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3. Finishing:
a)Dyeing process
Depending on the desired product, the hides then go through a dyeing process, which also involves adding moisture back into the skin. Hides which have been vegetable tanned are bleached and them soaked with oils, soaps, greases and waxes to make them more pliable.

b)Rolling
Rolling leather running the skins through a machine, which works to firm the leather and make it stronger. After the rolling process has finished, leathers are stretched, where they dry out in a heat controlled room.

c)Surface coating
Once trimmed, dependent on customer requirements, the initial application of surface water-based pigment is applied by machine roller coating. The final protective coats are applied by machine spraying.

d)Finishing (application)
A water-based pigmented finish is applied to give a uniform covering of colour. A concentrated pigment would not cover leather and so it is dispersed into a base solution of acrylic polymer, which contain binders to adhere to the leather. The pigment can be applied to leather in three ways;
•Sprayed on by hand using an airgun.
•Sprayed on by a machine on a conveyor belt.
•Often a combination of a base coat and a top coat are used.

After applying the pigment a polyurethane is then applied to seal the colour, to provide protection and durability, the polyurethane is also used to adjust the gloss level of the leather.

The above process is what lead me to the following statement on leather conditioners-
I have discussed this issue with many people in the leather tanning / preservation / care products industry and haven't received a definitive answer.
My issues are
(a) How much conditioner will permeate the thick polyurethane top coat on the leather

(b) Chrome tanned leather hide is sealed at the tannery and then pigmented; what could a conditioner do for the hide?

Approximately 90% of vehicle manufacturers are using a split-leather hide and a (thermoplastic) polyurethane covering for their interior upholstery. Vehicle upholstery is chrome tanned and uniquely treated with a light pigmented urethane coating and / or a vinyl covering to make it more viable for automotive seating. It retains the softness of natural top-grain leather but resists fading in direct sunlight, which besides body oil is leathers worst enemy.

The complex tanning process of chromed tanned hides results in the fat liquoring and oils necessary to keep the hide soft and pliable being locked in, this is further sealed by a durable polyethylene covering to protect the hide from abrasion from clothing as well as the dust / dirt introduced by the vehicle’s AC system.

“And now for the rest of the story”

Leather - The Manufacturing Process:
Leather is made from animal skins or hides which have been chemically treated to preserve quality and natural beauty. The chemical procedure used to ready raw animal hides for use is called "tanning." A piece of hide or skin which has been tanned produces strong, flexible leather which is able to resist decay and spoilage. The majority of leather made today is produced from tanned cattle hides, though many types of hides can be used, including those from horses, pigs, goats, calves, labs, deer, kangaroos, reptiles, seals, and walrus.

Stages of tanning (preparation)
Raw animal skins go through several steps during the tanning process. Depending on the type of hide used and the desired end-product, the steps taken during tanning can vary greatly.

1. Curing
a)Animal skins or hides are first "cured," a process which involves salting and/or drying the hide once it's been stripped from the animal. Because this step needs to be performed almost immediately upon removal from the animal, it often takes place inside the meat-packing industry or at a nearby factory. Hides can be cured in one of two ways:

b)Wet-Salting- is done by salting the hide and then piling many skins together until they form a moist bunch. They are then left to cure for one month, so that the salt can completely be absorbed into the skin.

c)Brine-curing - is more common than wet-salting, as it's considered a faster, easier method. During brine curing, hides are positioned carefully in vats and smothered with a mixture of salt and disinfectant. After 10-16 hours, the skins are completely cured and ready to move on to the next stage.

d)Soaking - once the hides have been cured, they are then soaked in water for several hours to several days. The water helps to rid the skin of salt, dirt, debris, blood and excess animal fats.

e)Liming - after being soaked, the hair and epidermis is removed by treating hides with a mixture of sodium sulphide and lime solutions.

f)Flesh removal - after soaking, animal hides are moved through a machine which strips the flesh from the surface of the hide.

g)Hair removal - the hides are then transported to a large vat, where they are immersed in a mixture of lime and water, which loosens the hair from the skin. After a 1-10 day soak, the hair is mechanically removed from the hide.

h)Scudding - stray hairs and fat which were missed by machine, are removed from the hide with a plastic tool or dull knife in a process known as "scudding." Scudding is done by hand.

i)Deliming - after the hair and debris has been cleaned from the skin, hides are delimed in a vat of acid. After the lime has been pulled from the skin, hides are treated with enzymes, which smooth the grain of the leather and help to make the resulting product soft and flexible

j)Splitting - the hide is split into layers; the grain layer (top) is used in the manufacture of ‘Top Grain Leather’ (premium upholstery, clothing, etc) the lower split is used for polyurethane protected automotive upholstery and the flesh split (bottom) is sold to other manufacturers for the production of suede, or to the food industry as a source of collagen.


