🧰 The Beamhouse, Part One: Soaking, Liming and Unhairing

The beamhouse is every step that takes a cured raw hide and turns it into a pelt: clean, hairless, swollen, chemically prepared, and ready for the tanning drum or the tanning pit. Nothing in the beamhouse tans anything. Every step in it is preparation. And almost every defect that a buyer later rejects a piece of leather for was either brought in from curing or created right here.

The order of work is fixed, and the order is not arbitrary. Each step is only possible because the one before it happened properly.

1. Soaking. Water and time bring the cured hide back to something close to its original water content, and wash out salt, dirt, dung and blood.
2. Liming. A strongly alkaline bath, slaked lime held in saturated solution, normally with sodium sulphide in it, loosens the hair and swells the fibre structure open.
3. Unhairing. The sulphide chemistry created in liming dissolves the hair's keratin so the hair and the outer epidermis can be removed.
4. Fleshing. The fat and loose connective tissue on the flesh side is scraped off, over a beam or by machine.
5. Deliming and bating. The alkali is brought back down and enzymes refine the fibre.
6. Degreasing, for fatty skins, and then pickling, which brings the pelt to the acid condition the tanning agent needs.

This module covers steps 1 to 3. The next module covers 4 to 6. But you need to know the whole list from the start, because the single most dangerous fact in the trade is that steps 2 and 3 are alkaline sulphide work and steps 5 and 6 are acid work, and they happen in the same workshop, often on the same day, sometimes in the same drum. Lesson 5 of this module is entirely about what happens when those two meet.

Why can a beamhouse mistake not be fixed later? Go back to what leather actually is. The layer that becomes leather is the corium, and it is a three-dimensional mesh of interwoven collagen fibre bundles. Tanning works by getting a chemical physically in between and around those fibres and then locking them together. Liming exists to prise that mesh open so the tanning agent can travel through it evenly. If a hide goes into liming only half rehydrated, it limes unevenly. If it limes unevenly, it tans unevenly. And an unevenly tanned hide comes out with a raw centre, tanned on both faces and untanned in the middle, which will rot from the inside while it looks finished on the outside. No dye, no fatliquor, no finish, no amount of skill later on repairs it. You cut the corner off a tanned hide and look at the colour through the thickness, and either the tan went through or it did not.

Now the honesty you are owed about the figures in this module. There is very little retrievable, measured, African small-workshop data on beamhouse chemistry. The process figures used here come from global industry technical literature and from UNIDO cleaner-production guidance written mainly for conventional, often larger, tanneries. Two sources that would have helped most, a university beamhouse process reference and a peer-reviewed paper on sulphide-lime ratios for tropical goat skins specifically, could not be retrieved when this course's reference material was compiled. So you will see industrial figures in this module clearly labelled as industrial. Use them for scale and for direction, never as a target your workshop must hit.

Here is what industrial scale looks like, so you can see the gap. A conventional tannery in a cleaner-technology reference used about 45 cubic metres of water for every tonne of hide processed, while below 30 cubic metres a tonne was the mark of a well-managed operation. Across a whole conventional operation, roughly 300 kg of chemicals go in per tonne of hides. And of every tonne of raw hide that goes in, roughly 450 to 600 kg leaves as solid waste and by-product rather than as leather.

Read that last figure twice. Most of the weight of a hide is not leather and never was. Your job in the beamhouse is to remove the parts that are not leather, in the right order, without damaging the part that is.

A cured hide is a hide with its water taken away on purpose. Salt curing pulls out roughly 20 per cent of the hide's water and puts 13 to 17 per cent of the hide's own weight back in as salt. Air drying takes the water out directly. Either way, the hide that arrives at your workshop is not in a state any chemical can work on. Soaking puts the water back and takes the salt, dirt, dung and blood out.

That is the whole job, and it sounds simple. It is the step most often rushed, because it is the step where nothing appears to be happening.

