🧰 Vegetable Tanning: Bark, Wattle and the Pace of the Pit

Everything so far has been preparation. Tanning is the single chemical transformation that turns a pickled pelt, which will still rot, into leather, which will not.

Understand the change properly, because the whole of the rest of the course depends on it. A pelt is a network of interwoven collagen fibre bundles, opened up by liming, cleared by fleshing, bating and degreasing, and brought to an acid condition by pickling. Wet and warm, that network is food. Bacteria break it down, the fibres separate and dissolve, and the pelt rots. It is meat with the hair taken off.

Tanning works by permanently cross-linking that fibre network. A tanning agent travels into the spaces between and around the fibre bundles and then binds them together chemically, so the fibres no longer separate and dissolve when wet and warm. That is what makes leather leather. It is not a coating, not a dye and not a dry state. It is a permanent change to how the collagen behaves.

There are two tanning families that matter for this course and the choice between them is the biggest single safety and business decision a small operator makes. Vegetable tanning uses plant tannins. Chrome tanning uses trivalent chromium salts. This module is the vegetable route; the next is chrome.

Now the one measurement that lets you compare tannages honestly: shrinkage temperature.

Shrinkage temperature is the temperature at which tanned collagen starts to shrink and degrade when it is wet and heated. It is a direct measure of how much heat resistance the cross-linking gave the fibre. The sourced figures are:

• Vegetable-tanned leather: around 85 degrees Celsius.
• Chrome-tanned leather: over 100 degrees Celsius.
• Glutaraldehyde or wet-white tanning: around 75 degrees Celsius.
• Alum tanning: in the range 65 to 85 degrees Celsius.
• Cod-oil or chamois tanning: around 50 degrees Celsius.

Read that list as a ranking of one specific property, not of quality. Chamois leather at around 50 degrees is not bad leather. It is soft, stretchy, highly water-absorbent leather made for a job that never involves hot wet conditions. Shrinkage temperature tells you what a leather can survive, not what it is worth.

Why does it matter commercially? Because footwear that is repeatedly wetted and dried, or worn in hot working conditions, punishes a low shrinkage temperature. So does anything that will meet hot water in use or in cleaning. This is the honest reason chrome dominates the world's footwear leather, and it is the reason a vegetable tanner should aim at belts, straps, saddlery, tooled work and structural leathercraft rather than fighting for the wet-boot market.

Two things tanning does not do, and confusing them costs beginners real money.

Tanning does not give the leather its colour, feel, softness or final performance. That is the job of the post-tanning sequence, neutralising, retanning, dyeing and fatliquoring, which comes after the tanning drum or pit. A skin that comes out of vegetable tannage is a crust: tanned, stable, but not yet a finished, sellable leather.

Tanning also does not repair anything. Every defect from the living animal, from flaying, from curing and from the beamhouse is still there and is now locked in permanently. A hide with a raw centre from bad soaking has a raw centre for ever. A pelt over-limed into looseness is loose leather. Tanning fixes the fibre network exactly as you handed it over.

That is why the tanning module comes after two full beamhouse modules and not before them.

One test you will use in every remaining module of this course. Cut a small corner from the thickest part of a tanned hide, usually the butt, and look at the cut face through the thickness. On a vegetable tannage you are looking for tannin colour that has travelled in from both faces and met in the middle. A pale, untanned band through the centre is a raw centre. It will rot from the inside while the outside looks finished, and it cannot be repaired.

Learn to make that cut without hesitating. A corner off the butt costs you a few square centimetres. A hide sold with a raw centre costs you a customer.

Vegetable tanning uses tannins, which are naturally occurring polyphenol compounds found in bark, wood, pods, leaves and roots. They bind to and cross-link collagen. That is the whole mechanism, and people have been using it for thousands of years without knowing the chemistry.

Start with what Africa actually produces, because this is one of the few places in this trade where the continent has a real commercial position rather than a gap.

Black wattle, Acacia mearnsii, is grown commercially in South Africa and its bark is processed into tanning extract by South African producers. In the leather trade that extract is sold under the name mimosa. Wattle and mimosa are the same tannin from the same tree; mimosa is the trade name. If you have been confused by seeing both words, that is why.

