Pet Lime Kiln Update, 10 Burns, 30 Gallons and Where to Go From Here

Here is my 100th video on youtube, an update on the last lime kiln I built.  It looks as though the main thing to address in this design is erosion of the edge.  I am thinking that a rim of cob-like material (probably just clay and sand) might do the trick.  That begs the question of why not just build the entire thing out of cob or similar material in the first place?  Well, that is certainly a possibility.  I don't think it would have the same insulative value, but that may not matter in the end.  It is impossible to know without testing the idea.  There are some advantages to the pet kiln under various circumstances though.  It is fast to build and can be built up all at once.  A similar cob structure would need either support or drying time between layers.  Less clay is required, which could be important sometimes.  My intuition is that the insulation value of thousands of tiny holes and grass stems is significant, but again, I can't know without testing that proposition.  Of course a similar list could probably be generated for the benefits of cob.  not need to make it one or the other.  The more tools we have in our box, the more we can adapt to varying needs and circumstances.

I may pursue some ideas I have with the pet kiln concept, but I have quite a few other lime burning projects I'd like to try as well, including scaling up to a bigger more sophisiticated set up.  I may even test the feasibility of burning lime for sale, but honestly, my interest is more in testing the proposition to assess the feasibility of lime burning as a cottage industry for other people to pursue, or the feasibility of producing moderately large quantities on site for projects, rather than for the actual money I'd make.  Curiosity is a curse and a blessing.

Also posted below, a recent video of my just walking around the homestead talking about stuff.  I could do that for days.

What Type of Lime Should You Use for Tanning and Rawhide?

I remember many years ago trying to understand what type of lime I should use for tanning and being really confused.  This video and blog post are an attempt to foster a basic understanding of lime as well as which type to use and where to find it.

Lime is used in tanning to loosen the hair for removal, and sort of clean the skin fiber of unwanted substances.  It is used in the same way for processing rawhide as well as skin for making glue.  While there are other alternatives, lime was most commonly used in 18th and 19th century tanning processes and by home tanners since.  It is easy to use, accessible, safe, predictable and does he job well.

What we call lime exists in a cycle.  There are three stages in the cycle and once the cycle is completed, it could theoretically be started over again.  Only one of the stages is useful for tanning and in the majority of other arts.

The first phase is the one in which lime naturally occurs.  That is as calcium carbonate.  Calcium Carbonate is what shells and limestone are made from, the natural materials from which we make the lime that we use.  This form would include chalk, limestone, dolomite, marl, marble, shells and coral.  Calcium carbonate is fairly inert and stable.  Ground limestone or ground shells can be used as a soil amendment to raise ph and provide calcium, but not for tanning.

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Shell, limestone and marble, three common forms of calcium carbonate.  Any of the stones may have any number of impurities, Magnesium being very common.  Shells are probably ideal for tanning use since they are very pure being almost all calcium carbonate.  They are also very easy to turn into lime.

If we heat calcium carbonate up red hot we end up with Calcium Oxide aka quicklime.  Qucklime is mostly an interim stage, though it has some uses in the arts and industries.  As relates to tanning, it is an interim stage.  If you read old tanning books that say to use quicklime, but that is because they acquired quicklime and slaked it immediately into the the next form for use.  Quicklime is easy to transport because it is very light, and it just made sense in the old days to order freshly burned quicklime and transport it that way. It would then be slaked immediately as it does not keep well.

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Quicklime, also known as lime shells whether made of shells or rocks.  As far as tanning goes, quicklime is just an interim stage.  Voraciously thirsty, unstable and highly reactive, lime shells should be processed immediately.  When quicklime is mentioned in old tanning literature it is often stated something like "take fresh burned lime".  Easy to transport because of their light weight, lime shells were often delivered fresh and slaked immediately in the liming pits.

When we add water to quicklime, it produces Calcium hydroxide in one of two forms.  If we add a lot of water to the quicklime we end up with lime putty.  If we add only a little water, the quicklime disintegrates into a fine powder that can be stored dry.  

The dry powder, Dry lime hydrate is what you can easily buy for processing hides for tanning.  It is available at hardware stores as "type S lime" or "builders lime" and according to some of my viewer/readers as "barn lime" in some parts of the country sold for spreading on barn floors.  Just make sure that you are not getting dolomite or agricultural lime which is just ground up rocks.

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Dry lime hydrate, the stuff you can get at the local building supply, is made using small amounts of water, which causes the burned quicklime to disintegrate into a fine powder.  This process doesn't always work on shells.  At least use hot water with shells, then it might work.  Really though, you should make lime putty at home.  It is more stable and more potent than the dry hydrated lime.

Lime putty can be made at home. It is more potent than dry hydrate and less apt to go bad since all you have to do to preserve it is keep it under a layer of water where it will keep indefinitely.  Either dry hydrate or lime putty can be used in tanning to equally good effect though, you just don't have to use quite as much lime putty.  For more on lime in general and burning lime at home, see the lime page.

