Experimental Homestead

LIME SQUAD! II: The Slaking





Having burned some sea shells into quicklime in photoessay part one, it was time to slake the shells into lime putty.

In the morning we went out to the kiln all excited and grabbed a few well-burned-looking shells for a quick experiment.  We put the shells into a dish and crushed them lightly with a spoon and added water.  In order to turn into calcium hydroxide, a.k.a. lime putty, quicklime has to undergo a chemical reaction with water which creates a good deal of heat.  Well, our quicklime just sat there cold in the water, no churning and boiling :(  what a let down…   After getting over our initial disappointment it seemed just not right that our lime didn’t react.  The shells were completely white through and through.  So, we put the shells and water on the stove to heat a bit and that seemed to slowly kick off the reaction and eventually they broke down into lime putty.  That was encouraging, but it wasn’t time for the champagne yet.

I had pretty much decided that we should use hot water to slake at this point, but considering the time and energy we had invested so far we wanted a more informed opinion.  I called Jeff Price at Virginia Lime Works who generously spent a small piece of his morning in the pursuit of my edification in regards to lime and lime burning.  Sure enough, he recommended hot water to start it off when using shells because the structure of shells is less conducive to the slaking process than the structure of stone.  The up side is that shells make more lime putty than an equivalent amount of burned stone.  I took my page of notes and was ready to rock.

The burned shells were added to an old wine barrel pre-charged with hot water.

It wasn’t long before the exothermic reaction began to take place.  Note the surface of the water is roiling in this picture and the shells are starting to break down.

The lime was stirred constantly.  Goggles should definitely be worn here.  The the lime is dangerous not only because it is literally boiling and splattering but also because the Ph is very high making it caustic enough to cause chemical burns to the eyes.

Partially slaked

Jeff had told me to keep the reaction going by constant stirring and addition of water as necessary.  A lot of water was added as the process continued.  The picture above shows the putty partially slaked, but still with a curdy, cottage cheese look to it.  The lime was stirred constantly to keep this stuff in contact with the water it needed to insure a continued reaction.

When the reaction had pretty much subsided the lime was broken down into a smooth batter-like consistency.

making milk of lime

A few days after slaking the lime was stirred up with extra water to suspend the fine lime particles in a thin liquid for straining through a screen.

Straining the lime

The lime was poured through a screen or two to strain out un-slaked particles which consist in this case wholly of under-burned shells.  The remaining milk of lime was stirred again in the barrel with a quantity of water to allow remaining particles to settle to the bottom as the stirring subsided. If any under-burned particles remain, they will make good aggregate for plaster or mortar, but for the finest finish plasters or lime washes (lime paint) these particles would be undesirable.

From here the lime sits under its blanket of water.  In time it will settle and congeal into a firm putty.  Hopefully the barrel will facilitate this process as the porous wood draws away water from the putty allowing it to evaporate out the sides of the barrel.

So far so good.  The seemingly small amount of shells expanded into a significant amount of lime putty… maybe 12 to 15 gallons?… and with the addition of at least two parts aggregate for almost any use, there is enough to do a significant level of experimentation in using it.  It the meantime it ages away improving with time in its wine barrel vintage 2011.

A quickie plaster/mortar experiment using under-burned shell as an aggregate. Tapping on it produces a high ringing sound. Yay! “artificial” “limestone”.


March 6, 2011 - Posted by | Uncategorized | , , , , , , ,



    Pingback by LIME SQUAD! I: A Photoessay on Lime Burning « Turkeysong's Blog | March 6, 2011 | Reply

  2. great information and we appreciate being able to benefit from your experimental process. Thanks, Steve!

