I had this in a file and cannot remember the source but it is very similar to the method I used to bark tan 25 or so years ago.
Here is some of my bark tan on a pouch I made when I had no gray hair
Bark-tanning
As in any type of tanning, it is preferable to use hides that have been stored frozen or wet-salted, or are fresh. Dried out hides are harder to re-hydrate and get good penetration by the buck and the bark liquor. For more, see the Storing Hides Tutorial.
Fleshing: Flesh as you would for brain tanning. Be careful not to damage the grain. You do not need to get every last bit of membrane off at first. Some people find it easier to membrane the hide after it has been soaked in the tannin for a few days.
Bucking/Liming: After fleshing, soak the skin in an alkaline solution of hydrated lime, wood-ash lye or commercial lye (see the book Deerskins into Buckskins for thorough directions) until the hair slips super easily. Limed hides, especially if they are limed for extended period of times, tend to come out somewhat less stretchy than bucked hides. This phenomenon has historically been exploited to create firmer leathers. If you don't want a firmer leather, you are better off bucking. See more below under 'Rinsing'.
Bucking or liming takes longer than it does to simply prepare a hide for brain tanning. Early on, the hair will pull out of the hide fairly easily, but you want it to be so easy that you can just push the hair out rubbing your hand over the hide. Deer hair slips more easily than many other types of hides.
De-hairing: To remove the hair it is best to use a wooden bar with a dull edge, or something similar. It should be more rounded than what you would use to scrape a brain tan, as you do not want to cut or mar the grain. You do want to remove the epidermis though, which is the dark pigmented layer just below the hair. Sometimes it'll just brush off with the hair, other times you'll need to make a conscious effort to remove it. The epidermis contains the pigment and is a darkish gray/black, especially in the summer. If patches of hair will not slip easily, return the hide to the alkaline solution.
Rinsing: When all of the hair and epidermis has been removed, rinse the alkali out of the hide by soaking it in running water for 12 to 72 hours (until all signs of swollen-ness are gone). Lime will leave some calcium in the hide, that to fully remove involves further processes that I prefer to gloss over here. These processes were employed when a limed hide was intended to come out more pliable and historically involved soaking the hide in fermenting materials such as hen or dog dung (known as bating), beer dregs, or fermenting grains (known as drenching). In brain tanning, one does notice that limed hides tend to come out less stretchy than bucked hides, particularly if the hide soaked in the lime for an extended period of time (more than three weeks) See Ancient Skins, Parchments and Leathers, for more on bating and drenching.
Preparing the Bark
The finer the bark is ground up the faster and easier it is to extract the tannin (its a surface area thing). In the old days bark was crushed using a large stone wheel, much like a millstone, powered by an ox or horse. It was ground until it was the consistency of cracked corn, wheat berries, or a coarse powder. Modern folks use grain mills on a coarse setting, chipper/shredders used for making garden mulch, or they use their hatchet as best they can to pulverize the bark into small pieces. It should be emphasized though that the smaller you can get it, the more tannin you'll get from a given quantity of material. And as mentioned before, buying shredded bark (sold as garden mulch) is an easy and cheap way to go.
Leaching the Tannin
It is ideal to use rain or other soft water. Tanneries were traditionally located on rivers and streams because they used so much water. I don't know if they treated their water to remove minerals. The main reason soft water is preferred is because the minerals will react with the tannic acid and create spots or blemishes on the skin. So will blood, if any has been left in the hide (the iron in blood can react with the tannin to make a black stain). I don't know of any other functional reason to use soft water... the hide will tan without it.
Tannin is water soluble. The warmer the water you soak the bark in the faster the tannin is extracted. Hot water darkens tannin resulting in a darker colored product. Boiling tannins especially darkens and dulls the color (like adding grays to it). Many sources recommend simmering the bark for several hours. Some traditions have you soak the bark in cold water for a few days to extract the tannin. This gives the lightest color. Your choice. Here are two recipes:
Lotta Rahme: "Fill kettle halfway with bark and totally full with water. Bring it to a boil and let it boil for at least an hour. Taste it. The more bitter and astringent the more tannin...like tea or coffee. Take half the liquor and mix with equal amounts of water for the first bath. Use a plastic or wooden tub."
Steven Edholm and Tamara Wilder: "Steven and Tamara use 1 to 3 gallons of shredded bark and soft water (rain or snow) to cover, in a 2-5 gallon stainless steel pot for several hours. Iron or aluminum pots will react with the tannic acid and cause stains etc., so don't use them. Plastic, wood, fiberglass, and masonry tubs are all suitable. Use wooden stirring paddles."
The First Bath
It is very important to use a very weak solution for your first bath. If the hide is put into a strong tannin bath, the outside gets tanned and shrinks. This inhibits the tannins from penetrating to the center of the hide, leaving the inner parts raw. This is called "dead tanning" or "case hardening"
The first pouring is too strong so put this aside. Add more water, simmer again and pour off. Many tanners will put this aside too, and use the third extraction and then add up to three parts water to the one part liquor for the first solution.
