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GUIDELINES & PROCEDURE OF HANDLING AND UTILIZATION OF ANIMAL’S SKIN & HIDES IN INDIA

December 10, 2022

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GUIDELINES & PROCEDURE OF HANDLING AND UTILIZATION OF ANIMAL’S SKIN & HIDES IN INDIA

The production of all leather involves several steps, from drying and curing to tanning and dyeing. We’ll look at the tanning process and discover just why the kind of tanning done on the hide impacts the final result so much.

Why Does Leather Need To Be Tanned

Tanning is necessary for leather because if the raw hide is not tanned, it will rot and putrefy. The tanning process stabilizes the protein of the raw hide or skin, preventing putrefaction, and preparing it for a wide variety of end applications.

The main difference between tanned and untanned dry hides is that untanned hides will putrefy after getting wet, while tanned leather will not. Tanning also changes the appearance of the hide. The change depends on how long it takes and the tanning agents used.

Tanning of hides for leather has been done for thousands of years by ancient civilizations like those in Sumer and India.

Preparation For Tanning

Animal hides go through several stages before they are ready for use in making products like belts, wallets, shoes, furniture and clothing. The stages before tanning may include fleshing, preservation or curing, soaking, liming, unhairing, splitting, deliming, bating, degreasing, bleaching, pickling, and depickling.

Once the hides arrive from the slaughterhouse or farm, the next step is to remove any scraps of flesh from them.

The next step is to cure the hides with salt. Curing prevents putrefaction by preventing bacterial growth on the hide between procurement and processing. It greatly reduces the moisture content of the hide. One way to cure the skins is by wet-salting, in which the hide is heavily salted and then pressed into packs for up to 30 days. Another way is to soak them in a saltwater brine for about 16 hours.

The steps after curing are called Beamhouse operations.

The first step is to remove the salts left over from the curing in a process called soaking. This process involves soaking the skins in water which has certain chemicals added to prevent bacterial and fungal growth.

After soaking, the hides and skins go for liming. This means soaking the hides in a drum or pit filled with milk of lime, an alkaline solution. Liming results in the removal of natural grease and fats as well as keratin and hair. It also causes the swelling and splitting of fibers to the desired extent and prepares the collagen in the hide to a condition that is ideal for tanning.

After liming, the pelt goes through a machine to remove fleshy tissue from the flesh side. This process, fleshing, involves removing connective tissue and meat residue from the flesh side of the skin. It uses rotating scraping rolls.

Unhairing follows fleshing, which sounds exactly like what it is; the removal of hair from the hides. After applying unhairing agents like sodium hydroxide and calcium hydrosulfide, most of the hair is then removed, first with a machine and then by hand using a dull knife.

Next comes deliming, the step in which the alkalinity of the hides is reduced by adding acids to the hides in a drum or pit. The swollen fibers of the hides shrink once more in preparation for the bating process. The deliming process can take about 2 hours for cattle hides.

The hides can then be bated, depending on the intended use of the leather. Bating involves the addition of enzymes to the hides to soften them.

After bating is pickling, which involves treating the hides by salt and then acid. The salt prevents the adverse effects of a sharp increase in acidity from the acids used in pickling. This process is important because it prepares the collagen for maximum penetration of the tanning agents in the tanning stage.

Degreasing comes next. This process uses solvents or water-based systems to remove excess grease before tanning.

Tanning Process

After the preparation stage comes the tanning. There are many ways to tan hides, but the end result is that the hides will not putrefy and are ready for dyeing and other uses such as manufacturing goods.

Chrome tanning is a relatively recent method of tanning, but easily the most dominant. It involves soaking the hides in baths containing acidic salts until they are ready for the next step, chrome tanning. The bath includes chromium tanning agents and the acidity of the bath is decreased until the surface of the hides starts absorbing the chromium agents. The wet hides produced from this process are blue, and known as “wet blue hides.”

Chrome tanning is faster than vegetable tanning and can take up to a day with modern machinery. After the initial tanning process, the leather is cut into various thicknesses as required by the leather craftsmen. It is then further processed for making leather goods. Being a highly automated process, this kind of leather is popular and costs less than more labor-intensive tanning methods.