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To make lime putty, just use more water.  The solids will settle to the bottom of the storage container.

For more on what lime is actually used for, tanning and pre-processing hides, you can see this video on de-hairing.

Related Videos  

Understanding Lime: an introduction to forms of lime and where they come from

By Steven Edholm

I used to be so confused about lime.  Some limes have more than one name and more than one use which can be difficult to keep sorted out in your head when you have no frame of reference.  Lime is super neat though, and worth understanding.  I’ll attempt here to present the types of lime and their uses in a way that is accessible to people without that frame of reference... or maybe offer an accessible frame of reference for understanding lime.  For more on lime burning and the lime cycle see The lime squad I and Lime Squad II posts on the Turkeysong blog.

First off, lime is cool, and so useful!  Understanding what uses there are for lime can help us understand the three basic forms of lime that we might have access to or make.  The basic use groups are these.

Reactive uses:  use the caustic nature of limes to chemically attack plant and animal substances in food processing and tanning of skins.  In other reactive uses, lime reacts chemically with other elements as in dyeing or to potentiate the drug alkaloids found in Coca leaves and Betel nut.

Plastic uses:  These are used for building, painting and in the arts.  the lime is shaped it how we want it before it dries and hardens.

Agricultural uses:  Used to adjust soil acidity into a range suitable for most plant growth, as well as to provide calcium.

Nutritional uses:  as a calcium supplement.

Filler uses:  Powder used as a filler in the arts for painting and sizing.

Fluxing:  Used to lower the melting temperature of unwanted materials during smelting of metals.

Thats enough basic uses for us to tackle in this short article about understanding the different forms of lime.  In future articles, I may address each use group more specifically to cement that understanding.

All lime comes from biological processes!  Wow!  that’s amazing.  All those billions of tons of limestone, chalk, marble, shells, coral and all that stuff was collected from the environment by living organisms.  When the organisms died they left deposits of their calcium rich shells which have changed form over time.  That's humbling.

Lime can exist in three basic forms in a simple “cycle”.  The lime can change from one form to the next in this cycle, and back again.  The basic material is calcium with variations in what is and isn’t attached to it.  We start the cycle at limestone.

Limestone:  Limestone, shells, marble, chalk etc.... there are various forms of limestone, but they are all basically the same material.  One type contains a lot of Magnesium in a similar form and we call that Dolomite lime.  Dolomite's uses are similar to regular limestone.  The limestone form, including shells of all kinds, is Calcium Carbonate.  It is calcium with 3 carbon atoms attached to it.  You know, carbon as in carbon dioxide the famous greenhouse gas.  We are all familiar enough to know what stone and shell are like.  They are hard and tough.  Maybe more important to understand is that they are not really reactive.  We can throw rocks and shells in water and they just sit there.  The take home message is that they are stable so they resist the elements.  Calcium Carbonate is the most natural and common form of lime and the one that other forms of lime will naturally turn into if exposed to the environment.

Quicklime:  If we take our Limestone rocks or shells and heat them up to a red heat, we drive off the carbon completely replacing those three carbon atoms with one oxygen atom to make CaO (calcium oxide) one calcium to one Oxygen.  If you're starting to get nervous about all that chemistry crap, never mind the chemistry.  The important thing to know is that the burned lime is highly unstable and very quick indeed to react to moisture.  Unless stored in a completely sealed container, it will quickly begin to react with the elements of the environment and start the process of turning back into limestone.  Quicklime is so unstable (and unsafe to have around) that it should be slaked processed into the next form) as soon as possible, preferably right after burning.  Quicklime is very light in weight without those carbon molecules which are now floating around in the environment somewhere as greenhouse gasses.  It is also thirsty for water which it can pick up from even the little bit in the atmosphere.  If left out, quicklime will usually absorb moisture from the air, often falling into a pile of fine powder which brings us to our next stage of the cycle...

Hydrated lime and lime putty:  When water is added to the thirsty quicklime, it absorbs H2O molecules rapidly forming Calcium Hydroxide.  One Calcium, two Oxygen and two Hydrogen.  In this process heat is given off and the mixture can even boil violently.  There are a couple forms of calcium hydroxide. If the calcium hydroxide is made in the form of a putty with excess water and stored in this wet state, it is known as lime putty.  Lime putty is the most reactive form of calcium hydroxide,  and the most  stable way to store it.  Lime putty is completely safe from conversion into limestone, as long as it is kept wet with no exposure to the air.  More commonly, Calcium hydroxide is found in the form of a fine powder known as builders lime, type S or hydrated lime.  This powder of lime is often used in tanning and building because it is convenient to store and sell in the dry form, but it is less stable because a portion of it will turn back into the limestone form with exposure to air, which also makes it less reactive.  Don’t worry if you are getting confused already, we are going to drive home this information with practical examples and if you ever use lime, you will begin to form a context for understanding and remembering the different forms.

