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Experimental Homestead

LIME SQUAD! I: A Photoessay on Lime Burning

wood and shells in a carefully laid pattern meet heat and oxygen.

LIME SQUAD! I:  A PHOTO ESSAY ON LIME BURNING

LIME SQUAD! II:  THE SLAKING.

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I’ve been interested in lime for a while now.  I use it in tanning hides and I want to use it in building.  Lime is also used in processing corn into hominy as well as masa for tortillas.  Doing things completely from scratch always interests me, so project buddy tonia and I set out to burn some lime and see just how much work and fuel is required for what results.  The following is a photo essay on our first lime burn, but first a few thoughts on lime, lime burning and lime users.

A BRIEF DESCRIPTION OF THE BASIC PROCESS

Limestone or shells (Calcium Carbonate CaCO3) are burned for a time until they are calcined, that is reduced to Calcium Oxide a.k.a. “quicklime” (CaO).  This releases carbon dioxide into the atmosphere.

The lime is now what is called quicklime and it is quick… quick to react with water in an intense chemical reaction creating heat and converting the lime to Ca (OH)2

To make dry hydrated lime, which is more stable than quicklime, the quicklime is sprayed with just enough water to cause it to undergo most of its reaction and crumble into a powder after which it is bagged.  Hydrated lime is used as an admixture to increase the workability of portland cement mortars which, even with the addition of lime, are horrid to work with.  Other people use hydrated lime as one would use lime putty that is mixed with water to form a lime “paste”.

To make Lime putty, the quicklime is mixed with a larger quantity of water whereupon the stuff boils like crazy and turns into calcium hydroxide, Ca (OH)2.  Lime putty remains as calcium hydroxide as long as it is kept from air under a cozy blanket of water.  Under water it only improves with age.

When the lime paste or putty is used it must be dried slowly. As the lime dries, it reabsorbs the Carbon which was driven away from it in burning back out of the atmosphere turning back into limestone (CaCo3) .  So, the lime putty is used wherever it is that you need limestone such as in mortar, lime concrete, plaster etc…  How cool is that?

This link is for a diagram of the lime cycle.

I’ve heard quite a bit of bad press on lime lately from natural builders, but I think if the facts are considered this trend indicates a lack of skill and knowledge on the part of users and/or the use of poor quality lime rather than any shortcoming intrinsic to lime as a building material.  After all, there are still buildings standing from Roman times that are built with lime.  There are also hella old cob houses in Europe that would be eroded into the ground if not for their protective lime skins.  In the book Building With Lime by Stafford Holmes and Michael Wingate there are numerous caveats about how the quality of lime products affects results in building projects and the care required in application and curing.  One of these is the recommendation against using hydrated bagged lime in building.  The impression I was left with after reading Building With Lime is that hydrated lime is an inferior product produced out of convenience and for specialized uses where powdered lime is required.  Freshly burned quicklime has a short shelf life, is highly reactive and is dangerous to handle which probably accounts significantly for its general absence from regular commerce.  Add a little water to get most of the dangerous reaction over with and voila- safe inferior product!

Lime putty is considered by traditionalists to be a greatly superior product, but it is hard to come by and what is available is quite expensive to buy as well as to ship.  It would be great if it was available locally where limestone or shells occur in sufficient quantity.

The Lime Midden

tonia and I had both done at least some homework on lime basics and lime burning.  I was looking around for local limestone sources (scant, yet promising) but then realized one day that I had a good sized pile of mussel and clam shells from foraging trips to the coast. I always dump my shells in a pile intending to someday grind them up for use as a fertilizer.

185(ish) Pounds of Shells

The shells were sorted out and weighed out at around 185 pounds.  (Note to self:  save the work of sorting by not dumping shells under deciduous oak tree.)  There were also a few periwinkle shells (a small sea snail) and a few ratty looking abalone shells were also thrown in.  (abalone shell dust is known to be toxic so I’m not sure whether this was a good idea or not.)  We realized that we didn’t really know enough, but decided to throw together a first run so that we could get our feet wet and possibly clarify questions to ask and find answers to.

