Burn Seashell Lime in a Primitive Straw/Clay Kiln!

In this project we use Earth, Water, Fire, Air, Animal, Vegetable and Mineral to make something with a multitude of practical uses. The text is detailed and stands alone, but the videos are very useful and entertaining. Video #1, the Quicky version, is the 7 minute general interest, entertainment version. Video #2, buildcult, is the long educational 20 minute version. I'd recommend watching the short version first, before reading this instructable, so that you have a visual reference. I did my best to make it worth 7 minutes of your life. I conceived this project for the Brave the Elements contest, which you can vote in here, hint hint.

The goal is to build a kiln from natural materials in order to burn and slake lime for seashells. Lime is immensely valuable! If lime disappeared tomorrow, civilization would fall hard! It is the main ingredient in cement and can be used to make mortar and plaster. It can also be used in the arts for limewash and fresco, in soap making, mixed with casein (milk protein) to make pre-industrial waterproof glue and paint, for smelting and refining metal, and to remove hair and prepare skins for rawhide, hide glue or tanning. Ever read the ingredients on your tortillas or tortilla chips and seen “traces of lime”? That’s because lime is used in processing corn kernels to make tortillas, hominy and grits, which is easy to do at home. Not only does it make corn more nutritious and digestible, but it also lends to the unique flavor of those products. Beet sugar is processed with lime too. Lime is also used to potentiate certain drug substances such as betel nut and coca leaves, a small bit of lime being chewed with the plant material to activate it. So, yeah, wow, lime is one of the most useful substances ever!

A WORD ON SAFETY: A lot of people think that lime is some deadly scary chemical that will burn you face off. It’s not… not really... well, maybe. Quick lime is dangerous, but that is a brief transient state. During slaking, the quicklime will give off heat and boil vigorously, so that is dangerous since the stuff can splatter around and is not only hot, but also highly alkaline. So, yeah, okay, maybe doing a face plant in a boiling tub of quicklime may burn your face off. Don’t do that! Otherwise, the stuff is not that horrible, and people have been making tortillas, mixing mortar, plastering walls and tanning leather without goggles and hazmat suits for a very long time. It is also non-toxic. You definitely don’t want your pets drinking lime water or your kids playing with lime, but that is due to it’s concentration mostly, and not to inherent toxicity. When diluted, it becomes less and less caustic and is at some point completely harmless. Once converted to calcium carbonate by drying, it’s just like egg shells, sea shells or stone, not only non-toxic, but actually used as a calcium supplement. So, don’t get it in your eyes, keep it away from children and pets, be careful when slaking and use common sense and everything will be fine. It will temporarily dry skin though skin though, so be aware of that.

Before we get to the fun stuff, let me explain how this works. Don’t be intimidated by the chemistry terms, they aren’t important. The changes lime goes through have a name, The Lime Cycle. By heating stone or shells red hot, about 900 Celcius (called calcining), we can change lime from it’s stable inert form, calcium carbonate, into Calcium oxide. Calcium oxide, aka Qucklime, is the most unstable and highly reactive form of lime. Quicklime reacts violently with water, giving off tremendous heat and boiling vigorously. This reaction with water makes it into Calcium Hydroxide, which is similar to lye, but not as strong. This is the form that is used the most in the arts and industries mentioned above. If the lime is kept under a layer of water, it will not only keep forever, but it improves with age! This stuff is called lime putty. You may be more familiar with the dry lime you can buy in a bag, which is dry calcium hydroxide. This bagged powdered hydrated lime is widely available, but inferior to wet slaked lime putty. You can hardly buy lime putty, and it is very expensive, but you can make it! For more on the forms of lime see my article, Understanding Lime.

Basically, lime putty is like liquid rocks. Once it is allowed to dry with exposure to air, it absorbs carbon from the atmosphere and turns back into a hard rock (or shell, which is pretty much the same thing). Think about that for a second. That is awesome!

And thus the lime cycle is completed from rock or shell, to quicklime, to lime putty, and back to rock/shell.

In my book, it doesn't get much funner than burning and slaking lime, so lets get this party started!

Step 1: TOOLS AND MATERIALS

The pet straw/clay kiln is so simple you could do it with almost no real tools, except maybe something to carry water, it would just take longer. A wheelbarrow is nice to mix in, but you could also just use a bucket or tub. A shovel is good for digging clay. Maybe some tools to cut and split wood.

STRAW: You need a type of straw that is flexible enough to curve around the kiln body. I prefer my wild straw, but bale straw from the feed store works fine too. Don't use hay. You want the hollow stems of the straws, not the flat grass blades which compose much of a bale of hay.