The sourced figures for wattle and mimosa are:

• Bark tannin content: about 30 per cent.
• Natural pH of the extract: about 4.6 to 4.8.

That pH figure is more useful than it looks. The extract arrives mildly acid on its own account, which sits sensibly against a pickled pelt, and it tells you that vegetable tanning is not an alkaline process. It belongs on the acid side of your workshop, with everything that implies about vessels, drains and separation from sulphide.

The other two major commercial vegetable tannins in world trade are quebracho, from South America, and chestnut, from Europe. Both are used globally alongside mimosa. Neither is an African-grown source. Know the names so you can read a supplier's price list, but understand that buying them means importing.

Now the honest part about wild bark.

There is a real tradition, in Africa and everywhere else, of tanning with whatever bark, pod or root is locally available. It works. It is how the trade began. But this course will not hand you a tannin percentage for a tree growing near you, because no such figure exists in the reference material behind this course, and inventing one would tell you to build a business on a number nobody measured.

What the reference does carry, and clearly flags, is a set of figures from a North American hobbyist bark-tanning guide, which gives natural tannin contents of about 6 to 12 per cent for temperate barks such as oak, birch, hemlock and willow, against 30 per cent or more for commercial extracts. That hobbyist guide is not African, not industrial, and its figures are for temperate trees that do not grow in most of this course's audience's countries. Use it for one structural lesson only, and it is a valuable one: raw bark carries far less tannin than a commercial extract, often by a factor of three or more. So a workshop tanning with gathered bark needs a great deal more bark, a longer time, or both, than a workshop buying extract.

How do you work with a local bark without a laboratory? By measuring your own results rather than guessing at chemistry.

• Pick one bark species and stay with it. Mixing species batch to batch makes every result uninterpretable.
• Record the weight of dry bark you use and the volume of water you extract it into, every time. That ratio becomes your own strength unit even without knowing the tannin percentage.
• Record the days each skin takes to tan through, against that ratio, and against skin type and thickness.
• After ten or fifteen skins you will have something no book can give you: a working strength-and-time table for your bark, your water and your skins.

That is not a second-best method. It is how tanning knowledge was actually built.

A practical caution on gathering bark. Stripping bark all the way round a living tree kills it. If you are gathering, take bark from trees already being felled for timber or clearing, or take partial strips, and understand that a tannery which kills its own tannin supply has a two-year business. Commercial wattle is grown as a crop for exactly this reason.

And a gap you should know about. No quantified African production or cost breakdown for vegetable tanning at tannery level was retrieved for this course's reference. What exists is data on the tannin supply side, from a South African commercial extract producer, not on what it costs an African tannery to run a vegetable tannage. So the costing in Lesson 6 is a method you apply to your own quoted prices, not a price list.

The oldest rule in vegetable tanning is that you start weak and you strengthen slowly. Everybody who ignores it produces the same defect, and it is worth understanding exactly why.

A tannin molecule is large. It has to travel from the outside of a pelt into the middle, between the fibre bundles, before it binds. But tannin binds to collagen readily, and it binds to the first collagen it meets. Put a pelt straight into a strong liquor and the tannin grabs the outer layers immediately, fills them, and effectively seals the surface. Now nothing can get past it. The pelt is case-hardened: tanned hard on both faces, raw in the middle, with a drawn, harsh grain.

Starting weak avoids this. In a weak liquor there is not enough tannin present to saturate the surface, so it penetrates further before it binds. As the pelt takes up tannin and the outer layers fill, you raise the strength, and the stronger liquor works its way into ground that is already partly tanned and no longer blocking. Weak to strong, over time, is how tannin reaches the centre.

The traditional pit system is this idea built in wood and stone. A run of pits holds liquors of increasing strength. Hides start in the weakest, spent liquor and are moved along the run, or fresh liquor is fed in from the strong end and works its way back down, so the freshest hides always meet the weakest liquor. Nothing about it is quick and nothing about it is complicated. It is the principle made physical.

Now the timings, and this is where the sourced figures disagree by an order of magnitude. That disagreement is the teaching point, not an error.

• Traditional pit tanning of heavy hides: up to about one year.
• Modern drum-based vegetable tanning: about ten days.
• Thin skins for shoe linings and uppers in a drum: as little as about eight hours.