Another method, vegetable tanning, is in use since ancient times and involves the tannins from the bark and leaves of trees and plants. After the preparation phase, the hides are placed in the tanning pit, where there are drums containing tanning solutions of various strengths. Over a period of two to three months, the tanners move the hides from drum to drum, with the solution getting progressively stronger. The tanning agents penetrate the skins and the fermentation and depositions process leaves the hides with a distinct color and appearance.

The fibers of the animal hide used for the leather are visible in the finished product, lending an authenticity and personality to the leather that makes it more personal. It also has a distinctive sweet, woody fragrance that is normally associated with leather.

Brain tanning is a labor-intensive and specialized way of tanning hides. It is usually practiced by hunters to preserve the hides of fresh kills. Animals such as deer, elk and moose have enough acids of the right type to tan their own hides. The process varies from person to person, but the method involves first cleaning the hide thoroughly by scraping off the flesh, fat and membrane covering the inside of the hide. After this, the tanner washes the hide thoroughly, wrung and stretched on a frame. A tanning solution, made by mixing warm water and the mashed-up animal brain, is applied by rubbing it onto the hide. Once the hide is ready, it is smoked to complete the process.

Tanning skins and hides with animal fats and oils is a very old method that involves fat-rich animal substances such as fish oil, sebum or marrow. Other substances used include soap, claw oil, yak butter or egg yolk.

Synthetic tanning is frequently employed as an alternative to chrome tanning. It is not employed in isolation, but in combination with either vegetable tanning or chrome tanning. It uses artificial tanning agents such as formaldehyde, glutaraldehyde, phenols and acrylates. After tanning, the wet and undyed leather has a bright color which gives it the name wet white. Tanning with aldehydes and oils produce very soft leathers like those used in car seats. This system can be used to produce dry-cleanable and washable fashion leathers as well as chamois leather.

How Tanned Leather is Sold

After the tanning process, the finished leather is sold to traders. The leather industry uses terms and measurements for the various ways that leather is sold. Some of these are a side, hide, skin, split or belly.

As the size of each side, hide, skin, split or belly varies, the size of the product varies as well. In some cases measurements are in square feet or square meters, while thickness is measured in millimeters and also in ounces.

Because the leather is split into various thicknesses having defined weights, conversion from width to weight is straightforward. Scrap leather is usually sold by weight.

Environmental Concerns

The tanning of hides has traditionally been done using methods that involve organic materials like tree bark and leaves. This kind of vegetable tanning had a smaller impact on the environment since it requires only water which is then discharged into the river or lakes. Because the effluent is organic, it eventually breaks down in the water and does not pollute.

Similarly, brain tanning uses the acids and fats in the animal’s brain to tan the leather, with the only impact being water use. Again, the biodegradable material is not a threat to the environment.

On the other hand, chrome tanning uses several chemicals which can be harmful to health and often end up polluting the air, soil and water. Effluent treatment is mandatory in many countries, but controls are often ax in developing countries, leading to high levels of pollution around tanneries. Tanneries are usually associated with a very strong smell due to the processes emitting a distinct stench. Historically, this meant tanneries were located some distance away from settlements and towns. Workers at tanneries are at higher risk of cancers and diseases linked to the chemicals they are exposed to.

Classification and Grading of Hides and Skins

CLASSIFICATION AND GRADING OF HIDES AND SKINS

Classification of buffalo hides

· In India, buffalo hides are generally classified as

· Buffalo (buff) calf skin

· Buff heifer hide

· Buff hide

· Buff bull hide

Classification of goat and sheep skin

· In India, this is done on the basis of weight and length (Baba and Bhote, 1966)

Class	Weight (kg)	Length (cm)	Region
Big goat skin	1.3	96.5 and above	Punjab, Kashmir-Punjab border, Orissa, Uttar Pradesh
Medium goat skin	0.91	96.4	Rajasthan, Maharashtra, Andhra Pradesh, Kerala, Karnataka
Small goat skin	0.86	76.2	Eastern part of India

· It may be noted that sheepskins are mostly of medium size without much regional variation.