CLAY: Clay or high clay soil is need to make clay slip for the kiln. Try to dig down at least past the top 6 inches of the soil. In many clay soil areas, there will be heavier clay about 1 foot down. Mix the clay with water. If dry, try to soak it for at least a day or two if possible. Mix by hand, squishing the clay between your fingers, or use a paint mixer shaft in an electric drill. Use less water than you think you’ll need. You can always add more! Make the slip thick, like a thin batter. When dipped, a lot of clay should stick to the grass leaving a good coating on each straw. Lumps, bumps and sand are no problem as long as the consistency is right.

FUEL: I have used all different kinds of scrappy wood for firing lime kilns. Use anything that is dry and not soggy or so rotten that it is very light in weight. pine cones, dead bark, nut shells, twigs, sticks and of course just plain old firewood are fine. You do want a proportion of small stuff in there as well as enough big stuff to keep the fire hot for a while.

We are dealing with two basic factors here-

Temperature: the shells or stone must reach a certain temperature to calcine.

Dwell time: The temperature must be maintained long enough to thoroughly penetrate and finish the job.

Having enough fuel and enough air flow will give you adequate temperature and having a large enough mass of wood, with some of it in larger pieces, will give you dwell time. (BTW, it is hard to drive the temperature of wood too high, certainly not without forced air.)

I don't use wood much over 2 inches in diameter in these small kilns. Use some around 2 inches, some small stuff to spread the fire and keep it moving, and a bunch in between. There's no need to over think it.

You can also use already prepared charcoal. Using charcoal will almost eliminate smoke, making this process accessible to backyard pyros who live in populated areas.

Feedstock can be either limestone or shells.

SHELLS: As far as I know, any shells will work. With shells, you know what you are getting, very high calcium lime, known as FAT LIME.

LIMESTONE: Limestone varies in composition and may contain either useful or un-useful impurities. High magnesium is common in limestone, but it has similar properties and magnesium limes can be used in building. I guess it either works or it doesn't. Just burn whatcha got and see if it works. I don’t have a lot of experience with limestone, because it is rare in these parts. The thicker the stone is, the more dwell time it requires, so break limestone up into smaller pieces. I try not to use any stone much over about 2 inches thick and I think smaller is probably better in a small kiln.

CHALK, CORAL AND MARBLE: Are other materials that are sometimes burned to make lime.

FOR SLAKING: After burning, the lime is slaked in water. You'll need a ceramic, galvanized metal or wooden vessel. A galvanized tub is good. A stainless stock pot will work for small amounts. The slaking lime gets very hot, so don't use plastic. You also need boiling water and a long stick to stir with so you can stay well back from the boiling splattering lime.

STORING: The finished lime should be stored in a container with a tight lid to keep it wet. A five gallon bucket with a lid is perfect.

CAMERA: You need a camera so you can take pictures or videos to show me later!

Step 2: ABOUT THE PET KILN

The kiln of straw and clay we'll be using has been given the name “The Pet” by my friend Yoshi because it’s shoooo cuuutte and fuzzy looking! awwwwwwww! It was inspired by a clay/straw wattle system innovated by my friend Michael Smith, author natural builder and author of The Hand Sculpted House. It turns out that he was inspired by a system used to build granaries in Mexico, similar to what I’m doing with the pet kiln. The granaries are structures of coiled bundles of straw mixed with clay. My concept was to create some insulation by using a lot of straw. There is a large proportion of straw in the kiln, which may seem foolhardy. I know, I was skeptical that my own idea would even work till I tried it. It is not idiotic, I swear, it just has to be considered in context. This is a temporary kiln. I believe the one I used the most lasted for 8 burns. The one I built for this has done three burns and looks like it could do a lot more. The clay protects the fibers to an extent and when heated cooks into something like pottery. Each burn erodes the kiln wall a little until holes appear and it’s time to build another one. The kiln may be dead, but the kiln fragments make a valuable soil amendment along with the ashes, lime fragments and any charcoal that is left. Burnt clay has a long history of use in soils. My last pet is buried in my saffron flower bed. I’ve found roots growing through the holes in the fragments, so apparently they like it!

Perhaps Using a larger proportion of clay may make a more durable kiln.

Having used a metal drum a lot to burn lime, I found that any shell near or touching the metal would not calcine completely, thus creating a large percentage of waste. The pet has insulation and mass, or at least conducts heat more slowly, resulting in a higher ratio of burned to un-burned shells. The mass absorbs heat and once up to temperature, should theoretically moderate the environment within the kiln by reflecting heat back into the interior creating a more even and probably higher all around, temperatures. A thin metal drum radiates heat rapidly outward and cools very fast, losing half of it's energy to the outside air. You often have to stand back from the red hot drum, while I can put my hand on the outside of the pet, even when it’s in full inferno mode.

