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