The ideal bath to start with is one that has already been used for another hide. That way all the large tannin particles have already been used up. This is known as a "spent liquor". There is another advantage to spent liquors. In an old bark liquor, the bark sugars have fermented, forming lactic and acetic acid, which help remove any traces of lime as well as help preserve the hide.
Soaking
As previously mentioned, you want to use wooden, plastic or masonry containers to do the soaking in. The bigger the container and the more solution that you have in it, the easier it is to evenly tan the hide. It is best to hang the hide(s) over sticks (crossbars) in the solution with as few folds and wrinkles as possible. If you don't, you should stir and re-adjust the hide more often.
Put the moist but drained skin in. Stir for the first 10 minutes and then once every 10 minutes for the first hour. Skin should be turned and agitated frequently in the first few days to assure even absorption. Epidermis will block the entry of tannin. If there are white patches it is from epidermis that was left. It must be scraped off, but be careful not to remove the grain. Lotta says that sheepskin epidermis is particularly difficult to get off.
Strengthening the Solution
You should use progressively stronger solutions. Different tanners recommend different timings for strengthening the bark solution. In general, you strengthen the liquor as you notice it looking weaker.
Lotta recommends strengthening it after the first several hours, removing the skin and membraning it again at this juncture. Mark Odle moves hides to the second run of liquor in a week or ten days. He then strengthens it weekly until the hide has been soaking for five or six weeks when the liquor can be used at full strength. One rule of thumb seems to be that you can push things faster if you are tanning thinner hides like deer and goat (which Lotta is), and that you need to be extra careful of case hardening with thick skins like cow, buffalo or bull elk.
After you start using full strength solutions, you can use the old bath to boil the new bark in, for added strength. However, bark liquor used as a first bath for skin that was de-haired with lime can contain residual lime and shouldn't be re-used. It should be thrown out. Once the whole skin has an even brown color, the bark can be left in with the skin, and you can leave it for longer times without stirring. If the hides stay in too weak a bath they begin to rot from the inside.
Mark Odle adds vinegar to further acidify and strengthen the solution. Mark adds three or four gallons to 80 or 100 gallons of liquor. Steven and Tamara used to this, but don't bother any more.
Smells, Molds & Textures
The solution should develop a somewhat pleasant fermented or vinegar like smell from the fermentation of the bark sugars. Smell can be strong but should never be putrid. A sulpherous smell indicates spoilage. At no time should the hide become slick, slippery or slimy. The texture will change from somewhat slippery to a firmer, textured grain. The pores and grain will become quite distinct. Mold may grow on the surface of the liquor, skim it off or stir it in. It is supposed to be (we haven't tried this) ok to freeze the skin in between baths.
Knowing When it is Tanned "Through"
Generally, to be considered 'through' tanned, the color should strike through to the center of the thickest part of the hide. To check this, snip a small piece off the neck. Lotta will also put a little saliva on the section that has been cut. Un-tanned skin will not absorb saliva easily and will appear wet, matted and glistening.
Another test is to fold the skin double two times and press the folded area between your fingers. When unfolded the fold should appear as light dry lines.
Some tanners say that the color should be even from the outer edge to the center. Doug Crist says he only has some color reach the center while the outer edges are much darker (much like the hide pictured above). However, he also says that softening the hide is a fair amount of work like brain tanning, whereas other folks say it should be much easier. This may be a factor of how much tannin you get into the center of the hide.
How Long
How long you soak your hide depends on the finished product that you desire. The longer you soak it the more it is "filled" with tannins. Once the color has penetrated to the center, you can either remove it from the solution and proceed to currying, or you can leave it in there longer to produce a 'fuller' leather. Getting color to the center of the hide means that some tanning has occurred throughout the hide. But you can always get more tannin to attach itself to the fibers and fill the spaces between the collagen chains. The amount of tannin can reach 50% of the weight of the finished leather.
The fuller the hide becomes, the thicker and less stretchy it gets. These are good qualities for saddles, belts and shoe soles, but may not be as desirable for other uses. Full-tanned hides are also easier to carve designs into the surface of. Contrary to this, one source says that thick hides used for sole-leather are sometimes left with an un-tanned stripe in the center which makes it more water tight and harder. This is also sometimes done for knife-sheaths.
Here are some ballpark figures of how long you should expect to soak your hides:
Mark Odle says deerskin sized hides should remain in a full strength ooze for three or four months in moderate temperatures. Cattle and buffalo will take five or six months. The warmer the temperature the faster the process. Once they are tanned through, there is no problem letting them sit in the bath as they will not rot. Looser fibered skins will take the tan more quickly than the tighter skins.
Lotta Rahme recommends as little as 7-10 days to tan a goatskin through, using around 1 3/4 to 2 1/4 lbs of dried bark. Goatskins are very thin. Cattle and elk, can take half a year or more.