    Comment by Cathy Farneman | March 7, 2011 | Reply

  3. Very cool. I’m curious to know how strong that lime “cookie” you made is.

    Comment by Gregory | March 10, 2011 | Reply

  4. Gregory: Its not very strong. This lime is almost pure calcium. Re-carbonation goes on for years, but most of the strength has developed by now and it will never be “very strong” in the sense that you mean. Strong can be a somewhat misleading term though. The idea of strong is very pervasive in our society and highly valued. The concept of strong can be divided into various characteristics like crush resistance, tensile strength, flexibility, hardness, toughness. A material can be hard as hell, but not at all tough for instance. One of the benefits of portland cement is greatly increased tensile strength. With that strength comes a price though, which is decreased flexibility. More importantly though, lime has free lime in it that can re-crystalize in cracks healing them as buildings move and settle over centuries. Once portland cement products are broke, they are broke for good. Another characteristic of lime is that since it is more porus and less waterproof it breathes better. It also allows water containing dissolved mineral salts to move through it instead of through the often softer building units (bricks etc…). The salts can form crystals near the surface of the lime joint eventually destroying it by expansive crystal growth. That sounds bad on the “surface”, but if you use concrete the moisture can take a route through the building materials instead of the mortar causing the same phenomenon. With a softer lime you might have to chip out the mortar every 50 years and restuff the cracks, but buildings repaired with cement mortar instead show damage to the building units instead of the mortar. And softer lime mortars can be chipped out and repaired *because* they are softer. The mortar is fixable, the building unit is not, therefore the less permanent mortar in these ways may end up contributing to the building’s longevity. This would be equivalent to painting the wooden siding on a house every so many years. So while pure lime mortars are not “strong” in the sense that they have a high tensile strength, when all things are considered they do have strengths in a more analogous sense and clearly have a place in a cultural context that is less short sighted than the one we live in.

    The relative softness of lime has to be accounted for in design and building of course, especially in seismic areas, but also just in general. Still there are many ancient buildings still standing that were built ages before the introduction of portland cement. Many of these will probably outlive the crap we build now with portland cement, but that may have as much to do with our short term mentality in design and execution as it does with the materials used.

    Various additives called pozzolans (like volcanic earths, diatomaceous earth or powdered brick dust) can be added to lime to make it have more of the characteristics of a cement. These hydraulic limes can set underwater, have increased tensile strength, or resist damp conditions better etc… These modified characteristics can be adjusted on a scale between portland cement and pure calcium lime. But they carry the benefits and drawbacks that come with increased or decreased hardness and waterproofness etc… like brittleness, lack of repair-ability and loss of self healing properties, so they should of course be tailored to their specific use. I’ve barely started to use the stuff personally, but this is how I understand it all at this point. I can say though that the workability of lime is a gijillion times better than portland cement mortars! Its actually fun to use. Its not a constant struggle to get it to stick to stuff, stay where you put it and flow under the trowel like portland cement mortar is.

    One last note. It is often claimed that lime is carbon neutral. That’s only almost true. As the lime cures, it reabsorbs the carbon dioxide that was released from it when it was burned. However, that assumes that the lime just exists which it doesn’t, it has to be burned with carbon releasing fuels. If wood is used one could consider that the carbon released from the burning wood would eventually be reabsorbed by new plants. Another factor though is that the lime only cures completely on the outside where it is exposed to the air and deeper layers of mortar joints and such do not carbonate thoroughly. So, don’t believe the hype. However, in contrast to portland cement lime does at least reabsorb significant carbon. If we add to this the fact that portland cement, by all accounts, requires far more energy to produce, then lime comes out way ahead in the carbon foot print aspect. Neither lime not portland cement are panaceas in building, but I’m gravitating toward lime whenever appropriate at this point. At least it has a long track record to go by.

    Comment by turkeysong | March 22, 2011 | Reply

    • Since this first burn, we’ve done another 8 or 10 or so. We scaled down to a 55 gallon barrel and have played with different ideas like slower burning, more and less densely packed fuel/shells and adding layers of wood and shells as the kiln burns down. Where we are roughly at now is that using an uninsulated barrel which has very little mass and no insulative value (quite the opposite I think), one should use some green or damp wood in the mix, allow it to burn freely by leaving substantial vents open in the bottom, keep the shells away from the sides of the barrel, and always leave a cap of wood on the top. The Barrel has top and bottom removed and sits upon two shallow trenches which intersect forming a basically symmetrical cross for ventilation. The best we’ve probably been able get is about 80% to 85% of the shells burned enough to pick out and slake. The worst was basically zero% when we shut the vents down all the way and it burned too cool. There is more burned lime in the left overs, but we slake that separately as a lower grade for tanning etc… The initial fire lay will have 2 to 3 layers of shells and up to 5 layers total can be burned by adding wood and shells 2 or 3 times in layers as it burns down. Each layer is about one milk crate full. We are still encouraged and planning to continue to work on home-scale lime burning so we can find efficient and accessible systems for micro scale home burners to share and use. Stay tuned for more….

      Comment by turkeysong | May 17, 2011 | Reply

  5. I wonder if it might be practical to calcine lime with a parabolic dish reflector, or with a Fresnel lens from, e.g., an old projection television set. It might require some sort of absorber…raku pottery maybe?