Deadwood from the understory

Manzanita was gathered from dead trees that had been overgrown by larger trees.  Manzanita is a pioneer plant.  It colonizes new areas where its dense shrubby growth shelters young tree seedlings of other species.  The sheltered tree seedling trees grow up like the ungrateful dependents of a wet nurse eventually shading out and killing the Manzanita.  The wood sits there for many years without rotting.  it has a very high fuel value and produces little ash.  Being winter, the wood was somewhat wet which may have been an advantage as its general tendency is to burn very fast and hot while lime should not be burned too hot.  Still, as woods go, Manzanita absorbs less water than most, dries back out quickly and burns pretty well when wet.

Ye ‘ol water tank and the beginning of the starting fire lay. Very small brush, twigs and fir pitch wood sticks were laid carefully to encourage a fast easy ignition. This initial pile was about 14 inches high and 18 inches in diameter with wood up to about 1.5 inches thick before the kiln shell was tipped up into place.

Half an old water tank with the bottom and top cut out was chosen for a kiln of sorts.  Originally bricks were considered, but there were not enough about and in retrospect loosely stacked bricks seems a poor idea while mortaring them up with clay seems like too much work for a one shot kiln.

A trench was dug to admit air to the center of the kiln and allow for ignition from the bottom.  A carefully laid crisscross of twigs and split pitch saturated fir was constructed to insure a quick and easy ignition.  This starter pile was then filled in from the sides.  The slate roofing scraps were to prevent excessive steam rising as the ground was wet and rain was expected, as well as to make a clean dry floor for the burned shells to settle on.

The first wood layer

Fuel was laid as densely as possible by careful stacking, but in retrospect, more care might have been taken in some sections.  It was somewhat difficult to reach in and place fuel carefully, but I think it is probably important.

as the pile progressed, larger pieces of wood were placed in the kiln to form a flat surface to receive the shells.

First layer of shells

The shells were placed in two layers.  Originally we had discussed keeping the shells to the center of the pile.  This idea was not carried out however and the shells were pushed all the way to the edge of the kiln.  With some experience now, I’m inclined to think that next time it would be best to form a flat layer of wood as we did, but then form a raised rim of firewood around the edge to contain the shells… something like building a gravy volcano in your mashed potatoes.

Loading the second layer of shells, note a few larger pieces of oak and bay split firewood were used in the upper layers of wood. (that’s not a recommendation)

The second layer of shells was left on top of the wood.  In retrospect I think this was a bad idea and they should have had a layer of wood on top of them.  I thought it was okay because there are traditional burns that put the limestone or shells on top, but Jeff Price at Virginia Lime Works said that those traditional open burn piles are built carefully in such a way that they collapse in on the shells.  Leaving these shells on top probably cost us 50% of the shells being incompletely burned.  Possibly too, with a proper kiln that had some insulation or at least some mass that could heat up or a narrower opening? the shells might be able to be on top without a layer of wood.  In this case though, I think a layer of wood on top would probably be better.

The fire lighting off from underneath. This view is through the airway trench.

The pile was lit from underneath with a torch of dry grass and slivers of pitch wood bound to a long stick.  It worked great and spread rapidly into the carefully laid fire base.

Smokey

We had decided to wait until dark to light the kiln as I knew there was likely to be a great quantity of smoke and steam produced.  Good thing.  I was right and I’m sure it would have worried the neighbors greatly if not the fire department although it was quite wet out.  I wouldn’t want to do this in our fire prone area in any but the very wet seasons.  The first 45 minutes or so were all smoke and steam though the side of the barrel near the air entrance was glowing red.

Yay, fire! Everyone likes a good conflagration.

At some point it was decided that we needed air on the other three “sides” to get things burning evenly.  Holes were dug under the container just enough to let some air in.  The air flow was controlled with broken pieces of roofing slate.

Sorting shells in the morning

The fire was cool enough by morning to start sorting and rain was expected, so we got right to work.  The burn was only partially successful in the goal of producing lime in that much of the shell was under-burned.  If making lime had been the sole goal we would have spent more time in researching and such.  The primary goal was to get our feet wet (dry?) and stimulate our questioning minds before bugging anyone with too many questions.

burned and under-burned shells

The shells that did not burn enough were darkly colored.  The fully calcined shells were white all the way through feeling almost painfully dry like fresh fired pottery.  Calcined shells are very light in weight and ravenously thirsty for some water which they will react with even in the quantities available in the atmosphere.  In short, freshly burned quicklime is unstable and should be processed immediately.