So there are the three forms of lime, but to close the cycle the last form, calcium hydroxide or hydrated lime, has to turn back into limestone.  This process is simple and we hinted at it already.  When the lime putty is dried it absorbs carbon from the air turning back into limestone.  In the case of hydrated, or powdered lime, the powdered lime is wetted first and then absorbs carbon as it dries, though it doesn’t carbonate as thoroughly as lime putty.  You might be getting the idea that I’m partial to the lime putty form rather than the powdered form, and you’re right, but the powdered form is useful too and often what is available.  More on that later...

BURN *So, we have gone from limestone rocks or shells which we heated up to drive off carbon causing the atmosphere to warm up, killing the planet.

SLAKE *Then we added water to the resulting thirsty quicklime which boiled violently or fell into a fine powder depending on how much water was added.

CURE *Then we let the spreadable, wet lime putty, or hydrated lime paste , dry slowly.  As the lime dries is reabsorbs carbon from the air saving the planet form carbon dioxide poisoning and forming limestone again.

We can use lime at these various stages for different purposes.  There are so many different uses for lime, that we’ll defer most of that discussion for another time, but here are the characteristics of each type of lime discussed in the context of some common uses.

Limestone, Shells, Etc:  (Calcium Carbonate).  Having been stabilized by the absorption of Carbon dioxide, Limestone, shell, marble etc.. are basically stone as we commonly think of it.  The stone can be used for building and paving of course.  Limestone and shell can also be used in agriculture in a powdered form, and while other forms of lime can be applied to the soil as well, it is usually the carbonate form that is used.  Lime increases the ph of soils by buffering soil acidity.  Calcium Carbonate is fairly stable and non-reactive, but acids, like Carbonic acid naturally found in the ground, slowly dissolve the lime in the soil which is washed out by rains.  Lime, usually as ground shell, can also be used as a nutritional supplement and in animal feeds, particularly to provide a source of calcium for chickens ensuring strong eggshells.

Quicklime:  Quicklime is dangerous to handle and store because it is highly reactive.  It heats rapidly and undergoes a violent reaction when water is applied.  This process is called slaking.  If the water is applied slowly and in measured quantity, the quicklime will fall into a very fine powder of hydrated lime.  If it is slaked with more water, the water may boil and spit hot lime putty and caustic alkaline solution all over the place.  It also expands a great deal during slaking, which is sometimes taken advantage of in building.  By using the still expanding quicklime to mortar walls, the lime, sand mixture is forced into all crevices in the stone.  Quicklimes uses  in the arts are actually minimal.  The most common use of quicklime is probably as a flux in smelting metals, though it is often added in the calcuim carbonate form as shells in primitive smelting, which then cook into quicklime in the smelter.

Hydrated lime and lime putty (Calcium Hydroxide): This is the form of lime with the most uses in arts, trades and cooking.  It is chemically reactive, alkaline and caustic.  Being highly alkaline, it modifies or attacks other materials like some proteins and cellulose, making it very useful.  it also reacts with fats to form soluble soaps.  When it dries it turns back into limestone as we said, which is non-reactive except with acids.  Calcium hydroxide can be used to prepare skins for tanning by attacking certain proteins to release the hair from the skin, and dissolving various substances in the skin fiber which can then be removed by washing.  It can also be used to process corn to make it more digestible in a process called nixtamalization (for more on which see this excellent page on corn processing and tortillas).  There are many many uses for Calcium Hydroxide.

• Type S or Hydrated Lime (aka, builders lime):  is a fine powder form of calcium hydroxide.  Hydrated lime is convenient to store and ship, but is not as reactive as lime putty and makes a relatively poor material for building purposes (even though it is often called builder’s lime which says more about our building ethic than anything else).  It’s convenient form however makes it the most common form of Calcium Hydroxide and it is perfectly fine for some uses like preparing skins and nixtimalizing corn.

•  Lime Putty:  Lime putty will keep indefinitely if stored with a thin layer of water over it and can even improve over time.  Some lime putty is stored for years to make fine quality limes for exacting uses such as fresco paintings.  It is the most caustic form of Calcium Hydroxide and should be the first choice for building purposes and arts unless a powder is required.  When used for its reactive properties, less is needed than when using hydrated lime.  When used in building, it has better workability, carbonates more thoroughly and consolidates into an all around more durable material upon curing.  Adding water to hydrated lime does not make lime putty, it has to be made during slaking by the addition of a larger amount of water.

RECAP:

*Lime is neat and useful!

*There are three basic forms of lime with different properties and two basic forms of the most used form, which is Calcium Hydroxide.

*Calcium hydroxide in the form of either powder or putty is caustic and reactive with many substances making it very useful.