🧰 The Beamhouse, Part Two: Fleshing, Deliming, Bating and Pickling

A hide has three layers. The epidermis on the outside carried the hair, and liming and unhairing have already destroyed it. The corium in the middle is the leather, and everything you do from here is in service of it. The hypodermis on the inside is fat and loose connective tissue that attached the skin to the muscle, and it contributes nothing whatsoever to leather strength. Fleshing is the mechanical removal of that inside layer.

Say that plainly to yourself: you are not trimming the leather. You are taking off a layer that was never leather and that will actively harm the hide if it stays.

Why does it harm the hide? Because fat and loose tissue on the flesh side is a physical barrier. Every chemical from here to the end of tanning has to travel into the corium from both faces. A film of fat on the flesh side slows or blocks the tanning agent from that side, so the hide tans mainly from the grain side and you end up with uneven penetration. Uneven penetration is how you get a raw centre, tanned on the faces and untanned in the middle, which rots from the inside while looking finished on the outside. That defect cannot be repaired by any later step.

Fleshing is also where a lot of the hide's weight leaves. Industrial figures give fleshings at roughly 70 to 230 kg for every tonne of raw hide processed, and total solid waste across the whole process at roughly 450 to 600 kg per tonne. Those are industrial-scale numbers from global technical literature, not measurements from an African small workshop, and they are here for scale rather than as a target. But the direction is true at any scale: most of the weight of a hide is not leather and never was, and a good part of what leaves does so at the fleshing beam.

Now the timing question. Why flesh after liming rather than before?

Standard practice is to flesh after liming because the tissue is loosened and swollen by that stage and comes away far more cleanly. A green or soaked hide holds its flesh tightly. A limed pelt has been swollen open by the alkali, the fibre bundles have been prised apart, and the hypodermis lifts off in sheets instead of having to be cut off shred by shred. Less cutting means fewer knife scores into the flesh side and a more even thickness across the pelt.

How it is done. In a small workshop it is a two-handled fleshing knife worked over a beam, which is a smooth curved log or a shaped board set at an angle. The pelt is laid grain side down, the worker leans into it, and the knife is pushed down the pelt so it shaves rather than digs. In a larger operation a fleshing machine does the same job between rollers.

The skill is entirely in the angle. Held too flat, the knife slides over the fat and does nothing. Held too steep, it cuts into the corium and you have thinned the leather, or gone through it. A hole is a hole; nothing repairs it. Learn the angle on a low grade hide before you use it on a good one.

Watch for the belly. The belly is thinner and more loosely woven than the butt, so it is the easiest place in the whole pelt to cut through. Ease the pressure there and go slower.

Splitting often happens near this point too. A thicker pelt can be split horizontally to bring it to a more even working thickness. Remember what that decision is worth: only the piece keeping the grain layer can be sold as full-grain or top-grain leather. The flesh-side piece is a split with no natural grain surface and it is priced lower for that reason alone.

Finally, plan the waste before you make it. Fleshings are wet fat and tissue and they putrefy within a day or two in a warm climate. Decide now where they go, who clears them, and how they are kept off the floor and out of your drains. A drain blocked with fleshings is not just a smell problem. It is how two liquors that must never meet find each other.

Deliming is the step where the workshop changes sides. Everything before it was alkaline. Everything after it is acid. That single sentence is the reason this lesson matters as much for safety as for chemistry.

What deliming does. After liming and unhairing, the pelt is full of lime and strongly alkaline. The sourced technical example puts the pelt at about pH 8.5 at the start of the deliming step. That alkalinity did a job, prising the collagen fibre weave open so chemicals can travel through it, and now it is in the way. Bating enzymes cannot work in it. Pickling acid cannot bring the pelt down through it economically. Tanning agents will not behave correctly against it. So deliming removes the lime held in and on the pelt and brings the pH back down toward neutral.

Understand that deliming is not a rinse. Lime is not simply sitting on the surface waiting to be washed off. It is held through the thickness of a swollen pelt, and getting it out means bringing a reagent all the way to the centre and letting it work. That is why deliming takes time and why the test that matters is a test of the centre, not the surface.