40 pounds of quicklime.

Partially calcined shell showing an under-burned core.

I was guessing that about 1/3 rd of the shells burned enough.  Considering that the shells lose a lot of weight and the end result was 40 pounds maybe I wasn’t too far off  base on that guesstimate.  I’m guessing that a final layer of wood on top, laying the shells in such a way that they are completely surrounded by wood, and or some insulation or at least a heatable mass to the kiln could shift that into a more acceptable figure.

Fun with charcoal. Wee…..  love the dirty work.

DON’T MISS PART TWO, SLAKING THE LIME! WOOHOO!

Having done this one simple, but largely successful lime burn, I am encouraged by the results.  I’m hoping that, aside from collecting the stone or shells, two people will be able to do a 200 lbs shell burn in one day including firewood collection.  How much this will yield is unknown, but I’m hopeful that it will be enough to make it “worth it”.  I have my own ideas about what is and isn’t worthwhile in terms of work v.s. returns.  It is difficult in the society we live in to step out of the monetary global economy enough to see value that is not directly weighed in dollars and cents terms regardless of other factors such as personal investment and satisfaction, meaningful work, broader impact, pride in workmanship and so on.  Neither view is correct in the strictest sense, but they rather reflect our values don’t you think?  And because the economy we live in is so persistently invasive and all encompassing in most of our lives,  it shapes our values without us even knowing it causing us to use an assumed “bottom line” as a measure of worth for nearly everything we do.  The trick is not so much whether we can notice this phenomenon, but rather whether we can actually embody an alternative idea to the extent that it can manifest change in our behavior.

Building things with my own lime burned here on the land rubs me right on many levels and that feeling pulls a lot of weight in deciding whether its “worth it” or not.  I’m hoping that I can actually make another significant impact in bringing the pieces of my life home to roost outside of a network of supply and demand that I can’t possibly grasp, understand or in the long run rely on.

Currently people use mostly concrete where lime was once used.  Lime however has some benefits that are overlooked by modern builders.  Concrete also has some benefits, but is more costly and energy intensive to produce.  A lime revival seems to be underway, though most of it in the US is still centered around hydrated lime paste.  The more encouraging part is the few dedicated people like those at Virginia Lime Works who are following traditional methods of production and use out of a sense of integrity.  I asked a Guatemalan friend what kind of lime they used in building and she said they just go buy the rock from a quarry cheap and burn it themselves.  I like that.

I think from here we will probably try another burn to correct some perceived mistakes and possibly build a better kiln designed to be re-used over and over.  As far as kiln size goes, I think I would favor the 200 to 300 pounds of shells (or whatever the limestone equivalent would be) range.  I have a particular interest though in figuring out how to make lime and bio-char at the same time.  People now are burning wood just to make bio-char without even using the heat.  What if we could burn biochar while making lime!?!  That would be awesome!  I have no idea of the real feasibility of this idea, but it is intriguing anyway.

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March 6, 2011 - Posted by | Building etc..., Infrastructure | , , , , , ,

9 Comments »

  1. […] LIME SQUAD! I:   A PHOTOESSAY ON LIME BURNING […]

    Pingback by LIME SQUAD! II: The Slaking « Turkeysong's Blog | March 6, 2011 | Reply

  2. great DIY. Ive juststarted looking atthe same processes myself for a cob structure im working on;weve got a biochar kiln were looking at mounting a lime barrel on over the tort. the limit we have is that he charge for the biochar burns about 1 hour and is out. those btus include syngas heat, so it can be an hour at 1100f. I have no idea how that heat/time/volume equation looks in our very beta designs… Ill be geting into it later this summer on my blog and look forward to seeing what ya’ll are coming up with!

