Charring Posts (Charcoal 5)
Charring Wood as a Preservative Treatment
To read about what charcoal is, see The History of Charcoal. To read about the mound method of producing charcoal, see Charcoal Clamps, and to read about the underground method, see Pit Kilns. To read about other uses for charcoal, see Micro-Charring.
Char chemically changes wood, making it more resistant to rot, insects and fire damage, and can be done easily at home without specialized equipment.
Background
In temperate regions, wood posts are likely to rot, particularly at the line where they protrude from the ground. The denser the wood, the slower the rot (which is why locust wood makes good fence posts), and certain species of tree can contain chemicals that fight mold and insect damage, but here in the mid-Atlantic region of the US, these are not enough. Even cedar posts rarely last longer than 20 years without treatment. Locust posts frequently survive 10 years, and other hardwood species last less than that. A poplar or a softwood, like pine, set as a post, is likely to break off at ground level in less than 6 months. This has been a concern for humans since we first began building with wood, and solutions are often simple.
Options to preserve posts: The simplest way to preserve wood is to keep it constantly wet and cool, or else constantly dry. For fence posts or post buildings, where rain can reach the base of the post, the first step is to make the hole drain well before setting the post. This means digging a much bigger hole, setting gravel in the bottom, putting a rock in the bottom on top of the gravel and setting the post on that, to make sure that no water pools in the hole. Backfilling the hole with large gravel or rocks makes sure that the water moves away from the post quickly. Still, over time the gaps between the rocks will fill with sediment and the post will be in contact with wet dirt. Also, making the hole this way is labor intensive and assumes you have easy access to rocks.
Note: some folk wisdom indicates that the wood posts will absorb less moisture if they are put into the hole "root-end up." The theory is that the tree is built to suck water up from the root end, so inverting the tree structure helps keep the wood dryer. I haven't found any scientific studies looking into this, but I know that tree sap flows both up and down the tree, especially in winter, which leads me to doubt that the absorption of water would be one-way, as this folklore suggests.
Since keeping the wood dry is thus impractical, it is more sensible to try to treat the wood to prevent it from rotting. Treatments in the past few centuries have included paint, tar, creosote and various poisons that discourage bugs and bacteria. Creosote, in particular, is cheap to produce and very effective. Any wood fire that produces smoke can provide creosote, which then can be melted and painted onto the posts. It saturates the outer layer of wood and forms a poisonous barrier that keeps rot from eating the treated portion of the wood.
History
Options to preserve posts: The simplest way to preserve wood is to keep it constantly wet and cool, or else constantly dry. For fence posts or post buildings, where rain can reach the base of the post, the first step is to make the hole drain well before setting the post. This means digging a much bigger hole, setting gravel in the bottom, putting a rock in the bottom on top of the gravel and setting the post on that, to make sure that no water pools in the hole. Backfilling the hole with large gravel or rocks makes sure that the water moves away from the post quickly. Still, over time the gaps between the rocks will fill with sediment and the post will be in contact with wet dirt. Also, making the hole this way is labor intensive and assumes you have easy access to rocks.
Note: some folk wisdom indicates that the wood posts will absorb less moisture if they are put into the hole "root-end up." The theory is that the tree is built to suck water up from the root end, so inverting the tree structure helps keep the wood dryer. I haven't found any scientific studies looking into this, but I know that tree sap flows both up and down the tree, especially in winter, which leads me to doubt that the absorption of water would be one-way, as this folklore suggests.
Since keeping the wood dry is thus impractical, it is more sensible to try to treat the wood to prevent it from rotting. Treatments in the past few centuries have included paint, tar, creosote and various poisons that discourage bugs and bacteria. Creosote, in particular, is cheap to produce and very effective. Any wood fire that produces smoke can provide creosote, which then can be melted and painted onto the posts. It saturates the outer layer of wood and forms a poisonous barrier that keeps rot from eating the treated portion of the wood.