How long? The sourced technical guidance gives a range that depends entirely on how the hide was cured. A wet-salted hide needs a few hours. An air-dried hide can need up to several days. That is not a vague answer, it is the correct answer: a dried hide has lost far more of its water than a salted one, and it has to take proportionally longer to get it back. If somebody tells you one soaking time for all hides, they are not looking at the hides.

So the first thing you do at intake is separate your hides by cure type. Wet-salted in one batch. Air-dried in another. Never soak them together on one clock, because you will either pull the dried ones out hard in the middle or leave the salted ones in until they start to go off. Remember that a hide in a soak pit is a piece of wet meat again the moment it rehydrates, and bacterial putrefaction starts again with it. Soaking is a race between rehydration and rot, and the longer the soak, the more that race matters.

Soak in plain water. A mild wetting agent helps water penetrate a stubborn dried hide, and a disinfectant is sometimes used on a long soak to hold back bacteria. Change the water at least once on a long soak, because the first water becomes a strong brine loaded with the salt and dirt coming out of the hide, and brine slows down further rehydration. This is the one step in the whole beamhouse that is not itself chemically hazardous. Use that fact: it is the right place to train a new worker.

How do you know when a hide is soaked? Nobody can give you a number for this, so learn the tests.

• Feel. A fully soaked hide is uniformly soft and flexible everywhere, including the thick butt and the neck. Any place that still feels stiff, board-like or papery is a place the water has not reached.
• Fold. Fold a thick part back on itself. A properly soaked hide folds without cracking or creaking.
• Thickness. The hide should have come back close to its green thickness and weight. Weigh a sample hide in and out if you can. Your own weight-gain record, kept over a few months, is worth more than any figure printed in a book, because it is measured on your hides, your water and your climate.
• Look at the flesh side. Salt caking, dung and dried blood should be gone. If they are not, the hide is not finished with the soak pit no matter what the clock says.

Why does this matter so much? Because everything that follows is a chemical trying to travel through the thickness of a collagen mesh. If part of the hide is still dry, the lime and sulphide do not reach it in the same concentration or at the same time as the rest. That part will be under-limed. Under-limed means the hair does not release cleanly, the fibre does not open, and the tanning agent later will not penetrate. You will see it as patchy hair remnants, as a stiff hard area in the finished leather, or in the worst case as a raw centre.

One more thing to plan for. Soak water is dirty water. It carries salt, dung, blood and dissolved protein, and it is a real part of your effluent load. An industrial average puts total tannery wastewater at about 35 cubic metres per tonne of raw hide processed, and soaking is a meaningful part of that. That figure is industrial and is here for scale only, but the lesson is small-scale and immediate: decide where your soak water goes before you fill the first pit, not after.

Liming is the step that decides what kind of leather you are going to end up with, and most people who learn it as a recipe never realise that.

A soaked hide goes into a strongly alkaline bath. The bath is slaked lime, which is calcium hydroxide, held in saturated solution, and in standard practice sodium sulphide is in there with it. The lime does the alkaline work. The sulphide does the hair work, which is Lesson 4. From this moment the workshop has a sulphide liquor in it, and everything in Lesson 5 applies from now on.

Liming does two jobs at once, and you should be able to say both of them without looking.

Job one: it attacks and loosens the hair and the epidermis. The epidermis is the thin outer layer that carries the hair. It contains no collagen worth having and it is deliberately destroyed and removed. It is not the layer that becomes leather.

Job two, and this is the one people forget: it swells and opens up the collagen fibre weave. The corium, the middle layer that actually becomes leather, is a dense three-dimensional mesh of interwoven fibre bundles. In a raw hide those bundles sit tight against one another. Alkaline swelling physically prises them apart. That space is what every later chemical travels through. Tanning agent, retanning agent, dye and fatliquor all have to get in between those fibres. If liming did not open the mesh, they would only ever coat the surface.

So liming is not a hair-removal step that happens to swell the hide. It is a fibre-opening step that happens to loosen the hair. Both jobs are the point.

How long? The sourced guidance says liming is typically a one-day operation, and says plainly that the duration varies. That is honest and you should teach it the same way. There is no single correct liming time, and here is why.