To summarize, the pet is temporary, but it is fast and simple to build with materials that are commonly available, it does it's job well and the burned out shell of the kiln is a good soil amendment. And it's adorable! You can do the same burn in a thin metal drum with the top and bottom removed, or similar structure, but expect 15% or more of under-burned shells.

Step 3: BUILDING THE KILN

Remember, keep the slip like a thin batter and add water or clay as necessary.

You need air vents. For this pet, I used half rounds of dried bark that supported the arch while it dried, but still allowed airflow. Another option is to dig a trench and cover it with a small board. Build the kiln over the board and it will leave a way for air to get under and into the kiln. By the time the bark or board burns up, the kiln will be dry enough to be self supporting.

I like to make the kilns with a belly, like a long pear shape. My fire intuition tells me that this shape will hold heat better and reflect more heat off of the curved walls and back into the center of the kiln. Also, the venturi effect may come into play when air moves from the large interior, through the narrow neck, and out into the large space of the open again causing an acceleration. That is all theoretical, but it looks cooler than a straight sided kiln anyway. It is also a pretty common shape for kilns off all sorts.

Fire intuition also tells me that the kiln should have a minimum height to width ratio. I’m not sure what the ideal is, but it should be taller than it is wide. Even taller is fine I believe, just not too short and wide.

The thicker the walls are, the longer the kiln will last. You can build them surprisingly thin if care is taken, but less so if the kiln walls curve outward. Don’t wring the clay out of the straw. You can squeeze it lightly, but the idea is to have the straw completely coated without any big air spaces in the body of the kiln. You should see clay both sticking to the individual straws after dipping, and also being held like water in a sponge between the straws. If the slip is very thin, the straws will barely be coated and the slip will all run right out of the straw "sponge" instead of adhering.

Have fun! I was going to say this is the fun part, but there are so many fun parts. We still get to play with fire and dangerous boiling stuff!

Step 4: DRYING, PRE-FIRE

The kiln can be fired the same day it is built, but it should have one fire run through it first to dry the inside a little bit. Fill it up with scrappy wood and let it burn down. No need to go slow, crank that sucker up.

Step 5: LOADING THE KILN

Shoot for around 25% shells/limestone and 75% wood...somewhere in that ballpark. Definitely not more than 30% shells. I would recommend erring on the side of more wood at first, and then experiment with skimping to see what you can get away with. I usually use a mix of wood sizes, and rarely anything much over about 2 inches in diameter. Using some small stuff mixed in gets each wood layer off to a fast start, while the larger pieces give you a longer burn time to make sure the material is heated all the way through.

You can either load the kiln and light it, or light a fire and then add layers gradually.

Whether loading gradually or all at once, the wood must be stacked loosely enough to allow air to travel up through the kiln. A criss cross pattern is good, with space between each piece of wood. You want to fit in a lot of wood, but if you pack it in tightly, the burn will be sluggish.

If loading the kiln gradually, start a fire in the bottom and get it going very well before adding shells. Use quite a bit of wood so that the shells will be left resting on a large coal bed. Add a layer of shells or limestone, but not too thick The limestone or shells may very well start exploding right away, so be careful. Immediately add a layer of wood. If you stick around to manage the fire, adding one layer at a time as each previous layer begins to burn cleanly, you can lessen the amount of smoke, but not by that much. I usually set the kiln up and light it, then add a few more layers as the kiln burns down.

To load all at once, put a layer in the bottom that is neatly stacked and designed to light quickly and spread the fire rapidly. Use a log cabin criss cross design with lots of small splinters and twigs worked in. Other good additions are the fluff from cattail reed seed heads, shredded bark, dry grass and pitch saturated wood from conifer trees. Around and on top of this put a good layer of wood, enough to form a nice coal bed on the bottom, and then add a layer of shells. Add another layer of wood with some large and some small fuel, then more shells. Continue layering, maintaining your 1:3 proportion of shells to wood until within about 6 inches of the top, Cap off with a nice layer of wood. The last wood layer can be mounded over the top of the kiln in such a way that it will fall in as the contents burn and sink. If you prepared your base fire well, it should be easy to light the fire, but you may have to make some kind of little torch or something to shove into one of the vent holes. If it is hesitant to light, insert a glob of burning pine pitch.

You can keep adding alternate layers of shells/limestone and wood until you start running out of room. Just be sure to put a hefty layer of wood on each layer, including the final one. When you are done, simply allow the kiln to burn out. If rain is an issue, cover the kiln. It is important that the freshly burned lime stays absolutely dry!

Step 6: FIRING THE KILN

This method produces a lot of smoke, but it is not as bad as it looks. If the smoke is very white, chances are a lot of it is actually just steam. But it is still a lot of smoke, so be warned. After a while, when the fire reaches the top of the kiln, the gasses will begin to flare off. Smoke is basically fuel, it just needs the right conditions to burn off.