History
In many ancient and medieval archaeological sites, wood post holes remain our best evidence of structures, palisades, causeways and other human buildings that have long since decayed. An amazing number of these post holes include a layer of charred material that indicate that the wood posts had burned at some time. Too many of these include burns far below what would have been the surface of the ground at the time, which suggests that they did not burn once they were in the ground, and some show no evidence of the structure above having burned. Conversely, in cases like the Must farm, or some crannogs in Ireland, there is a large amount of surviving wood from the upper portion of the structures specifically because those structures burned. The charring, combined with the waterlogged environment, preserved them for us to study. Together, these suggest that humans have used charring to preserve wood since at least the Bronze Age in a variety of cultures/regions, and that they were right to guess that it would work.
In Egypt and the later Mediterranean cultures, posts for docks and other wet structures were charred before being set, to preserve them in their marine environment against rot.
In Asia, examples of char-preservation still exist above-ground, and the process is still in use today. A wooden Japanese temple built in the 8th century, for example, has survived hundreds of years thanks to the periodic re-charring and oiling of all of its surface wood. Shou sugi ban is the name for this charring treatment. It can supposedly preserve wood against shrinkage, rot and insects in the Japanese humid, temperate climate for 80-100 years without the oiling, but for above-ground wood that will dry out from years of exposure to air, a light oiling is recommended (every few decades). There are also claims that the surface charring of above-ground structures helps prevent fire, because it requires a higher temperature to ignite char than dry raw wood.
Charring
In Egypt and the later Mediterranean cultures, posts for docks and other wet structures were charred before being set, to preserve them in their marine environment against rot.
In Asia, examples of char-preservation still exist above-ground, and the process is still in use today. A wooden Japanese temple built in the 8th century, for example, has survived hundreds of years thanks to the periodic re-charring and oiling of all of its surface wood. Shou sugi ban is the name for this charring treatment. It can supposedly preserve wood against shrinkage, rot and insects in the Japanese humid, temperate climate for 80-100 years without the oiling, but for above-ground wood that will dry out from years of exposure to air, a light oiling is recommended (every few decades). There are also claims that the surface charring of above-ground structures helps prevent fire, because it requires a higher temperature to ignite char than dry raw wood.
Charring
However, the most direct way to treat wood is by fire. This also simplifies the removal of bark, which is another critical factor in keeping the wood from rotting. Bark & sap wood are less dense than heartwood, so they are more vulnerable to moisture and rot. Burning the bark makes it flake off with minimal effort, using just a shovel or other scraping implement. Removing bark by other means may include swinging an adze (harder and more dangerous and slower) or using a drawknife (slower) or a specialized bark knife (harder and you need to own one).
Charring can be done slowly by simply mounding the coals and ashes in a pile over the log to create an anaerobic environment. Just make sure to pile ashes all the way out from the hot zone, to prevent the log from burning aerobically where the ashes end. This is a useful way keep the char going while you take a break.
Once the bark is burnt and scraped off, the inner wood (likely still green) can be burnt as well. The purpose is to deliberately carbonize the wood, like making charcoal. Charcoal is relatively pure carbon, and not edible or even appetizing to most bacteria, fungus and insect species. However, turning too much of the wood into charcoal weakens the structure of the wood. Also, charcoal absorbs water very efficiently (as gardeners who add crushed charcoal to their compost rely on). Having too deep a layer of charring on the post, therefore, is counter productive (because it will now hold moisture against the unburnt wood and rot faster, not slower). The trick is to get a balance: char deep enough to chemically alter the wood by burning out the stored sugars & nutrients in the cells that would feed rot, but then scrape any build-up of charcoal that forms too deeply. The depth can be judged by the texture; a cracked, chunky crust of charcoal should be scraped free, leaving a smooth, shiny black wood, now essentially coated with its own creosote, without too many cracks that would allow water to penetrate.
