it catches and holds a flame where fresh oak smokes and chokes. To be honest: fresh, it burns poorly – part of the energy escapes as water vapour, efficiency collapses, the flue gets fouled – but it burns. This is a very real tolerance, not a method to recommend.
Splitting. One last tip from the sawmill: dry robinia is very hard to split. Split it fresh, just after felling – and above all, don't wear yourself out with an axe. On dense hardwood, only a machine can keep up: our factory log splitter turns out 40 tonnes of logs a day, over two ten-hour shifts. For a few steres for the winter, hiring a hydraulic log splitter will always cost less than a bad back.
Should you really burn robinia?
Let's say it frankly, even if it does the timber seller no favours: making firewood out of robinia is a waste. It is too noble a material to end up as a single evening's fire. And behind that statement lies an inescapable logic, which anyone who splits logs confirms without thinking about it: a robinia log that splits well is a straight log, knot-free, with no star shake – exactly the one that would have made a stake, a decking board or a paver. The one that resists the log splitter is crooked, knotty or hollow: it would never have made a finished product. In other words, the best robinia firewood is precisely the wood that should never have been burnt.
That is why, with us, robinia never goes for firewood logs. On the felling site, stems too crooked or too thin for sawing supply the local wood-energy sector; in the workshop, everything the sawmill cannot turn into cleft palings, decking, pavers or stakes is chipped. The order of magnitude of one load: a walking-floor trailer of wood chips – mixed species – leaves the factory at about 25 tonnes gross for 18 tonnes of dry matter, nearly 320 GJ of energy, the equivalent of some 90,000 kWh. Seven tonnes of water out of twenty-five, barely 30 % of the weight carried: the wood's low natural moisture shows right down to the weighbridge.
Shipping firewood across Europe would make no sense in any case: the transport would cost more than the wood. Robinia firewood therefore remains what it is – a Central European tradition, where the resource is abundant and the stove is a stone's throw from the forest.
Key points
· Excellent firewood: ≈ 18 GJ/t oven-dry, ≈ 14.4 GJ/t net (4 kWh/kg) at 20 % – on a par with oak and beech, with long-lasting embers.
· It starts with little water: about 40 % moisture content standing, against 60-80 % for oak – hence faster drying and few splits.
· Seasoning 18-24 months split, under cover, target < 20 % moisture content – it dries faster than oak and keeps for a very long time.
· Split it fresh, and by machine: dry, it is one of the hardest woods to split.
· At Acacia Bois: robinia never goes for firewood logs – whatever does not become a finished product is chipped for the local sector.
· A log that splits well is a straight log – so the one that would have made a stake or a board. The best robinia firewood is the wood that should never have been burnt.
Frequently asked questions
Does robinia give as much heat as oak or beech?
Yes, it is in the same league – and the comparison is more honest in gigajoules per tonne than in kilowatt-hours per kilo. Oven-dry, robinia yields about 18 GJ per tonne, like most timbers, hardwoods and softwoods alike: at absolute dryness, a tonne of wood is a tonne of wood. At 20 % moisture content, the state of wood ready to burn, about 14.4 GJ per tonne remains as net calorific value (NCV) – 4.0 kWh per kilo. Beware of the most common trap: the figure of "16 GJ/t at 20 %" often quoted is a gross calorific value (GCV), which also counts the energy lost up the flue as water vapour. It is the net calorific value that heats the room. The real difference between species is therefore not per tonne but by volume: at 720 to 800 kg/m³, robinia outdoes oak, and a stere contains more combustible matter – in the region of 1,900 to 2,100 kWh per stere, depending on the source. Its embers also last particularly long.
Is robinia bark a good firelighter?
Excellent, and it is a use that regulars know well. Robinia bark is thick, deeply furrowed and naturally very dry: it comes away in fibrous strips that catch fire instantly and burn with a lively flame, without the need for paper or shop-bought firelighters. So keep the strips that come off the logs during splitting and storage rather than sweeping them away: they make the best fire starter, before the kindling and the logs.
How long should robinia firewood be seasoned?
18 to 24 months for split wood, stored out of the rain and ventilated, until it drops below 20 % moisture content. That is faster than oak, despite a higher density. It tolerates being burnt early better than other species – see the next question – but insufficiently seasoned robinia fouls the flue like any other species.
Can you burn robinia while it is still fresh?
Partly, yes, and it is not an American myth: in Hungary and Romania, robinia is the everyday firewood precisely because you can buy it and burn it in the same year. Its heartwood naturally contains little water – around 40 % at felling, against 60 to 80 % for oak – and it takes a flame where fresh oak smokes and goes out. But it burns poorly: efficiency collapses, part of the energy goes into evaporating the water, and the flue gets fouled. See it as a stopgap, not a method.
Can robinia offcuts and sawmill waste be burnt?
Yes, without reservation: robinia is never treated – no pressure treatment, no biocides – so its offcuts are raw timber that can be burnt or chipped quite safely. It is the exact opposite of pressure-treated timber, which must never go on the fire. At the sawmill, that is exactly what happens to our edgings: wood chips for the local wood-energy sector.
Further reading
Reference guide: "Robinia outdoors – the complete guide".
See also: "Robinia forests in Romania – felling, brash windrows and root suckering" (sorting on the felling site) · "Robinia or pressure-treated pine (use class 4) – the comparison" (why treated timber must not be burnt).
Our products: Round stakes · Sanded posts · Unedged boards – structural timber first.
Contact: Looking for robinia to build with rather than to burn? The sawmill will price stakes, boards and decking – keep the fire for the offcuts.
Disclaimer. Non-contractual article, provided for information only (photos and drawings included). Robinia is a natural material: the values given are orders of magnitude and may change without notice; in case of discrepancy, only the product’s technical data sheet and the general terms of sale apply. Always check the regulations that apply to your project.
© Xylo Impact SRL — all rights reserved, products marketed under the Acacia Poles brand. Any reproduction, even partial, is prohibited without prior authorisation.
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