Those three figures come from the same global industry technical guide. They differ because they describe different equipment on different material. A heavy cattle hide hanging in still liquor and a thin goatskin turning in a drum are not the same problem. A drum forces liquor through the pelt mechanically; a pit waits for it to soak in.

The course reference also carries a hobbyist soak progression, and it is flagged heavily for good reason. It describes starting at about three parts water to one part liquor, strengthening weekly, and reaching full-strength liquor only from about week six, with total soak times of 7 to 10 days for a goatskin, up to 5 to 6 months for cattle or buffalo hide, and about 6.5 months for sole leather. That guide is North American, not African, not industrial, and its times are for buckskin-style hand tanning of small skins. Do not run your business on those numbers. Take from them the one thing they genuinely show, which is the structural shape of the method: begin very dilute, strengthen on a schedule, reach full strength only late, and expect thickness to drive time far more than anything else.

So which end of the range are you at? Answer it honestly with three questions.

1. What is your equipment? A drum forces liquor into the pelt and cuts time enormously. A pit or a tub does not. If you have no drum, you are at the slow end and no recipe changes that.
2. What thickness are you tanning? Goatskins and sheepskins are thin and tan comparatively fast. Cattle hide for sole leather is the slowest thing in the trade.
3. What can your cash stand? A pit of hides is money you have already spent and cannot sell. Six months of stock in a pit is six months of capital tied up, and that is a genuine, quantifiable disadvantage against chrome tanning that no amount of enthusiasm for natural methods removes.

A workshop that answers those three honestly will usually find its sensible starting point is thin skins, goat and sheep, in the smallest vessel that will hold them, aiming at weeks rather than months. Build the skill and the records there, then decide whether heavy hide work is worth the capital it locks up.

One practical rule for running any progression. Change one thing at a time and write it down. If you strengthen the liquor and lengthen the soak and switch bark species in the same batch, you will never know which change did what. Vegetable tanning is slow, which means you get few experiments per year, which means each one has to be readable.

This lesson is the working day. You have a pickled pelt, you have tannin, and you have whatever vessel you can afford. Here is how the tannage is actually run and, more importantly, how you prove it worked before you spend any more money on the hide.

Setting up. Make your liquor by extracting bark in water, or by dissolving commercial extract to the strength your supplier's sheet specifies. Start deliberately weak. If you are using extract, your supplier's technical sheet is the right authority for starting strength and offer, and this course does not print a substitute figure, because none exists in its reference material for a small workshop. If you are using your own bark, use your recorded bark-weight to water-volume ratio from the previous lesson and start at the diluted end of it.

Getting the pelt in. Pelts must move in the liquor, or must have liquor moving past them. A pelt lying flat and folded on the bottom of a tub tans on the outside of the fold and not inside it. In a drum, the drum does the work. In a pit or tub, hides are hung, or laid flat and separated, and turned and moved by hand on a written schedule. Turning is not optional and it is the single most common thing a beginner skips.

Strengthening. Raise the liquor strength on a schedule you decide in advance and write on the batch card, not when you happen to remember. Each time you strengthen, record the date, the change made and what you observed on the pelt.

Now the test that matters more than any of it.

Cut a small corner from the thickest part of the hide, usually the butt, and look at the cut face straight on through the thickness. Tannin colour is visible. You will see brown travelling in from the grain face and brown travelling in from the flesh face, with a paler band between them where the tannin has not yet arrived.

That pale band is your progress meter. Cut a fresh corner at each strengthening and watch the band narrow. When it closes and the colour is continuous across the whole thickness, the tan has gone through.

What different cut faces tell you:

• Even brown right through, corner to corner: a good tannage.
• A pale core band: not finished. Keep going. Do not take it out because the calendar says so.
• Deep dark colour on the faces with a sharply defined pale core that stops narrowing: case hardening. The surface has sealed. This is what starting too strong produces, and once it has set it is very difficult to fix.
• Colour reaching through in the belly but a pale core in the butt: normal and expected. The butt is thicker and denser. Judge the whole batch by its thickest part, always.