The test is simple and it costs nothing but a corner of pelt. Cut a small piece from a thick part, usually the butt, and look at the cut face. An indicator solution such as phenolphthalein, dropped on the cut face, stays pink where the pelt is still alkaline and goes colourless where deliming has reached. A pink band through the middle means the centre is still limed and the pelt is not ready, whatever the clock says. If you have no indicator, you learn to judge by the change in the pelt itself, which loses the plumped, rubbery, alkali-swollen feel and goes flatter and softer as the lime comes out. The indicator is far better. Get some.

The reagents. Ammonium sulphate is the traditional deliming chemical and is still very widely used. It works well and it is cheap. It also has a real cost that a course must be honest about: ammonium compounds put nitrogen into your effluent, and nitrogen load in wastewater is a genuine pollution problem. For that reason cleaner-production tanneries are increasingly replacing ammonium salts with carbon dioxide based deliming systems. Carbon dioxide deliming does the same acid-side job without the nitrogen, but it needs gas supply and equipment that most small workshops do not have.

What should a small workshop do with that? Not pretend. If you are using ammonium sulphate, use it knowingly, keep the float separate, and understand that your effluent carries a nitrogen load you may one day be asked about. If you can move to a carbon dioxide system as you grow, it is the direction the industry is moving for a real reason.

No deliming dose is given in this course. The course reference does not carry one, and a made-up dose is exactly the kind of number that produces a half-delimed pelt or a damaged one. Get your dose from the technical sheet of the product you actually buy, or from a working tanner who will show you on your own hides.

Now the safety part, and it is the reason deliming is taught as its own lesson rather than folded into pickling.

Deliming is the first acid-side process after liming and unhairing. It happens close in time and usually close in physical space to the alkaline sulphide process it follows. Often it is the same day. Sometimes people are tempted to do it in the same drum. That proximity is exactly where the trade's deadliest mistake lives.

You already know the mechanism from the previous module. Sulphide liquor is stable and safe only while it stays alkaline. Bring acid to it and it releases hydrogen sulphide gas immediately, a gas that kills quickly and destroys your ability to smell it before it kills you.

So at deliming the rule becomes procedure, not a warning:

1. The liming and unhairing float is fully drained before the pelt goes anywhere near a deliming reagent.
2. If a vessel must change duty from alkaline to acid, it is drained, rinsed twice and inspected by a second person. Uncertainty counts as dirty.
3. Deliming float goes to its own drain, decided before the work starts, never into a channel carrying sulphide-bearing water.
4. Everything is done in the open air or in genuine cross-ventilation. Never in a closed room. Never inside a pit, for any reason at all.

Bating is the quietest step in the beamhouse and the one that most decides whether a buyer picks your leather up and keeps holding it.

Here is the problem it solves. A pelt coming out of deliming is clean of hair, clean of flesh, and back near neutral. But it is not only collagen. It still holds soluble non-collagen proteins and degradation products left over from liming and unhairing, sitting in and around the fibre structure. Those leftovers do not become leather. They fill the spaces between the fibre bundles, they hold the grain slightly tight and slightly coarse, and they get in the way of the tanning agent doing an even job.

Bating uses enzymes to remove those remaining soluble proteins and to further loosen the fibre structure. Enzymes are biological catalysts. They break specific proteins apart and leave others alone, which is exactly the selectivity you need here, in the same way that sulphide unhairing was selective between keratin and collagen. Bating enzymes work on the leftovers, not on the collagen you are keeping.

What you get for it is grain feel. A properly bated pelt has a smoother, silkier, more relaxed grain under the thumb, and the finished leather drapes and breaks more finely instead of standing stiff and coarse. That difference is not decoration. It is one of the first two or three things any buyer tests, along with tan-through and thickness, and it is very hard to add back later.

How do you judge it? By the pelt, not by a clock. There is no bating time in this course's reference material and none is invented here. Enzyme products differ enormously in strength, temperature range and dose, and the only honest instruction is to take your dose, your temperature and your time from the technical sheet of the product you actually buy, and then learn your own end point by feel.

Learning the feel is a real, teachable skill and it is free.