Once the contents settle down, you can begin adding more layers of shells and wood. Start each layer with some new wood, followed by shells and a cap of wood. Adding one layer at a time will reduce smoking a little bit. You can just keep adding shells and wood as long as there is room, but always leave each layer with a good cap of wood.

Allow the kiln to die and go out. If night is falling, cover the kiln before you go to bed to make sure the quicklime stays dry until morning, when you should slake it as soon as possible.

Step 7: SORTING

Not everything in the kiln will calcine completely or evenly. When it comes to shells, it is fairly obvious what is and is not burned thoroughly. You don't absolutely have to sort burned shells, but I prefer to. Shells are sometimes hesitant to slake and having a proportion of under-burned shells, while it may or may not hurt very much, certainly won't help. Also, for most uses, you'll probably want to sift off dirt, ashes and fragments of the kiln wall anyway, so it's easy enough to toss aside under-burned shells while you're already sifting through the remains.

Under-burned shells will have a grayish cast to them, or may be gray or black inside. Sometimes you can break off well burned parts for your good pile. No need to obsess. No matter what, there will be plenty of little bits of un-slaked stuff, but we'll screen all that out later, so just pull out the obvious ones.

Get a container for the well calcined shells and a container to put everything else in. All of the other stuff, charcoal, burned clay, ashes and chips of shell or limestone are valuable first rate fertilizers or soil amendments! If you already have highly alkaline soil, you may want to avoid lime and ashes, but those soils are the exception rather than the rule. These things are not waste, or even leftovers, they are a valuable product of the burn. I've even envisioned using this kiln or a similar system as a way to intentionally produce these products specifically for agricultural use.

Step 8: SLAKING

Now you have a pile of burned lime, or quicklime. Don't put off slaking it. Old technical instructions and recipes will often specify "freshly slaked lime" or say to take "freshly burned lime", because quicklime has a short shelf life. If you must store it, keep it sealed in a vapor-proof container.

Do not slake in plastic. It will melt. I am not sure whether you can use aluminum or iron.

If all goes well, the lime will boil. Hot things are dangerous! Not only that, but it is a caustic substance, so that's doubly bad. Absolutely, do not get this stuff in your eyes! Keep children and pets away. Once slaked, lime is less dangerous, but still better keep kids out of it since, frankly, the stuff looks good enough to eat.

If slaking shells, heat some water. Stone seems to do okay with cold water, but shells are hesitant to slake and need a jump start. Once they get going, the reaction will usually create plenty of heat and you can start to add cold water. If you add too much cold water, you can stall the process. You can also stall the process by not adding enough water, because the water is the catalyst. For your first time, I'd have a large quantity of hot water to make sure you keep the ball rolling. The quicklime will use up a lot of water, so be ready with several times the quantity of water as you have shells to slake.

If the lime is well burned and fresh, it may start to hiss and react immediately. Other times it may take a little while to get going. After it really begins to react, be sure to keep adding water as it is used up. If the water level gets low the slaking will slow down. Keep stirring the lime as it slakes, to insure expose of all areas to adequate water.

Eventually, the slaking will really slow down. Make sure it is pretty wet and you can leave it for 30 minutes or so to finish up.

Dry hydrated lime is made by wetting quicklime in the open air. The quicklime will swell and crumble to a powder. Shells will not always slake in this manner, though they sometimes will. Dry lime hydrate is of a lower quality for most uses and I don't make it. It's really fun to try though. See the accompanying video for an example.

Step 9: SIEVING, STORING AND CONCLUSION

The slaked lime will have bits of under-burned shell and stuff that you probably don’t want in there. Sieve them out through a fine screen or mesh of some kind by first mixing extra water with the lime to make a thin solution that will pour easily. I use a screen that is about half the hole size of a window screen, so maybe 1/32 inch. Fine particles may or may not matter depending on the use, but if the lime is made into a liquid by adding a larger amount of water (called milk of lime), it can be run through a coarse cloth or some such thing to strain out all but the smallest particles. As the lime is stored in water, it will settle to the bottom of the container forming a firm putty which can be dipped out as needed. Again it will improve with age. Just make sure it always has a layer of protective water over it! The standard five gallon plastic bucket is perfect.

There are so many places to go from here, like more efficient and less smokey designs, or arrangements that utilize the heat produced, or using heat that is already produced for heating water or houses to burn lime.

Subscribe to my blog and youtube channel for future projects using lime for various things, and If you build a pet, or do any backyard/homescale burning, I’d very much like to hear how it goes.

And for much more on lime, it's traditional use and it's often great advantages as a binder, read the excellent Building With Lime, by Holmes and Wingate, which I can unreservedly recommend in spite of one of the worst book covers I've ever seen in my life!

Understanding Lime

Lime Squad III

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