Grading of hides and skins

· FAO Expert, Aten (1995) suggested a useful system of grading hides based on the degree of faults

S. No	Grades	Properties
1.	First grade	· Shape or pattern regular and symmetrical, Minor scores and gouges may be over looked
2.	Second grade	· Good shape or pattern · Reasonably free from knife damage i.e. upto 1/16th of area may show concentrated scores or gouges or 1/8th of area may show dispersed scores or gouges. · One or two cuts may be allowed on the edges of belly
3.	Third grade	· Irregular shape or pattern · Upto 1/2 of the area damaged by knife showing cuts, scores or gouges.
4.	Reject grade	· Irregular shape or pattern · Extensive damage of the back or butt.

Terminologies

TERMINOLOGIES

· The skin from a fully–grown large animal is called as hide

· Those from small stock (i.e., sheep, goats and pigs) is called a skin.

· The terms skin is also applied to calves.

· The slunk is the term applied to the skin of an unborn calf, which is often used for parchment, light suede or drumbeats.

· The skin from the older calf, which has not yet reached maturity, is called a kip.

· There are a number of terms in each country, which varies with terms of the countries.

· The nomenclature of hides and skins expressed in green weight is as follows:

Calf skins	15 lb.
Overweight kip/heifer skin (heifer)	25 to 30 lbs.
Hides (cow)	Over 30 lbs
Extremely light steers (steer)	30 to 48 lbs.
Light steers	48 to 58 lbs.
Light cow hide	Less than 53 lbs.
Heavy cow hide	Over 53 lbs
Heavy steers	Over 58 lbs.
Bull hide (bull)	60 – 100 lbs.

· The United States has its own terminology.

· The term native is used to denote hides, which have not been branded.

· Colorado is one, which has one or several brands.

· Texas is referred to the compact, narrow and plump steer hide.

· Packer hide is a term reserved for hides originating in large meatpacking plants.

· These are usually of the finest quality because of the excellent conditions under which they are flayed and preserved and also because of the high skill of the operators.

· Frigorifico is the term used for the ones originated in South American meatpacking plants, which are of even better material.

· These are prepared by a good method of brining prior to salting and hence have an improved quality.

· Freezer is the term applied to goods produced by a meat-freezing establishment, particularly in Australia or New Zealand.

· They are of uniform high quality because; they are prepared from centralized slaughter and supervision.

· Big packer hides – Hides obtained from highly skilled flayers.

· Small packer hides – Hides obtained from less skilled flayers.

· Big packer hides – Flayed by highly skilled workers.

· Small packer hides – Flayed by less skilled workers.

LEATHER INDUSTRY IN INDIA

In the past, India had been one of the largest exporters of hides and skins.
However, a few decades back, the policy of exporting only value added material was adopted and presently finished leather is exported to many countries.
Interestingly, there is a marked difference in sources of hides and skins in our country.

Source of hides

20-25% of hides are obtained from slaughtered animals.
75-80% of hides are obtained from fallen animals.

Source of skins

80% of skins are obtained from slaughtered animals.
75-80% of skins are obtained from fallen animals.
Skin and hides are sold for use as raw material for leather making.
The tannery is the ultimate destination.
Skins from healthy animals without the slightest blemish properly removed, preserved and delivered to the tannery without damage in transit is in great demand and fetches more money.
Tanning is the conversion of hide or skin in to a rot– and insect–resistant material, leather.
Leather must be durable, flexible and elastic.
The tanning process is a combination of mechanical and chemical actions.
The yield of hides and skins varies between species.
In cattle, the average yield of hide is 7 percent of the live weight, whereas in sheep and goat the average yield of skin is 11 percent of live weight.
As per FAO estimates (2001), India produced the following quantities of fresh hides and skins.

Hides and Skins	Weight (MT)
Cattle hides	4,66,000.00
Buffalo hides	5,10,000.00
Goat skins	1,28,880.00
Sheep skins	52,380.00
Sheep skin with wool	524.00

STRUCTURE AND PROPERTIES

Structure of hide

· Epidermis

· This is the outer layer of the hide, consisting of an outer-pigmented surface and tubular invaginations of hair follicles.