• Before bating, run your thumb across the grain of the delimed pelt and press it. Note that it feels a little tight and slightly harsh.
• After bating, do the same thing on the same place. The grain should feel smoother and softer, and the pelt overall a little more relaxed and flatter.
• The classic workshop test is the thumb-print test. Press your thumb firmly into the grain of the bated pelt and lift it. On a well-bated pelt the print stays visible for a moment and then relaxes out. On an under-bated pelt the surface is too tight to take the mark. On an over-bated pelt the print sits there and the grain feels loose and lifeless.

Over-bating is the real risk, and you should understand what it is. Enzymes do not stop when the leftovers are gone. Left too long, or dosed too heavily, or run too warm, they begin to work on the structure you wanted to keep, and the pelt goes loose, flabby and weak in the grain. That is fibre damage. Like over-liming and like a curing failure, it is not recoverable. Every later step you carry out on that pelt is money spent on a piece of leather that will never grade well.

So bating is a step you stop early rather than late when you are unsure. An under-bated pelt makes slightly coarser leather. An over-bated pelt makes weak leather.

Match the bating to what you are selling, in the same way you learned to match liming to it.

• A soft product, a garment leather, a lining, a drapeable bag panel, wants a fuller bate for a finer, more relaxed grain.
• A firm structural product, a belt, a strap, a saddle part, wants less bating, because firmness and tightness are the properties you are selling and a heavily bated pelt gives them away.

That is the same commercial logic as liming: the same chemicals, run differently, produce different products from the same hides. It is one of the few levers a small workshop controls without buying anything new.

One practical note on the workshop. Bating is an acid-side operation, run after deliming, and everything in the previous lesson applies unchanged. Its float goes to the acid drain. Its vessel never shares duty with a sulphide vessel without a full drain, a double rinse and a second person's inspection.

Not every hide needs this lesson. Every sheepskin and pigskin does, and if you buy those and treat them like cattle hide you will lose money on defects that only appear at the very end, when all the cost has already been spent.

Start with the fact. Fat is not part of the collagen structure and is not what makes leather strong. Natural fat content varies enormously by species. Bovine hides carry relatively little natural fat, and how much varies with the animal's feed. Sheep and pig skins can carry natural fat as high as 30 per cent of the skin's own weight. That is nearly a third of the skin, and it is not spread on the surface where fleshing could take it off. It is held within the skin, between and around the fibre bundles, in the places every tanning chemical needs to travel through.

So the problem is not cosmetic. A fatty skin has a third of its internal pathways occupied by something that repels water. Tanning liquors are water-based. Water and fat do not mix. Wherever fat sits, tanning agent does not go.

What inadequate degreasing produces, in order of when you notice it:

• Uneven tanning, because the tanning agent penetrates freely in the lean areas and poorly in the fatty ones. On a sheepskin that usually means the belly and flank areas tan differently from the back.
• Patchy dyeing later, because dye follows the same pathways as the tanning agent, so an unevenly tanned skin almost never dyes evenly.
• Grease staining. This is the one that hurts commercially. Fat left in the skin migrates over weeks and months, works its way to the surface, and appears as dark greasy patches or a white fatty bloom on finished leather. By then the skin has been tanned, retanned, dyed, fatliquored, dried and finished, so every cost has been incurred, and the defect cannot be corrected.

Now how degreasing works. It is a wash, but not a plain one. Two things do the work together.

Warm water. Fat softens and becomes mobile with heat. The sourced technical example uses water at around 45 degrees Celsius for degreasing. That is warm, not hot. Understand why the temperature is limited: collagen is damaged by wet heat, and every tannage has a shrinkage temperature above which the fibre structure starts to shrink and degrade when wet and hot. An untanned pelt has no such protection at all, so a degreasing bath run hot to speed things up will damage the skin you are trying to sell. Warm enough to move the fat, never hot.

Emulsifying agents. An emulsifier is a chemical that lets fat and water hold together instead of separating, so the softened fat can be carried out of the skin in the wash water rather than simply moving around inside it. Without an emulsifier, warm water alone mostly redistributes fat.