    Comment by Deston | May 26, 2011 | Reply

    • Hmmm…. well, you really want right around 900c (1650f)j, which is about what we’ve measured everytime we’ve checked. I’m unsure what the lower limit is, but I think 1100 will probably be too cool. That is also assuming 100% efficient retention of that heat. Have you actually measured that temperature? Do you have any info posted on your biochar set up?… Post a link? The other factor, as you already mentioned, is dwell time. This is a point I’m pretty unclear on at this point, but it keeps coming up. The dwell time we are getting in the hottest part of the kiln is more than an hour I’m sure, but not 12 or more as a lot of sources mention. I have a feeling an hour will be somewhat inadequate. I’m not sure how this plays out, but I’m guessing that with more dwell time the shell or limestone will be calcined more thoroughly. How this really practically and functionally affects the quality of the lime I’m not sure. Maybe you would just get a little more reactive and effective product.

      I’m kind of shying away from the biochar/lime thing just now. Its still my ideal, but I’m think that its looking somewhat less feasible or practical. One problem I’ve already encountered working through it mentally is that, just as you mentioned, there is a limit to the burn time of the biochar unit charge which means a loss of sustained heat. I’m not 100% sure, but I have a feeling we want to bring the shells up to temp and keep them there… not heating and cooling in cycles. If a way can be figured to discharge some of the char quickly from the pyrolitic zone while keeping a new charge going maybe it could work out.

      Right now I would say that stopping the reaction in our barrel burner midway would not really work out so great. It burns thoroughly and there is little charcoal left over. The ash in the bottom is full of small bits of calcined shell which I use as fertilizer, but I would like to find a way to use in building as well. I’m leaning toward a rocket stove now as an indirect heat source… maybe even two separate burners that feed the same hot gas kiln which is insulated and has at least a thin layer of refractory earth or stone as a mass to moderate the temperature inside the kiln a little. One other option I can think of for the biochar is if you can have two biochar burners and switch them out mid process, and probably more than once.

      One final thought here. The shape of the shells and the density with which they pack down might inhibit the flow of gasses through the kiln somewhat. I would think that, especially if you are tending to burn cool (as with any wood fired process actually) conservation and thorough use of that heat would be critical. I am thinking of keeping the kiln size very small, having at least a thin layer of refractory material on the inside of the kiln for some mass to keep the temperature more constant and a good insulative layer outside of that. Even with all that it is a little hard for me to imagine the heat required developing inside that kiln for a very long time using wood as an indirect heat source. I hope I’m wrong. It might just be a matter of scale. I wonder if, on a home scale, that mixed fuel and shells isn’t just going to be a better method. I suppose too it depends on your incentive. There is a lot of wood around these parts that is just waiting for the next forest fire. Even if it wasn’t, there is a high potential for sustainable forestry because there is just a lot of wood and trees around. I have to say though that I’m moved to work toward cleaner burning systems regardless. The barrel method is always very dirty. We’re probably done burning till fall when the rains return, but after that I think we’ll be moving on first to a refined, but still low tech version of what we’ve been doing before moving on to more complicated stuff. I’ll look forward to seeing how things go for you and don’t hesitate to contact me for anything I might be able to help with.

      Comment by turkeysong | May 26, 2011 | Reply

  3. Excellent post and excellent pictures. Thank you for this. In Europe when we burn limestone we always always use coal (either rock or bio). It’s a little bit tricky to make coal out of wood (what you call bio-char?) but it is possible to do it well if you are willing to spend a few days doing it. I have never seen anyone attempt to use the heat generated in burning the wood to make coal, but I think the reason is that in flatland forests the energy you would spend hauling the wood to your “heat capture place” would be far more than the worth of the captured heat. In Europe’s flat forests we just burn it on the spot in earthen kilns (packed ferns and underbrush under a thick layer of earth dug up from the side of the fire pit). In Japan where I live now however wood for charcoal making is more plentiful in the form of bamboo and can usually be very easily transported the very short distances down the mountain to the kiln which is often near the house. Presumably you could use it in winter to warm the house, but staying up all night to watch the fire in winter would be counter productive and in the summer you wouldn’t need the heat. Hence not even the Japanese have figured out anything to do with the heat from making wood-coal.

    The kilns for making the lime would usually be sunk into the earth rather than built up, think old stone walled wells with a hole in the bottom (obviously on the edge of a cliff or something so you wouldn’t need to dig another hole next to it) to add the fire and scrape the finished lime out. You could probably build one like that by using reclaimed fire bricks (bricks made for hearths and ovens) to make a “well” in the side of a hill, about as tall as yourself perhaps, and a yard across. Then be very careful to get the right proportions between shells/rocks and coal.

