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!
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!