· Corium or dermis

· This layer has Elastin, reticulum and collagenous fibres.

· The inner portion of corium is interwoven with bundles of collagen. It is this part, which makes up the leather.

· Subcutis

· This is a loose membrane network and contains fatty deposits.

Properties and utilities of leather

· It has an excellent flexibility due to high tensile strength over a wide range of temperature and moisture.

· It has high tear and is puncture resistance due to in-built fibrous network.

· It has the ability to breath, which provides coolness in hot weather and insulation in cold weather.

· It has the moulding ability.

FLAYING

Flaying refers to the skinning operation performed by skilled workers following a uniform pattern.
Being a valuable byproduct, improper flayingwill lower its value.
In modern large animal abattoirs, skinning of buffalo and cattle is done on pritch plates.
These are steel plates of 20 cm diameter about 120 cm long and 60 cm wide, anchored to the floor with bolts.
The animal is lowered onto this plate for skinning and is held in position by a pritch bar – a metal rod.
On a smooth floor, skilled workers can very well accomplish this task.
If this facility is not available, it can be efficiently done on a skinning cradle made up of steel tubings or smooth round timber rods.

FLAYING OF BUFFALOES AND CATTLE

The hide is first opened from the neck or slaughter incision with the help of a flaying knife and continued straight along with the middle of dewlap and belly to the middle of the tail.
By making an encircling cut between knee and hot joint each leg is opened. The cut on the foreleg is continued to the breastbone and that of the hind leg upto scrotum or udder to meet the longitudinal cut as mentioned in the above step.
The hide is then separated from the carcass by cutting from the lower part of the breast towards the neck on one side and towards the navel on the other side. This is done by careful use of the knife.
Now with the help of tail grip and gambrel, which are connected to the hook of the hoisting gear, the hind legs are hung and hide is removed from the tail.
Carcass is further raised and hide is pulled off the back to the hump and then to the shoulder and neck separating the thick subcutaneous tissue with the help of a knife.
The hide is severed from the carcass by giving a cut behind the horns.
A properly flayed hide has
- rounded rumps
- equal width from centre line of the back to the belly edge on each side
- Medium length in the shanks
- regular dewlap and
- square outline

CASE-ON FLAYING OF SHEEP AND GOAT SKINS

This technique is quite popular in tropical and subtropical countries.
It has the advantage of giving skins with minimum cuts and excellent keeping quality.

Procedure

After bleeding the carcass, a small incision is made on the inner side of the hind leg above the hock joint.
A narrow steel rod or a smooth wooden rod is now inserted in this incision and pushed under the skin towards the grain for about 45 cm. (This rod tears or loosens some of the connective tissue between the skin and body, thus minimizing the use of knife which could cause cuts, scores or gouge marks).
After removing the rod, a hand cut is made around the incision hole and air is blown inside with the help of bicycle pump or car foot pump.
Due to inflow of air, carcass is uniformly inflated like a balloon.
Now, the carcass is lifted from the ground and hung up by the hind leg on the hook or tripod.
The skin is then removed from the carcass in a case form.

The following cuts are made in this technique:

Circular cuts around the knee and hock joint.
Cuts on the scrotum, udder, etc.
A circular cut on the neck to severe the head skin.
Now the fingers followed by fist are inserted into the incisions on the hind legs.
The skin is pulled downward and the knife is used only when it is absolutely necessary.
When it reaches the chest, the skilled worker holds the skin with the both the hands and rigorously pushes with his foot to remove it from the carcass.
The only disadvantage in this method is that asymmetrical skins are obtained many a time.
Careful flaying a small part of the belly by knife can prevent this defect.