    Would love to see more posts on your thinking and building about the differences between commercial cement and lime!

    Comment by tokyobling | September 10, 2012 | Reply

    • tokyobling: It would be awesome to come up with a way to use the heat from char production in the field. I would like to work on coming up with ways to heat water or ovens etc… with wood, while making biochar. Ideally if we could burn even a small amount of lime at the same time, that would be triple duty in a way. I’m thinking along the lines of homescale production where you would fire a boiler every so often or something along those lines and burn a few shells at the same time while producing a small batch of biochar. We have a lot of wood here, so that’s all feasible if the technology can be worked out. Production would be small, but incidental and at least partly free relative to traditional wood burning. We have used charcoal and it worked fine. The lack of smoke was nice, but I think we can eliminate that for the most part while still using wood. Have you seen the Rumford lime kiln? it looks awesome. I’d like to work off of that design if it can be scaled down and use wood. I’ve made charcoal by various means, but not very good at it yet. I think it would be easier/more feasible to use the heat from charcoal production using some sort of kiln arrangement, but you may be right about the transportation factor. Also to consider in remote field locations is getting the limestone or shells to the site, or the wood to them. Regardless, there is heat there to be tapped.

      We are somewhere in lime 2.0 right now. When the rains come we can think about burning again and another kiln revision. We took the barrel as far as we could without adding insulation. I had a blog post mostly finished summarizing what we learned from and about barrel burning, but I never got it finalized and posted as of today. Since then we’ve built two small kilns of dry straw and clay slip to test some ideas. I hope to write about that by the end of this winter. Suffice to say for now that barrels are easy to get and use, but not ideal otherwise. Some degree of insulation and/or mass is definitely desirable and increases efficiency notably. We hope to keep working on backyard scale lime burning from micro batches for making tortillas or tanning a few hides, to making enough to build with. As for applications, I hope we will be doing a little plastering this winter, but we’ll see.

      Comment by Stevene | September 10, 2012 | Reply

      • I guess if you made a Rumford Furnace as part of the wall of your main living accommodations, you could have it function as massive stove wall, absorbing heat and releasing it into the room to heat the air. It would be great heat as well, not the kind you get from electric appliances. Or you could water mantle it and have the excess heat warm up water that could be made to circulate through the inner walls of your house with a simple tube system built into the earthen walls (cob, loam etc.). On the side of the lime kiln you would burn wood to produce coal that would then be fed directly or by hand, into the lime kiln. If you built up a separate room for the operation it would be possible to have it a continuous winter time operation, place a reading chair in front of the kiln/coal furnace and read away while keeping an eye on the fire while warming water for cooking or baths or heating the other occupants/critters in the next door rooms. It would be a lovely set up to see for real! Organic wood char coal is easily refined into other products, such as solid but nice looking logs of coal that can be placed inside any room to clean the air, I also use coal in cooking rice, as it helps clean the tap water used in boiling the rice (just drop a piece into the rice cooker, lasts for 1-2 months of daily use if dried in the sun periodically and makes the rice taste better as well). The lime produced could be sold as artisan lime for traditional building conservation or to local farmers using it for their walls and critter barns (as we do in Europe, not so much in Japan). It could become a nice source of cash or for trade with farmers!

        I would love to see more posts on the subject, and see what you do with the lime you produce! It would also be interesting to try out barrels made of different materials (plastic, oak, pine, etc.) to see if you get better results. You could also produce batches for aging, 1 year lime, 2 year lime, 5 year lime, 10, 15, 25, 50, 100 etc. I hear aged lime is even better than aged wines!

        Comment by tokyobling | September 10, 2012

  4. What type of klin did you use?

    Comment by Cas | August 13, 2013 | Reply

    • The first time around just a steel sleeve basically. I’m working on follow up post about kilns used since then, which include a steel drum with both ends removed and a clay/straw stack.

      Comment by Stevene | August 13, 2013 | Reply

  5. […] first burn (see lime squad #1) was done in an open ended (both ends open) section of old rusted water tank.  After that burn, […]

    Pingback by Lime Squad III: Burning lime in metal drums. Advantages, limitations and where to go from next. « Turkeysong | October 27, 2013 | Reply


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