2. Tanning
Leather is made from animal skins or hides which have been chemically treated to preserve quality and natural beauty. The chemical procedure used to ready raw animal hides for use is called "tanning." A piece of hide or skin which has been tanned produces strong, flexible leather which is able to resist decay and spoilage.

Hides and skins are often treated several times during the process of tanning. Which type of tanning procedure is used, depends largely on the hide itself and the resulting product intended. After de-liming and pickling to make the hide more receptive, the hides are tanned to stabilise the collagen fibres and prevent its decay. This can be done either using a solution of recyclable chromium salts producing 'wet blues' or various organic solutions producing 'wet whites'. Subsequent to the chrome tanning process (using chromium salts to preserve hides and prepare them to absorb dyes) automotive leather is pigment coated and finished (and some leather then has a polyurethane protective coating applied)

Leather is hygroscopic (naturally absorbs and retains water), meaning it’s also susceptible to losing the moisture necessary to keep it pliant and soft. The denaturing process of leather tanning removes moisture from the hide; water-based products restore the lost moisture of leather hides to maintain its natural flexibility.

a)Vegetable Tanning
Hides which have been tanned with a vegetable tanning agent solution produce flexible, but stiff leathers, such as those used in luggage, furniture, leashes, belts, hats, and harnesses. Vegetable tanning consists of stringing hides on large frames, located inside large vats, and exposing them to tannin, a natural product found in the bark, wood, leaves and fruits of chestnut, oak and hemlock trees. Hides are transferred to many different bins during this step, each containing a stronger solution of tannin. Vegetable tanning prevents the skin from decay and shrinkage.

b)Mineral Tanning
Mineral or chrome tanning is performed on skins which will be used for softer, stretched leathers, such as those found in purses, bags, briefcases, shoes, gloves, boots, jackets, pants, and sandals.

c) Chrome-tanned leather
Invented in 1858, is tanned using chromium sulphate and other salts of chromium. It is more supple and pliable than vegetable-tanned leather, and does not discolour or lose shape as drastically in water as vegetable-tanned. Also known as wet-blue for its colour derived from the chromium. More esoteric colours are possible using chrome tanning.

d) Fat liquoring
All cowhides are naturally oily, unfortunately, these natural oils are stripped away in the tanning process (tanning is a process using a water vat and chromium salts to preserve hides and prepare them to absorb dyes) and some equivalent oils must be re-introduced after tanning. This step, the replacement of oils, is called fat liquoring.

3. Finishing:
e)Dyeing process
Depending on the desired product, the hides then go through a dyeing process, which also involves adding moisture back into the skin. Hides which have been vegetable tanned are bleached and them soaked with oils, soaps, greases and waxes to make them more pliable.

f)Rolling
Rolling leather running the skins through a machine, which works to firm the leather and make it stronger. After the rolling process has finished, leathers are stretched, where they dry out in a heat controlled room.

g)Surface coating
Once trimmed, dependent on customer requirements, the initial application of surface water-based pigment is applied by machine roller coating. The final protective coats are applied by machine spraying.

h)Finishing (application)
A water-based pigmented finish is applied to give a uniform covering of colour. A concentrated pigment would not cover leather and so it is dispersed into a base solution of acrylic polymer, which contain binders to adhere to the leather. The pigment can be applied to leather in three ways;
•Sprayed on by hand using an airgun.
•Sprayed on by a machine on a conveyor belt.
•Often a combination of a base coat and a top coat are used.

After applying the pigment a polyurethane is then applied to seal the colour, to provide protection and durability, the polyurethane is also used to adjust the gloss level of the leather.

Information resource - Leather tanning information

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