DEFECTS OF HIDES AND SKINS

Defects due to disease and insects

The animals in tropics fall a prey to a number of parasitic, bacterial and viral conditions affecting the hide and skin.
The damage that results depends on the duration of the infection and its severity, and may range from slight marring of the grain to destruction of the corium itself.
Such damage is often attributable not so much to the disease itself, as to secondary infection, rubbing and scratching because most skin diseases are accompanied by severe itching.
Starvation during excessive drought and lack of green fodder, combined with heavy internal parasitic infestation, aggravate skin diseases.
Tick damage and damage due to strong concentration of tick killing drugs are some of the causes of defects of hides and skins due to disease and insects.
Follicular or demodectic mange
- One of the most commonly seen disease defects is made by a mite called Demodex folliculorum, a parasite burrowing deep into the hair follicle, where it establishes its nest; this disease is called follicular or demodectic mange.
- The damage is clearly visible on the flesh side of the hide or skin, in the form of raised whitish spots.
- These lesions are erroneously called pox marks by the trade.
- True pox is caused by a virus and leaves very slight marks only, located mostly on the udder and on the inner surface of the thighs.
- When a vesicle bursts, the lesion may occasionally become infected, because of the severe irritation and the subsequent scratching by the animal.
Sarcoptic and soroptic mange
- Sarcoptic and soroptic mange (commonly called scab) are widespread because of the warm climate and primitive management.
- As these parasites tunnel between the fibres of the corium of the skin, the result is rough pitted leather with damaged grain.
Streptothricosis
- Streptothricosis is another widespread disease caused by a microorganism of the genus
- It is known in tropical countries by various local names such as Krichi in Nigeria, Senkobo in Northern Rhodesia (Zimbabwe) and Uasin Gishu in Kenya.
- The lesions made by this disease vary from slight inflammation of the skin, resulting in leather damage on the grain side only, to large areas deeply affected; in severely damaged areas, scar tissue is formed and there may be general thickening of the hide, giving it the appearance of elephant or rhino hide and making it quite unsuitable for leather.
Nodular dermatitis
- Nodular dermatitis is a disease most probably caused by a virus and principally affects goats.
- It causes lesions similar to those made by
- Other minor skin lesions may be caused by a fungal infection of ringworm of the Tricophyton
- Ringworm itself does not produce heavy damage to the skin, as the pathological changes are restricted to round, bald patches.
- However, due to rubbing and secondary infections, deeper lesions may appear, affecting the resulting leather.
Lumpy skin disease
- Lumpy skin disease, which occurs widely in the continent of Africa south of Sahara, also contributes to damage on cattle hides.
- This appears in the form of lumps and nodules or round patches, deprived of grain, or as button-like defects reaching deep into the corium.
Photosensitization
- The ingestion of certain plants or drugs may sensitize the skin to sunlight; this is known as photosensitization.
- The areas usually affected are those where the actual skin is unpigmented; these are generally covered by white hair, and therefore known as white-haired animals.
- Particular breeds such as Ayrshire and Friesian, imported from the temperate zones, suffer the most.
Dermatitis
- Dermatitis is caused by strong concentration of tick-killing drugs or by their improper use, also contributes to the list of lesions to be found in hides and skins derived from territories where tick control is practiced.
- In addition to the diseases mentioned above, certain insects (as mentioned below) attack the living animal and damage the skin.
Warble Fly (Heel Fly or Grub)
- Warble flies cause very great losses in the temperate zones than in tropical and subtropical zones, since, these insects do not live in the true tropics.
- The fly belongs to the family Hypodermidae and atleast three, namely, Hypoderma bovis, H.lineatum and crossi, exist on the northern fringe of subtropical countries.
- The ova deposited by the fly on the hair of an animal hatch into very small larvae, which penetrate the skin and, after migrating through the body, settle on the back in the subcutaneous tissue, causing visible lumps, often referred to as
- The parasite breathes through small openings, and after one or two molts, emerges as a whole warble, falls to the ground, molts again and appears as the mature parasite, starting the cycle again.
- The damage to the hide skins depends on the stage of this cycle when the animal is slaughtered.
- Open grubs are unhealed places where the grub was imbedded or from where the warble emerged, resulting in holes, while wounds leave scarred tissue.
Tick damage
- Tick damage is extremely frequent.
- Ticks leave the hides and skins doted with pinhole spots at each site where they were attached.
- Tick-eating birds often cause damage to the deeper layers, especially if bacterial infection of the wound occurs.
  - Lice damage, biting flies and other stinging insects may leave spots on the grain, but these are of minor importance.
  - Insects may cause extensive damage to hides and skins after they have been removed from the animals. Of these insects, the hide beetle (Dermestes) is responsible for tremendous losses to hides and skins, which have not been properly protected. The beetle itself and its voracious larvae are able to consume a large part of the entire hide in a very short period.
- Treatment
  - In the past, the only drugs suitable for destroying skin parasites were nicotine, derris root, rotenone, lime, sulphur and creolin.
  - Now modern chlorinated hydrocarbon insecticides such as DDT, lindane, BHC, toxaphane and chlordane, which allow mass control of the parasite, are used.
  - Thus give great hope for the reduction of damage to the hides and skins due to parasitic infestation.