Scraping is relatively easy work, using a regular spade shovel, but it is very dirty. The black dust that breaks free contains charcoal and creosote, so make sure you don't breathe it in! The fire is ideal if it burns hot, with a good bed of coals, but is not roaring. Theoretically, the slower the burn, the better, but decide your level of speed based on your own time constraints and practicality. The fire to avoid is one that burns the post too quickly and creates cracks or weak spots before you can rotate the post. You want to char evenly around the entire log, to a height greater than the post will sit in the ground. If you plan to sink the post two feet deep, char at least 2'6" to protect that most vulnerable area, where the moist wood in the ground will meet the oxygen above-ground.
Be careful in rolling the log in the fire; working with hot, green wood is heavy and potentially dangerous. Think about ways to prop up your log and levers to control it when rolling. Wear appropriate shoes or boots to protect your feet in case the log rolls onto them.
Build your fire in a pit if possible, to make rolling the log above it easier. Ideally, you want to keep the part of the post that you do not plan to burn outside of the fire. It may help to make a pile of ash or sand to heap on that part of the post, to stop your fire from getting greedy. Once you have hot coals, a good bed of ash and some fresh, burning wood on top, put the post(s) in. The part in contact with the coals may not char much, while the fire licks rapidly up the sides of the log and does little on top. Rotate it every few minutes to get it charred evenly all the way around. If you have a long-handled shovel with a wood haft and unpainted metal head, you can even start scraping & removing the bark after the first rotation, while the next side is burning. If you can, it makes the process much faster, since the bark doesn't need to be fully burnt on the log and you can start on the sapwood right away. Just be careful not to get burned!
Scrape the char until the log is black and shiny and relatively smooth. If you scrape too deeply, the lighter color of uncharred wood will show through. If you don't scrape enough, the built up chunks of charcoal will appear cracked and flaky.
Charring a green locust log 2'6", with a diameter of 8" with the bark on, and about 6" after the final scraping, took about an hour total. However, with a large firepit and multiple posts in the fire simultaneously (and a helper who can rotate the logs and remove any that are ready from the fire), this can be streamlined. I would guess that it could be simplified to 15 minutes of labor per post, and perhaps less, with practice.
This is a great solution for those who have spare/scrap wood to burn outdoors (like pine, that cannot be used in the woodstove), free hardwood for cutting posts (like young trees to cull from a mature woodlot for the health of the forest) and more time than spare money (or else, a desire to be self-sufficient and to avoid using synthetic poisons such as are in modern pressure-treated lumber). It is very sustainable and, compared with replacing rotten posts more often, is well worth the input efforts (15-60 minutes to treat the post, vs digging a new hole, setting a new post and re-making fences or structures). However, it is definitely not easier or faster, nor any more effective, than using pressure treated posts if they are available and not contrary to the goal of your project.
Charred locust logs set in the ground to make a butchering stand for hanging carcasses.
Update: the charred posts set into the ground in the Fall of 2017 remain untouched by carpenter bees, ants and other bugs, as of summer 2019. They show no further shrinkage or checking since they came out of the fire, and no sign so far of any rot or damage, in spite of the extreme humidity and rainfall of the year 2018. We will continue to check on it every year to monitor how it fares, but already it has done better than a dogwood post of equal diameter that was simply stripped of its bark before being set into the ground. That post, set very near these charred posts, shows shrinkage and the beginning of rot, even after just one year.
Charring can be done slowly by simply mounding the coals and ashes in a pile over the log to create an anaerobic environment. Just make sure to pile ashes all the way out from the hot zone, to prevent the log from burning aerobically where the ashes end. This is a useful way keep the char going while you take a break.