Hold on to what a raw centre actually means, because it is easy to be tempted. A hide with an untanned core will rot from the inside while the outside looks finished. It may be weeks or months before the customer finds out, and when they do it is your name on it. There is no repair. The only correct response to a pale core is more time.

The batch record. Every tannage gets a card that stays with the hides and records, at minimum: pickled weight, tannin source and supplier or bark species, starting liquor strength, each strengthening with its date, each cut-face observation, turning done, and the total elapsed days at tan-through. That card is your entire technical library after a year.

Why it matters so much here in particular: vegetable tanning is slow, so you get very few complete experiments per year. A drum tanner running short cycles can afford sloppy records and learn by volume. You cannot. Twelve careful records beat a hundred vague ones.

And keep the safety discipline that the pace makes easy to forget. A tanning float is acid-side. Its vessel and its drain never share duty with anything sulphide-bearing unless that vessel was drained, rinsed twice and inspected by a second person. Never pour a spent tan liquor into a pit or drain that has held liming or unhairing float, for any reason, including that it is nearly empty at the end of the day. A vegetable tannery is far safer than a chrome one, but it still stands in a workshop where sulphide was used a few steps earlier, and that is where the danger has always been.

One last thing. A tanned hide out of the pit is a crust, not a product. It still needs the post-tanning and finishing work covered later in this course before it is leather anyone will pay a good price for.

This course recommends vegetable tanning as the safer starting route for a small rural workshop, and it will say so again in the next lesson. But a recommendation you cannot argue against is not a recommendation, it is a sales pitch. So here are the three real costs of choosing this route, stated plainly and without softening.

The first cost is colour, and it is permanent.

Vegetable-tanned leather is cream to dark brown. That is the tannin's own colour, held throughout the leather rather than sitting on top of it, and the leather darkens further with age and light exposure. You can dye vegetable-tanned leather, but only within and on top of that base colour. You cannot make it a bright, saturated red, a clean white, a strong blue or a pale pastel, because you are always dyeing over brown.

Chrome-tanned leather starts pale, so it can be dyed almost any colour. That is not a small advantage in a market where fashion colour sells.

Say the consequence out loud, because it is one of the most useful sentences in this course: a workshop targeting bright fashion colours has already chosen chrome tanning, whether or not it realised it. If the products you intend to sell need colour range, this module is not your route and you should read the next one carefully.

Against that, the natural tannin colour is genuinely attractive, ages well, and is exactly what the tooled and carved leather market wants. That is a real market, not a consolation prize. But it is a different market from fashion colour, and you must choose.

The second cost is wet-heat resistance.

Vegetable-tanned leather has a shrinkage temperature of around 85 degrees Celsius, against over 100 degrees for chrome. In practice that means it is less tolerant of hot, wet conditions. It is generally not the first choice for footwear that will be repeatedly wetted and dried, or worn in a hot working environment.

Be careful how you read that. It does not mean vegetable leather falls apart in the rain. It means that when leather is both wet and hot, the vegetable tannage gives up its structure at a lower temperature than chrome does. For belts, straps, saddlery, tooled goods, bags and structural leathercraft, all of which value its firmness and natural handle, it remains an excellent traditional choice. For a work boot that gets soaked daily, chrome is technically better and pretending otherwise helps nobody.

The third cost is time, and therefore cash.

A heavy hide in a pit can take up to about a year. A drum vegetable tannage takes about ten days, against a chrome process of around 15 hours from deliming through to completed tanning. Even at the fast end, vegetable tanning is slower.

Translate that into money the way a business owner must. Every hide sitting in a pit is money you have already spent on the raw hide, the salt, the beamhouse chemicals, the tannin and the labour, and it earns nothing until it comes out, is finished and is sold. Six months of pit stock is six months of your working capital locked in a hole in the ground. A chrome tanner with the same money turns it over many times in that period.

That is a genuine commercial disadvantage. It is not cancelled by the fact that vegetable tanning is safer. Both things are true at once, and a business plan has to carry both.

One more difference worth knowing, and it cuts the other way. Vegetable tanning's effluent is a high-BOD organic tannin effluent, and it carries no chromium at all. Chrome tanning's effluent carries chromium, in quantities that depend heavily on the technology tier the tannery can afford. High organic load is a real pollution problem and your national environmental regulator will still have something to say about it, so do not read this as an exemption. But an effluent with no chromium in it is a genuinely simpler problem to manage than one with chromium in it, and that difference matters more at small scale than at large.