DEFECTS DUE TO FAULTY HANDLING

Fallen hides
- Hides removed from animals died of natural causes are known as fallen hides.
- These are inferior not only because the hide substance was reduced due to fever or starvation but also, because the flayingof a dead animal is difficult as subcutaneous connective tissues hardens after death.
Ground drying
- This is a defective method of preservation and leads to visible and invisible damages such as hair slip, taint, blisters which leads to cracks etc.
- Smoke damage: occurs due to tanningmaterials, if they are kept in the vicinity of open fires.
Used hides and skins
- Hide and skins are used as sleeping mats or garments before selling them.
- By this, they are generally torn, cracked, smoked, oil-tanned or damaged by insects.
Brand marks
- Brand mark when carelessly made, seriously reduces the value of the hides.
- The hot iron should be used only on the less value part. Branding for identification or for disease reduces the value of the skin.
Wire damage
- Inserting wires into the brisket, dewlap, and shoulder area or side region causes this.
- This damage should be differentiated from animals, which suffer scratches when confined to areas fenced with barbed wire.
Cracks
- Cracks occur when over dried hides are folded during transport, which leads to damage.
Pressure sores
- This is also called as deductible gangrene, which is a very common defect seen on hides derived from animals due to illness or starvation for a prolonged periods.
- The hip and shoulder blade regions are mostly affected.
Bad shape or patterns
- Damage by vermin, such as hyenas, rats and dogs with subsequent trimming of the damaged parts contribute to an asymmetrical hide.
- Knife Damage: This occurs due to use of sticking knives for flaying or knives with spear shaped blades leads to hide damage.
- Carelessness, unskilled speed, lack of experience, poor visibility and the flaying of cold or undressed carcasses also lead to hide defects and damage.
Bruises
- Bruises are common either before they reach the abattoir or during the slaughtering operation.
- Insufficient Bleeding: In incomplete bleeding, the blood vessels are not drained; such hides are called as “veiny” leather an undesirable defect.
Dragged or rubbed grain
- Pulling the carcasses over rough ground or damaged cement floors causes this fault.
Thorn scratches
- During the dry period, the animals browse on the bushes and thorn trees and in consequence, thorn scratches and grain damage results.
Infrequent transport
- Transport defects and damages occur due to difficulties in transport of hides during the rainy seasons, when the goods are stored for long periods without proper protection from rain and insect.
- Rubbing and soiling during transport may contribute further damage.
Adulteration
- By smearing and plastering the hides with mud, manure, ashes or by leaving excessive amounts of fleshing is a common custom in a number of less developed countries.

To overcome the defects and damages of hide and skins the following are to be taken into consideration.

Programs to improve the hide should be drawn.
Training and demonstration of proper methods of flaying should be given.
Abattoirs should be provided sufficient facilities for hide and skin handling as it fetches substantial revenues.
Skinning cradles should be provided.

Treatment after flaying

Hide and skin may leave the abattoir premises either in green or preserved form.
- Green hide or skin:Green hide or skin is the term applied to hide or skin, which have only been flayed, fleshed, trimmed and washed.

PRESERVATION OF SKINS

Preservation of hide or skin is done either by suspension drying or by curing, i.e. salting.
- The green skins contain approximately 62% water.
- As most of the hide substance is protein, which forms good nutrient for bacteria, all that is needed for them to develop is time and favourable temperature.
- Contamination with blood, manure or dirt will increase bacterial growth.
- The deterioration, which occurs during the curing of hides and skins, may be attributed to the fact that the bacterial growth was not checked in time.