The sourced relationship is directional, not numeric: the more alkaline the liming, or the longer the liming time, the softer and looser the resulting leather. Read that carefully, because it is a lever you control with no extra equipment and no extra chemical purchase.

• A workshop making soft glove leather, a drapeable bag panel or a garment leather wants a looser, more open fibre structure, and will lime longer or more strongly.
• A workshop making a belt, a strap, a saddle part or a sole wants firmness and tightness, and will lime less hard.

Same chemicals. Different product. That is a real commercial decision made in a pit of white liquid, and it explains why two tanneries buying the same hides sell very different leathers.

What this course will not give you is the sulphide-to-lime ratio. The one source that would have answered this properly for tropical conditions, a peer-reviewed paper on sulphide-lime unhairing ratios for tropical goat skins, could not be retrieved when this course's reference material was compiled. It is named in the reference so you can go and find it yourself. No ratio is supplied here in its place, because a made-up sulphide dose is exactly the kind of number that gets someone hurt. Get your dosing from your chemical supplier's own technical sheet for the product you actually buy, from a working tanner near you who will show you, or from that paper.

Three practical points that come out of the chemistry rather than from a recipe.

First, hides that were badly cured lime badly. Bacterial action that already began breaking down collagen leaves hair that will not grip properly for chemical unhairing, and leaves loose, weak grain. Liming does not repair a curing failure, it exposes it.

Second, fatty skins behave differently. Sheep and pig skins can carry natural fat as high as 30 per cent of the skin's weight, far more than a typical bovine hide, and that fat physically blocks chemicals from reaching the collagen. Those skins need the degreasing step covered in the next module.

Third, splitting a limed pelt horizontally to a more even working thickness is commonly done at this stage for thicker hides. That decision is worth money: only the piece that keeps 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 accordingly.

Unhairing is the moment a hide stops looking like an animal and starts looking like a material. It is also the step where the whole hazard of this trade sits, and the two facts are the same fact.

Hair is made of keratin. The leather you want is made of collagen. Both are proteins. A method that removed hair by brute force, by scraping or by heat, would damage the collagen at the same time. Sulphide unhairing works because it is selective: in the alkaline sulphide and hydrosulphide environment created during liming, the sulphur chemistry attacks and dissolves the keratin of the hair, while the alkaline lime environment protects the collagen the leathermaker wants to keep.

That is the entire trick, and it is a good one. Destroy one protein, protect the other, in the same bath, at the same time.

Now hold on to the second half of it, because you will need it in the next lesson. The chemistry that dissolves keratin harmlessly is reactive sulphur chemistry, and it is only stable and only harmless while the liquor stays alkaline. It is not tame. It is held in a condition where it behaves. The moment that condition is broken by acid, the same liquor releases hydrogen sulphide gas and becomes a fast-acting poison. The reason sulphide unhairing works is exactly the reason sulphide liquor is dangerous. They are not two separate things to learn.

What unhairing physically removes is the hair and the epidermis, the thin outer layer that carried it. The epidermis contains no collagen worth having and is deliberately destroyed. What is left is the corium, the layer that becomes leather, with its grain surface newly exposed.

What to look for on a pelt coming out of the liming and unhairing process.

• Hair release. Rub a thumb across the grain in several places, including the neck, the butt and the belly. Hair should come away cleanly and easily everywhere. Hair that still grips in patches means those areas were under-limed, which usually traces back to uneven soaking or to a badly cured hide.
• Colour and feel of the grain. The exposed grain should be clean and even. Remnant hair roots left in the follicles show as dark specks and will show in the finished leather.
• Swelling. The pelt should be noticeably plumped and rubbery from alkaline swelling. That swelling is the open fibre structure you paid for.
• Over-liming. A pelt that has gone too far comes out slippery, loose and flabby, with a grain that feels weak under the thumb. That is fibre damage. Like a curing failure, it is not recoverable.