So where does this leave a small workshop? Choose products that fit what vegetable leather actually is: firm, natural-coloured, holding a stamped or carved design well, excellent for belts, straps, saddlery, sheaths, tooled bags and structural goods. Do not choose it and then try to sell bright soft fashion leather with it, because you will lose to chrome-tanned imports on both colour and handle, and the loss will be blamed on your skill when it was actually a choice made months earlier in a pit.

The course reference behind this module says plainly that vegetable tanning is the safer route for a small rural operation. This lesson explains exactly what that means, in hazards removed rather than in general reassurance, and then makes you cost it.

What vegetable tanning removes from your workshop.

It does not remove the sulphide hazard. That belongs to liming and unhairing, which every hide goes through whatever it is tanned with. Sulphide liquor and acid liquor must still never meet, and vegetable tanning float is acid-side. So the separation rules from Modules 5 and 6 apply to a vegetable tannery in full and without discount. Be very clear about this, because the phrase natural tanning has misled people into thinking the whole workshop is safe.

What it does remove is the chromium hazard entirely. No trivalent chromium in the drum. No hexavalent chromium risk in the finished leather. No chromium in your effluent. No chromium-bearing solid waste in your trimmings and shavings. The whole of the next module's subject matter simply does not exist in a vegetable tannery.

That is not a small thing for a workshop with no laboratory, no effluent treatment plant, no trained chemist and no capital to buy any of them. Chrome tanning's environmental and cost performance depends heavily on which technology tier you can afford, and conventional low-exhaustion chrome tanning in a small, undercapitalised workshop is the worst combination for effluent chromium load. That is precisely the tier most small operators would start at. Vegetable tanning takes that whole problem off the table.

So the honest summary is this. Vegetable tanning removes one of the trade's two major chemical hazards completely, leaves the other one exactly where it was, and charges you time, colour range and wet-heat resistance for the privilege. That is the real trade and you should make it with your eyes open.

Now cost it, because a safety argument that loses money is not a plan.

Build your cost per finished skin like this, using your own prices, this month, from your own suppliers.

1. The raw hide or skin, including its curing cost. Salt is a real input at 30 to 40 per cent of green weight for standard wet salting, or about 25 per cent for a reduced short-term regime, so price the salt you actually use.
2. The beamhouse chemicals: lime, sodium sulphide, deliming reagent, bate, degreasing emulsifier where needed, pickling acid and pickling salt.
3. The tannin, priced from the quote you obtained in Lesson 2, at the quantity your own records show you actually use per skin.
4. Labour, including your own time at a real rate. A plan that forgets to pay the owner is not a plan.
5. A fair share of capital cost, spread over the realistic working life of your vessels and tools. No sourced working-life figure for small tannery equipment under African conditions exists in this course's reference, so estimate it locally and conservatively and write down the assumption you made.
6. Waste and effluent handling.

Then divide by actual usable output, not by hides started. Measure your losses; do not assume them.

Every price in that list is local and current and this course gives you none of them. No current African price for raw hides, tanning chemicals, tannin extract, finished leather or leathercraft products was retrieved for this course's reference material, and the older figures that do exist are pre-2000 and are used only to show the direction of value gaps, never as prices. Anyone who hands you a universal price for these things is guessing.

Finally, the market question nobody has answered for you, and you should know it is open.

In some world markets, chrome-free and vegetable-tanned leather commands a price premium. Whether such a premium currently exists and is accessible to African small-scale tanners, and how large it is, was not established in this course's reference material. It is named there as a real and valuable gap. So do not build a business plan on a premium you have not confirmed. Go and find out for your own market: ask your buyers directly whether they will pay more for vegetable-tanned or chrome-free leather, and get the answer in numbers, not in enthusiasm.

If the answer is yes, you have a genuine differentiation built on a real safety and environmental advantage. If the answer is no, vegetable tanning is still the right starting route for a small rural workshop on safety grounds alone, and you now know to compete on product fit, firmness, tooling and the natural finish rather than on a premium that is not there.