Once the bark is burnt and scraped off, the inner wood (likely still green) can be burnt as well. The purpose is to deliberately carbonize the wood, like making charcoal. Charcoal is relatively pure carbon, and not edible or even appetizing to most bacteria, fungus and insect species. However, turning too much of the wood into charcoal weakens the structure of the wood. Also, charcoal absorbs water very efficiently (as gardeners who add crushed charcoal to their compost rely on). Having too deep a layer of charring on the post, therefore, is counter productive (because it will now hold moisture against the unburnt wood and rot faster, not slower). The trick is to get a balance: char deep enough to chemically alter the wood by burning out the stored sugars & nutrients in the cells that would feed rot, but then scrape any build-up of charcoal that forms too deeply. The depth can be judged by the texture; a cracked, chunky crust of charcoal should be scraped free, leaving a smooth, shiny black wood, now essentially coated with its own creosote, without too many cracks that would allow water to penetrate.
Scraping is relatively easy work, using a regular spade shovel, but it is very dirty. The black dust that breaks free contains charcoal and creosote, so make sure you don't breathe it in! The fire is ideal if it burns hot, with a good bed of coals, but is not roaring. Theoretically, the slower the burn, the better, but decide your level of speed based on your own time constraints and practicality. The fire to avoid is one that burns the post too quickly and creates cracks or weak spots before you can rotate the post. You want to char evenly around the entire log, to a height greater than the post will sit in the ground. If you plan to sink the post two feet deep, char at least 2'6" to protect that most vulnerable area, where the moist wood in the ground will meet the oxygen above-ground.
Be careful in rolling the log in the fire; working with hot, green wood is heavy and potentially dangerous. Think about ways to prop up your log and levers to control it when rolling. Wear appropriate shoes or boots to protect your feet in case the log rolls onto them.
Build your fire in a pit if possible, to make rolling the log above it easier. Ideally, you want to keep the part of the post that you do not plan to burn outside of the fire. It may help to make a pile of ash or sand to heap on that part of the post, to stop your fire from getting greedy. Once you have hot coals, a good bed of ash and some fresh, burning wood on top, put the post(s) in. The part in contact with the coals may not char much, while the fire licks rapidly up the sides of the log and does little on top. Rotate it every few minutes to get it charred evenly all the way around. If you have a long-handled shovel with a wood haft and unpainted metal head, you can even start scraping & removing the bark after the first rotation, while the next side is burning. If you can, it makes the process much faster, since the bark doesn't need to be fully burnt on the log and you can start on the sapwood right away. Just be careful not to get burned!
Scrape the char until the log is black and shiny and relatively smooth. If you scrape too deeply, the lighter color of uncharred wood will show through. If you don't scrape enough, the built up chunks of charcoal will appear cracked and flaky.
Charring a green locust log 2'6", with a diameter of 8" with the bark on, and about 6" after the final scraping, took about an hour total. However, with a large firepit and multiple posts in the fire simultaneously (and a helper who can rotate the logs and remove any that are ready from the fire), this can be streamlined. I would guess that it could be simplified to 15 minutes of labor per post, and perhaps less, with practice.
This is a great solution for those who have spare/scrap wood to burn outdoors (like pine, that cannot be used in the woodstove), free hardwood for cutting posts (like young trees to cull from a mature woodlot for the health of the forest) and more time than spare money (or else, a desire to be self-sufficient and to avoid using synthetic poisons such as are in modern pressure-treated lumber). It is very sustainable and, compared with replacing rotten posts more often, is well worth the input efforts (15-60 minutes to treat the post, vs digging a new hole, setting a new post and re-making fences or structures). However, it is definitely not easier or faster, nor any more effective, than using pressure treated posts if they are available and not contrary to the goal of your project.
Charred locust logs set in the ground to make a butchering stand for hanging carcasses.
Update: the charred posts set into the ground in the Fall of 2017 remain untouched by carpenter bees, ants and other bugs, as of summer 2019. They show no further shrinkage or checking since they came out of the fire, and no sign so far of any rot or damage, in spite of the extreme humidity and rainfall of the year 2018. We will continue to check on it every year to monitor how it fares, but already it has done better than a dogwood post of equal diameter that was simply stripped of its bark before being set into the ground. That post, set very near these charred posts, shows shrinkage and the beginning of rot, even after just one year.
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