The University of Oxford Botanic Garden News - Harcourt Arboretum

8Botanic Garden News | No. 77The ancient art ofcharcoal productionat Harcourt Arboretumby MalcolmRutherfordMalcolm Rutherford is originally from New Zealand and is a trainee at theBotanic Garden. He studied botany at university and later worked in rarebird conservation in New Zealand with the Kakapo and various seabirds,and taught ecology at Fota Wildlife Park in Ireland.It is half past five on a Wednesday inDecember. The sky is clear and thetemperature is around minus four.Buses don’t run at this time of day so I’mwaiting to be picked up by Guy, an arborist atHarcourt Arboretum. He is delayed a little bysnow but soon arrives and we are on our wayto Nuneham Courtenay. The time has cometo create some charcoal.Charcoal production is an ancient craftwhich, through the centuries, has led tosignificant steps forward in civilization.Charcoal burns at a hotter temperaturethan wood so was used to smelt metal frommined ore. This was first carried out on alarge scale in the Bronze Age. Improvementsin the efficiency of burning charcoal allowedhotter temperatures to be reached so thatharder metals like iron and alloys such asbrass could be smelted.Metal working relied on charcoaluntil a process to turn coal into coke wasdeveloped in 1735; within 100 years charcoalproduction in Great Britain had declinedArborist Guy Horwood tends the charcoal kilnsignificantly, despite the developmentof new uses for charcoal as horticulturalgrowing media, filtration media and as amedicine. Production was at its lowest pointin the mid 1980s but there has now beenan increase with the recent renaissance inwoodland crafts.By the time we arrive at the charcoal kiln,Karl (the other arborist at the Arboretum)has already started a bonfire which isa welcome source of heat on this crispmorning. Once there is a glowing pile ofembers we start to shovel these into atrench we have dug around the base of thekiln. The kiln is a large drum about 6 feet indiameter with a slightly peaked lid and airvents around the base. The previous day westacked it with wood from a recently felledtree, leaving air channels in a star shapeon the bottom and a space up the middleto allow air and heat to circulate evenlythroughout the stack. Some of the woodsoon catches alight and a huge amount ofsteamy smoke starts to come out of the kilnMalcolm Rutherfordas water is driven out of the wood. The steamrising is mirrored by large flakes of snowwhich have started to fall.Charcoal is produced by heating organicmatter in a low-oxygen environment. Thisdrives off the water and other volatilecompounds leaving the carbon skeleton ofthe plant. If burning is too rapid then thewood will burn away leaving a kiln full of ash,and if it is too slow then dry, slightly charredwood is left. To find the right balance we haveto control the amount of air entering the kilnthroughout the day by putting the lid on thekiln and blocking up the gap around the base.There are six vents, three of which we turninto chimneys by adding tall pipes and threeof which we leave as air intakes. Over thenext ten hours we check the embers burningthroughout the base of the kiln, adjusting theairflow as necessary. Ideally we would staywith the kiln until the smoke changes fromwhite to blueish grey. Unfortunately we aren’tcamping on site so we have to leave beforethis occurs, when the sun has gone down andthe working day is over. We block up all thevents to stop any oxygen from entering thekiln; now we have to wait a few days to seethe outcome of the burn.The production of charcoal is one ofthe most damaging practices in the world,responsible for deforestation, poor publichealth and greenhouse gas emissions. Anestimated 40 million tons of charcoal areproduced worldwide each year, half of themin Africa. In approximately 70% of EastAfrican homes charcoal is the sole energysource because electricity and fossil fuelsare unavailable or too expensive. Charcoal isoften produced and burned very inefficiently,and smoke and carbon monoxide fromcooking on indoor charcoal, wood, and dungfires cause an estimated 400,000 deaths(mainly of women and children) in subSaharan Africa every year. There are currentlyinitiatives trying to address these problems