Principles of preservation and its merits

The basic principles of preservation, therefore, lies in creating such conditions that bacterial flora cannot multiply.
This can be achieved either by immediate delivery of the hides and skins to the tannery.
A condition very unreliable in developing countries is by reducing the moisture to a point where bacterial growth stops.
A simple way of reducing moisture is by exposing the hides and skins to free air circulation.
Another method is to absorb the moisture by salt.
This combined with the penetration of the salt into hide substance acts as a kind of dehydration.
The method of preservation had definite influence on the final weight of the hides.
The final percentage of the moisture in the air-dried hide and skin will range from 10 to 12 percent.
Green hide weighing 100 lbs. will contain 62 lbs water and 38 lbs of hide substance.
In Air-dried method there is no loss in hide substance and so it will be 38 lbs.
The final percentage of moisture will be 10 to 12 lbs.
Therefore, the final weight of air-dried hide will be 38 + 10 = 48 lbs.

METHODS OF PRESERVATION

Wet salting

100 lbs. Green hide will loose approximately 35 lbs of water and takes up 6 lbs of salt. Thus the final (weight) field will be 71 lbs. of wet salted hide.

Dry salting

This process consists of only a few days salting, followed by air-drying. 100 lbs. of green hide will yield about 55 lbs. of finished goods. These are only approximate calculations. Many factors are responsible for the variations. The shrink is very important to calculate the yield. The air-drying is the high test as seen above.
The two main methods of preservation are air-drying and salting.
So, to get good qualities of hide and skin, the preservation should be done immediately.
The methods followed for air-drying and salting are many and varied.
- Air drying
  - Ground drying
  - Suspension drying
  - Drying in sheds.
  - While drying the hides and skins are protected from insect damage with the help of benzene hexa chloride and arsenicals.
- Salting
  - Wet salting
  - Dry salting.

AIR-DRYING

It is an age-old method for the areas where relative humidity is low it is done in three different ways.

Ground drying

It consists of stretching out the hides with their flesh side up.
Though it is the cheapest and easily adoptable for rural people with fallen hides, there is problem of blemishes, which become apparent on tanning.
There may be problem of hair-slip and blisters due to incipient putrefaction of epidermis and hair follicles.
However, in summer months, the hides become much wrinkled and fetch only half the value of salt cured hides and skins.

Suspension drying

It is simple, cheap and effective way of drying in tropical countries.
It allows free circulation of air, sunrays strike the hides and skins obliquely and cooling of hides take place rapidly.
The hides and skins become light after drying, so transportation is cheap.
Suspension drying can be done in several ways

Frame drying

This is done on an angled frame, which is appropriately oriented to the sun.
The frame may be in the form of hoop, tripod or bamboo square.
Line drying is more suitable for sheep and goatskins.
Here skins are spread on horizontal cords with their flesh side up.
In tent drying, the hides are supported over the ground in the shape of a tent by cords or wires.
In any case hides and skins take a minimum of seven days to dry.

HIDE CURING

In this case, drying is performed with the help of uniform and finally ground salt.
It should be done immediately after flaying by either of the following two ways:
- Dry salting
  - This is widely used in tropical countries.
  - Salt packing helps in initial removal of moisture, the remaining being removed by exposure to air.
  - Ideally, the store should have a temperature of about 15°C, a relative humidity of 85-90% with good ventilation and slated platform.
  - It involves stacking the hides flesh side up and applying fine salt (2-3 mm) evenly.
  - The quantity of salt applied matches with the weight of the each hide.
  - Stacking should not be more than one meter in height.
  - The moisture drains onto the floor. Such hides retain only 12% water.
- Wet salting
  - This is done by preparing a curing solution consisting of 23 kg pure salt and 62 kg of water for 100 kg of hides.
  - A saturated brine solution can also serve the same purpose.
  - The hides are soaked in this solution in a pit of 1.25 meters for a few days.
  - The time varies from 48 hrs for fleshed hides and 2 weeks for unfleshed hides.
  - The hides are put on the slatted platform for draining. Such hides are left with only 35% water.