Very fatty skins are commonly degreased in more than one stage, because a single bath removes what it can and then becomes loaded with fat and stops working. Two moderate baths beat one long one.

No degreasing dose or bath time is given here. The course reference carries the temperature example and the principle and nothing more, and no emulsifier dose is invented in its place. Take the dose and the number of stages from the technical sheet of the emulsifier you buy, and judge the result on the skin.

How to judge it. Take a degreased skin, squeeze a thick fatty area firmly between finger and thumb, and look for any greasy sheen or slipperiness left on your fingers. Hold the skin up to the light: a still-greasy area often looks slightly translucent and darker against the light than the lean areas around it. If in doubt on a valuable batch, run one more short warm bath rather than tanning and hoping.

One last commercial point. Because sheep and goat skins are such a large part of African raw material, this step matters more here than a general industry text might suggest. Africa holds about 27.7 per cent of the world's goats, the single largest African share of any livestock category, and goatskins are about 16.8 per cent of global goatskin output at 44.9 million pieces a year. Sheepskins run at 50.3 million pieces. A workshop that learns to handle fatty skins properly is working with the raw material Africa actually has most of.

Pickling is the last step of the beamhouse. It takes a delimed, bated pelt from its mildly alkaline state down to the acid condition a tanning agent requires. It is also the step where the largest single quantity of acid in the whole process arrives in the workshop, and everything in the next lesson follows from that.

Start with why acid at all. Chrome tanning in particular will not work properly on a pelt that is not acid. The chromium has to get into the collagen structure before it starts binding, and at higher pH it binds too fast at the surface, so the outside of the pelt is tanned and blocked while the centre is still raw. Bringing the pelt down to a low pH first lets the tanning agent travel all the way through before the pH is raised again and binding is allowed to happen. Vegetable tanning also requires the pelt to be brought to an acid condition, though the pickling regime differs from that used ahead of chrome.

So pickling is a penetration step disguised as an acid step. That is the sentence to remember.

The sourced technical example gives these parameters. They come from global industry technical literature and are not measurements from an African small workshop, so use them as the shape of a correct pickle and confirm against your own supplier's guidance.

• Float: 40 to 60 per cent on pelt weight.
• Neutral salt: at least 5 per cent on pelt weight.
• Acid dose: 1.0 to 1.8 per cent on pelt weight, using sulphuric and/or formic acid.
• Final pickle liquor: pH 2.8 to 3.0.
• Duration: 1 to 2 hours.

Now the single most important operating rule in this lesson, and it is the one that gets forgotten.

The salt goes in first, and the pelt runs in the salt float before any acid is added.

Why? Because acid on its own at pH 2.8 to 3.0 causes acid swelling of the collagen fibre. Acid swelling is destructive. The fibre structure swells violently and unevenly, the grain is damaged, and the pelt comes out with a drawn, coarse, weakened grain that no later step repairs. Neutral salt in the float suppresses that swelling. That is the entire reason at least 5 per cent salt is specified. Salt is not a filler and it is not there to save acid. It is there to stop the acid destroying the pelt it is supposed to prepare.

So the order is: make up the float, add and run the salt, check the salt is dissolved and distributed, and only then add the acid, slowly and diluted, never in one pour.

Add acid diluted and add it in stages. Concentrated acid dropped into a drum lands on whatever pelt is nearest and burns it. Two or three additions with running time between them gives an even pickle and a safer workshop.

How do you know the pickle is right? Two checks.

First, the liquor. Measure the pH of the float with paper or a meter and confirm it has come to the target range, and confirm it holds there rather than drifting back up, which tells you the pelt has stopped consuming acid.

Second, the pelt itself, which matters more. Cut a corner from a thick part and check the cut face with an indicator, in exactly the way you learned for deliming. What you want is a pelt acid all the way through, not a pelt with an acid skin and an alkaline core. A pelt pickled only on its faces will tan only on its faces.

One genuinely useful commercial fact. Because the salt in the pickle liquor preserves the pelt as well as protecting it, pickled pelts can be held for a period and even sold on as an intermediate product. This is recognised trade practice, particularly for sheepskins. That matters to a small workshop with limited tanning capacity: you can process the beamhouse on a batch, hold it pickled, and tan it when you have the drum, the chemicals or the order. It also gives you a saleable product at a stage earlier than tanning if cash is tight, though remember the value ladder: every step you sell out of is value someone downstream captures instead of you.