Remember why hair grip failure often is not a liming problem at all. A hide that partly putrefied before curing, or that was cured with too little salt or dried while folded, has already had bacterial action begin on its collagen. That hide will not unhair cleanly however good your liming is. Liming exposes bad curing rather than fixing it, which is one more reason to refuse a bad hide at intake instead of hoping the beamhouse will save it.

Hair itself is a waste stream and it must be planned for. Hair and fleshings putrefy and smell within a day or two in a warm climate. Industrial figures put trimmings alone at roughly 32 to 120 kg per tonne of raw hide depending on the processing stage. That is an industrial figure and your workshop is not industrial, but the stream exists at your scale in proportion. Decide before you start where hair goes, how often it is cleared, and how it is kept off the floor and out of the drains where it will block them.

One last piece of honesty. There is no African-specific count of hydrogen sulphide injuries or deaths in the tanning trade. The figure used in the next lesson comes from United States national workplace-injury data across all industries. The absence of an African statistic is not evidence that the chemistry behaves differently here. It means nobody has published the count. Treat the mechanism, not the statistic, as the thing that must be respected.

This is the lesson that has to be learned before you are allowed near a pit, a drum or a drain. Read it as procedure, not as a warning.

The mechanism first. Sodium sulphide, and the sodium hydrosulphide it partly turns into in the liming and unhairing liquor, is chemically stable and does its work safely only while the liquor stays alkaline. Expose that liquor to an acid and it reacts and releases hydrogen sulphide gas, written H2S. Not slowly. Immediately.

Now look at where the acid is in your own workshop. Deliming, bating and pickling are all acid processes. They come a few steps after liming and unhairing, on the same hides, often in the same week, sometimes in the same drum after a rinse. Weak acid also sits in a poorly rinsed drum, in shared drain water, in acidic wash water. The two liquors are not in different buildings. They are within a few metres of each other by the ordinary design of the trade. That proximity is the hazard.

Why this gas in particular. Hydrogen sulphide is dangerous in a way that defeats a worker's own instincts.

At low concentrations it has a strong foul rotten-egg smell that most people can detect easily. That sounds like a safety feature. It is not. Exposure, at low concentrations over time or far more quickly at high concentrations, causes olfactory fatigue: the sense of smell stops registering the gas as the concentration rises or the exposure continues. The warning disappears exactly when the danger is greatest. A worker can lose the ability to smell the gas that is about to kill them.

Say that back to yourself in the form you will need it in: if the rotten-egg smell suddenly fades while people nearby feel unwell, dizzy or short of breath, that is not the danger passing. That is very often the danger arriving.

The toxicity is dose-dependent and it is severe. The sourced technical literature gives these figures:

• At about 10 ppm, irritation and headache.
• At about 100 ppm, capable of causing unconsciousness and possible death.
• At about 300 ppm and above, rapid unconsciousness and death.

Understand the shape of that scale. It is not a gentle slope where you get steadily more uncomfortable and have time to decide. The step from irritation to unconsciousness is a factor of ten, and the gas that takes your sense of smell away is the same gas that closes that gap. United States national workplace-injury data recorded 46 worker deaths from hydrogen sulphide exposure between 2011 and 2017 across all industries where it occurs. That figure is not tannery-specific and it is not African. It is here to settle one question only: this gas kills ordinary working adults in ordinary workplaces. It is not a laboratory curiosity.

Now the rule. It has no exceptions and it takes no qualification.

Sulphide-bearing liquor, meaning the liming and unhairing float, and acid-bearing liquor, meaning deliming, bating, pickling and tanning float, must never be mixed, never share a drain without dedicated separate treatment, and never be stored, moved or disposed of through the same channel.

In practice, that becomes four working habits.

1. 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. This is the exact shortcut that has killed tannery workers.
2. Liming and unhairing float must be fully drained and the vessel rinsed before any acid process begins in that same vessel. If you cannot be certain a drum is clean, it is not clean.
3. Never enter a pit. A pit is a confined space with heavier-than-air gas that 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. This has happened repeatedly in confined-space incidents in many industries.
4. Ventilate. Liming and unhairing pits release some H2S even under correct alkaline operation, especially when agitated or when a cover is lifted after standing. Work in the open air or with genuine cross-ventilation. Never work in an enclosed pit room with the door shut.