CONDITIONING OF HIDES AND SKINS

The preserved hides and skins are first conditioned at the tanneries before tanning under the following steps:
- Washing and soaking:
  - It is done for several hours in water containing zinc chloride and a mix of soda ash and borax.
  - During this process, salt is removed along with proteins of blood and lymph.
  - The hides absorb water and restore their original shape and dimension.
- Fleshing:
  - It is done on a convex wooden beam by scrapping the flesh with a serrated knife.
- Liming and dehairing:
  - A saturated solution of lime and 0.1% sodium sulphide remove and loosen hair and epidermal cells.
- Washing and deliming:
  - The hides are now washed with weak acid to neutralize the lime.
  - A mild heat can also be used if necessary.
- Bating:
  - The hides are treated with proteolytic enzymes (pancreatic juice mixed with saw dust) at pH 8.5.
  - It results in soft and pliable pelt, which can be tanned by vegetable tanning.
- Pickling:
  - It is done for chrome tanning wherein pelts are pickled in a bath of 1% sulphuric acid and 10% salt in water at pH of 2 to 2.5 for 2 to 3 hours.

TANNING

Tanning is the conversion of hides and skins into insoluble and nonputrescible leather without destruction of the original structure.
Leather posses many desirable properties such as flexibility, heat resistance, chemical resistance, abrasion resistance, dimensional stability, etc.
It can also withstand repeated wetting and drying.
Tanning comprises of two types
Vegetable tanning and
Chrome tanning

VEGETABLE TANNING

This is the traditional process and takes a lot of time.
It involves immersion of hides and skins in the infusion made from the extracts of tanning bearing plants and barks.
Some tanning producing plants are:
- Avarum (Cassis auriculata)
- Babul (Acacia Arabia)
- Myrabalan (Terminalia chebula)
- Konnan (Cassia fistula)

CHROME TANNING

This is the modern and quite popular technique since it yields soft, supple and strong leather, which is permeable to air.
It also takes less time.
It can be done by any of the two processes.
In single bath process,basic chromium salt (chromic sulphate or chrome) is directly applied in solution to the skin in gradually increasing strength.
In double bath process, chromium salt is formed on the fibres by interaction of chemicals.
Usually sodium bichromate is reacted with a reducing sugar maltose and sulphuric acid to get chromic sulphate.
It is applied in 1.5 to 3% range initially at a pH 2.8, which is then increased to pH 3.5 to increase the affinity of the collagen for the chrome.
Chrome tanning takes places due to the formation of cross linkages between chrome ions and free carboxyl groups in the collagen side chains.
Tanning operation is accomplished in 5-6 hours.
Post-tanningoperations are also necessary to get finished leather.
- Setting out or wringing is done to remove excess tan liquor or moisture by passing the hide between two large rollers.
- Splitting and shaving is done to adjust the leather thickness for the desired ultimate use.
- Dyeing of the leather is done to produce the desired colour.
- Fat liquoring is done to adjust the firmness or softness of the leather by lubricating the fibres with oil.
  - It also increases the tensile strength.
- Staking refers to the softening and making the leather more pliable.
  - It decides the final temper of the product in combination with fat liquoring.
- Buffing is done to smooth the grain surface of leather for the better appearance and to diminish the blemish.
  - Buffed leather is called corrected grain, which is otherwise called full grain.
- Glazing is done on chromic leather after seasoning an drying.
  - A glass cylinder clamped to the end of a moving arm is made to roll on the leather.
  - Heat generated by friction softens the wax in the finish and produces a continuous plastic coat of high lusture on the grain surface.
- As the leather gets ready, it is graded for temper, uniformity of thickness, colour, etc.
- The graded leather is properly packed for convenience and to fetch a better price.
- With the use of strong tan liquid and revolving drums, the speed of tanningcan be improved.
- This type of tanningis good for the manufacture of sole, belts, harness, saddlery and other heavy leathers.

Conclusion

As we have seen, tanning is an essential part of leather manufacturing. Though it involves several processes, each one contributes to the finish and other qualities that make leather so desirable. Since there are many methods of tanning ranging from brain tanning to chrome tanning, the leathers that are produced will come in a range of prices, colors, thicknesses, and qualities. Hopefully you are now better informed to make a selection based on the tanning type of the leather.