And the warning that belongs beside that fact. A pickled pelt is a pelt full of acid. It must be stored well away from anything sulphide-bearing, in its own labelled place, and never near the liming chemicals.

The previous module gave you the rule and the layout. This lesson turns it into how the acid side of the beamhouse is actually run on an ordinary working day, and then hands the pelt over to the tanner.

First, remember exactly why this matters here more than anywhere else. Liming and unhairing put sulphide liquor in your workshop. Deliming, bating and pickling put acid in it. Sulphide liquor is stable and does its job safely only while it stays alkaline. Bring acid to it and it releases hydrogen sulphide gas immediately, a gas that kills quickly and that deadens your sense of smell before it kills, so the warning fades exactly when the danger is greatest. Every rule below is that one fact turned into a habit.

The acid-side daily routine. Write it out and pin it above the vessels.

1. Open every door, hatch and vent before anything else. Ventilation is a control, not a comfort.
2. Confirm in writing that every vessel today's acid work will touch has not held sulphide liquor since it was last drained, rinsed twice and inspected by a second person. If nobody can confirm it, it has not been done.
3. Check that every drain the acid floats will run into is the acid drain, unblocked and clear. A blocked drain redirects liquid to the nearest low point, which is exactly how streams that were separated on paper meet on the floor.
4. Work one side at a time. Do not run an alkaline operation and an acid operation at the same low point in the floor at the same moment.
5. Add acid diluted, in stages, downward into water. Never add water into concentrated acid, which spits and can throw hot acid back at the person pouring.
6. Drain every float to its named drain. Nobody decides where a float goes while carrying it.
7. At the end of the day nothing stands in a shared channel and nothing acid is left unlabelled.
8. Nobody works alone on either side.

Storage. Acids in one locked, labelled, ventilated place, off the ground and out of the rain. Lime and sodium sulphide in a different one, equally locked and labelled. Not different shelves in the same cupboard, different places, so a leaking container on one side cannot physically reach the other.

Protective equipment for acid work is gloves, boots, an apron and eye protection, because acid at pickling strength burns skin and blinds eyes. Have clean water for immediate flushing at the vessel, not in another room.

And the absolute prohibitions, which have no exceptions.

Never pour spent pickle or tan liquor into a pit, drum or drain that still holds sulphide liquor, to save water, or because it is nearly empty, or because it is the end of the day. That is the exact shortcut that has killed tannery workers.

Never enter a pit. A pit is a confined space and hydrogen sulphide collects in it. There is no task inside a pit worth a life, and a rescuer who climbs in after a collapsed worker becomes the second casualty.

If the rotten-egg smell suddenly strengthens, or suddenly fades while anyone nearby feels unwell, dizzy or short of breath, get everyone out into fresh air immediately. A fading smell with people feeling ill is the danger arriving, not the danger passing.

The things this course must send you elsewhere for, because inventing them would be worse than admitting the gap. Your occupational exposure limit for hydrogen sulphide is set by your own national occupational health and safety authority. No number is given here as if it were yours. A first-aid and emergency-response protocol for hydrogen sulphide exposure written for a small workshop could not be sourced when this course's reference material was compiled, so get it from that authority or a competent occupational physician before you open. Your effluent discharge consent comes from your national environmental regulatory authority, the body that issues industrial discharge and environmental permits. In Kenya that body is NEMA and it requires a valid effluent discharge licence; every country has its own equivalent and you must find yours by name.

Finally, the handover. Before a pelt goes into the tanning drum or pit, it should pass these checks.

• Fleshed clean, with no fat film on the flesh side and no knife holes.
• Delimed to the centre, confirmed on a cut face.
• Bated to the grain feel your product needs, and not beyond it.
• Degreased, if it is a sheepskin or pigskin.
• Pickled to the target pH, confirmed in the liquor and confirmed through the thickness.

A pelt that passes those five is ready. The next two modules are about what you do with it.