    Another potential source of pozzolans is rice hull ash. Because rice hulls tend to form silicon carbide, they might be a reasonable consumable solar absorber: the SiC would stay black despite exposure to air and high temperature, although it would end up adding significant silica and grit to the finished putty.

    Because under-burned shells have some soot in them, it might be straightforward to grind them coarsely, spread them out, and finish them off with a parabolic lens.

    Comment by Joel | May 18, 2011 | Reply

    • Joel: I doubt it would be practical to use direct solar energy to calcine any quantity of shells. I’ve spent a lot of time wondering about solar potential for smelting and stuff like this and I don’t think its that promising. I’m not saying impossible, but it seems to me that there are a few serious pitfalls. The quantity of shells or stone required to actually have enough lime to do much of anything with is quite large. Freznel lenses and parabolic mirrors like all other high temperature solar heat concentrators do just that… concentrate heat. We can only concentrate what is available from the sun and if we want to concentrate more heat per square inch or heat a larger area we have to make our lense bigger and bigger. The temperature required is in the range of 900˙ C and to heat say a milk crate full of shells to that temperature is going to take a pretty damn big lense. Also, the heat has to be even and sustained which could be challenging when your heat source is moving across the sky which also adds another level of complication And is the day even long enough? There are also other factors such as the lack or the availability of oxygen and atmospheric pressure inside the kiln which I don’t understand enough to speak to, but suspect might come into play. As far as regrinding and calcining half burned shells, I was told that I should bother trying to recalcine them because it wouldn’t work well. I’m not sure why though. For me it is no great loss to have 20% of unburned shells because here on my homestead there is zero waste. There is some burned lime in the collection of under burned shells and I either slake them separately for use in leather tanning or I add them to gardens and trees where they can contribute lime to my acid soils for a long time to come. Same with the ash and lime fragments in the bottom of the kiln.

      Now here is what I think would be elegant and cool as hell. First, let the plants collect your solar energy which they are remarkably good at. Then adopt or adapt the simplest possible system of combustion that is near 100% efficiency (rocket stoves and gassifier stoves come to mind). Come up with a simple earthen or junk pile kiln design that can use this heat as efficiently as possible without any gadgetry or excessive expense. The kiln itself won’t be anywhere near 100% efficient, but at least you are releasing carbon that was already in the atmosphere before the plant grabbed it out. As a topper if we could halt combustion half way through and collect the charcoal to use as biochar or in building etc…. then we could also sequester half of the absorbed carbon and yield a second useful product. This would require more wood of course (twice as much?), but that can be grown or managed in a woodland coppice system. Black Locust comes to mind. In my forest at my scale, enough wood is produced for me to burn a whole lot of lime by just collecting overcrowded trees and stuff that dies naturally. I should think that these goals could be achieved without electricity or mechanization of any kind and with no other high tech stuff.

      I’ve heard of using the rice hull pozzolan. Quite a lot is required. I wonder if horsetail might be useful or even better, as it has a very high silica content, enough to use as sandpaper even.

      Comment by turkeysong | May 19, 2011 | Reply

  6. I found your articles and fotos very enlightening and interesting. I have a couple questions though, a few years ago i was reading Joseph Maxons Encyclopedia of trades, (that is not its name, but i cannot remember exactly what it is, and it is halfway ’round the world from where i am at the moment so i cannot lok it up) This book was writen around 1680 (or so) and deals with buildings and trades mostly. I bought it primarily for the bit on cabinet making, as i am a cabinet maker myself, and also an historical enthusiast. I then read his bit on building, and so forth, and i remember reading that the lime was mixed with the sand, and kept dry under cover of straw under a roof for ‘at least 6 months’ prior to using it. He then went on lamenting that ‘ the reason no good work was being done in buildings nowaday was that the builderw wanted to rush the lime mortar, and did not let it age long enough, thus rendering poor quality modern buildings’ (my paraphrasing, but i put the quotations because of the significance of th emeaning) I remember being amused by someone complaining in 1600;s that ‘modern’ buildings were inferiorly built!!! (wonder what he would think of the rubbish built now?!) I also remember him saying that if it was left for several years it would be best. My question is, how this fits in with what you are doing? I am guessing that his lime which his stirs dry with the sand is ‘quick lime’ but as you stated it must be used up at once (or what?) any comments ?

    Also i read somewhere that some native groups who used to burn shells for lime burned from the top down so as not to blacken the shells, but your shells seem quite white anyway. Have you experimented with that method of burning?

    Thanks, cheers, and keep up the good work!

    Comment by Johann | June 20, 2011 | Reply

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