A.B. Farnham: Harness and belting leather takes four 1/2 months (for cow), and 6 1/2 months for sole leather.
Currying
"Currying is defined as "the preparation of tanned skins for the purpose of imparting to them the necessary smoothness, color, luster and suppleness".
Traditionally in Europe, currying was carried out by specialized tradesmen in an entirely different shop as it is an art unto itself. How exactly this is done, again depends on what you are looking for in the final product. First we'll give you the general rundown, and then cite specific methods for specific types of leather.
Once the hide is tanned as thoroughly as you want it, rinse it in fresh water for a couple hours. Between each rinse use a slicker and a beam to squeegee out the liquid. (We've done this using our wetscraping tool on the flesh side of the hide, putting a towel between the hide and beam to help protect the grain layer from tearing). Your are trying to remove as much unfixed tannin as possible. A slicker can be a round, smooth rod or a hand-sized, rounded edged slab of glass. Slickers can also have slate, brass, copper or heavy glass blades. You want to be real careful not to tear the grain off.
Next the hide is dried a bit, then greased and softened. Any dyeing should be done before oiling the skin.
Dyeing
At this point your bark tanned hide will be whatever color was imparted by the tannins, usually a tan or reddish brown. Once the hide is oiled, this color will darken somewhat. If you want to change the color of the skin, you can soak the hide in any tannin based dye. There is a good chapter on dyeing in Steve & Tamara's Wetscrape Braintanned Buckskin.
Oiling
Oiling the bark tanned hide makes it dry softer, darkens it and prevents it from cracking...much like oiling a pair of leather boots. Neatsfoot oil, olive oil, tallow, brains, bear fat and fish oil have been used to finish bark tanned leather. Using tallow (a waxy body fat from deer, elk, cows and other ungulates) imparts a heavier feel and more water resistance to the leather. Using a light oil such as neatsfoot, fish, bear or brains results in a lighter, stretchier leather.
The hide should be damp with all excess water expelled by working it on both sides with the slicker. Stretch the hide in all directions. Oil is then spread evenly on the hide and it is either worked soft as it dries or not depending on the type of leather desired. When the hide is dry, it can be lightly dampened or "damped back" by rolling it up in a damp towel. This process of oiling, working and drying can be repeated until you get the softness you desire. When the hide has dried, any surplus oil or tallow can be removed with a rag. To smooth the flesh side, it can be "sleeked" with a slicker.
A.B. Farnham, describes different finishing methods
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To finish sole leather, lay the sides or strips down and press out most of the water by covering with some old dry cloths and treading over the whole surface to compress the fibers, then hang up until they are only damp. While still damp give them a good coat of oil on the grain side only, and hang up again until fully dry. Sole leather can be waterproofed by greasing heavily. Recipe: 3 parts tallow to 1 part fish or neatsfoot oil.
Harness and belting are finished by taking the still quite damp hide, pressing out the rinse water, slick over the grain side thoroughly and give it a liberal coat of neatsfoot or fish oil. Hang up or better, take out, spread smooth and let dry slowly. When dry, damp back by wetting or rolling up in wet burlap until damp and limber all over. Prepare a stuffing of equal parts tallow and neatsfoot oil (or fish). Heat them together, and allow to cool until soft and pasty but not liquid. Apply a thin coating to the grain side while it is warm and hang them up to dry. When dry remove the surplus stuffing by working over the grain side with the slicker. If there isn't enough grease in the leather yet, dampen back again and repeat the process of greasing, drying and slicking. Finally rub over with sawdust to remove a surplus of the grease.
Softer leathers are finished by oiling the damp leather, stretching out and drying, damping back, slicking, staking and drying. Repeat if necessary. Do not apply tallow or heavy grease to light skins and spend plenty of time slicking and staking it.
Softening
"All softening processes begin when the tanned skins are partly dry and are continued until they are fully dry and sufficiently flexible."
Mildly Soft: Hides that you want mildly flexible can be rolled on a table with the hands using considerable pressure. How you do this rolling will affect the grain's texture. According to Steven & Tamara, "If the hide is rolled up each way with the grain inside, an "orange-peel" texture results. If the hide is maintained flat or rolled with the grain out, the surface remains flatter." The grain side can also be rubbed with a weak soap solution and then scrubbed with a piece of glass to produce a tight and shiny grain. In the old days, grain patterns were made with specialized tools called "grain-rollers".
Softer: Only hides for which you want a soft and stretchy texture are 'softened' anything like brain tan is. This is usually limited to thinner skins that haven't been tanned very 'full'. They may be softened by drawing them across a dull edge (like a staker) or wire, as well as pulled and stretched by hand. You probably wouldn't want to use a cable as that will rough up the surface. Up to a point, the more you work it, the softer the finished skin will be. If you have just barely tanned it through, you may need to soften it as rigorously, or nearly so, as you would for brain tanning. That is Doug & Lynx's experience at least.