One thing this course cannot give you. A specific, sourced first-aid and emergency-response protocol for hydrogen sulphide exposure in a small workshop setting could not be retrieved when this course's reference material was compiled. No substitute is invented here. Obtain it from your national occupational health and safety authority or a competent occupational physician before you open a liming and unhairing operation. That is not optional preparation. It is part of opening the workshop, like the roof.

The previous lesson gave you the rule. This one turns it into a workshop that follows the rule on a bad day, when it is raining, when you are short-handed, and when the person doing the draining is new. A rule that depends on everybody remembering it every time is not a control. A layout that makes the wrong action physically difficult is a control.

Start with the layout principle, which follows directly from the chemistry.

Alkaline sulphide processes, meaning soaking, liming and unhairing, and acid processes, meaning deliming, bating, pickling and chrome tanning, should be physically separated wherever the size of your workshop allows. Separate pits or drums. Separate drains. Separate chemical storage. The reason is not tidiness. It is that a spill, an overflow or a moment of inattention must not be able to bring the two liquors into contact.

A cramped workshop where the same drum, the same channel, or the same low point in the floor serves both processes has a permanent, built-in version of the hazard, independent of how careful any individual worker is on any individual day. That is the honest test of a layout: not whether careful people can work in it safely, but whether a careless moment in it is survivable.

What separation looks like in a small workshop with limited money.

• Two drains, not one, even if the second is a shallow lined channel cut in the floor and led to a separate settling tub. Separating streams at source is far cheaper and easier than separating them after they mix.
• A raised kerb, or even a mud and cement lip, around the alkaline side so an overflow runs to its own drain and not across the floor.
• One vessel, one duty, wherever you can afford it. If a vessel must change duty, it is drained, rinsed and inspected first, and one named person signs that off.

• Chemicals stored apart. Lime and sodium sulphide in one place, acids in another, both locked, both labelled, both off the ground and out of the rain.
• Doors and openings that give real cross-ventilation, and a working rule that nobody works alone on the alkaline side.

Now the daily routine. Write it out and pin it up.

1. Before starting, open everything that ventilates. Then check that every vessel is holding what the label says it is holding.
2. Handle the alkaline side first or the acid side first, deliberately, and never both at the same low point in the floor at the same time.
3. Drain every float to its own drain, named in advance. Nobody decides where a float goes while carrying it.
4. Any vessel changing duty is drained, rinsed twice, and inspected by a second person.
5. Clear hair, fleshings and trimmings off the floor and out of the drains the same day. They putrefy fast in warm weather and they block drains, and a blocked drain is how liquors find each other.
6. At the end of the day, nothing is left standing in a shared channel.

On effluent, keep the two streams separate right through to disposal. This is not only a workshop rule, it is an effluent rule: a mixed effluent stream is more hazardous, harder to treat, and usually not compliant with whatever consent applies to you. Simple settling and screening is worth doing even before full compliance is achievable. A cleaner-technology reference gives primary treatment alone removing 25 to 50 per cent of biological oxygen demand, 50 to 70 per cent of suspended solids and about 65 per cent of oil and grease. Those are industrial figures, and they show that basic settling is genuinely useful and equally that it is not a substitute for proper treatment to whatever standard applies to you.

Finally, 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 authority, normally the national occupational health and safety body. No number is given here as if it were yours.

Your first-aid and emergency-response protocol for hydrogen sulphide exposure could not be sourced for a small workshop setting. Get it from that same authority or from 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. Other countries have their own equivalent, an environmental protection authority or a ministry of environment. Find yours by name and ask them directly.

And there is no sourced, costed equipment list or layout specification for a small African tannery. Every workshop figure in this module is scaled down from industrial sources and labelled as such. Your layout has to be designed from the separation principle and your own site, not copied from a plan in a book that does not exist.