There are many types of leather in the market, and why they differ can be due to how they are made. How are they made? Let’s go through the steps a factory goes through to prepare the leather into the final product we see.
The leather manufacturing process is divided into three sub-processes: preparatory stages, tanning and crusting. Most of the Real leathers will undergo these sub-processes. In the next few posts, we will be developing on the different steps and the sub-processes.
The preparatory stages are when the hide is prepared for tanning (the next step). During the preparatory stages many of the unwanted raw skin components are removed. Many options for pretreatment of the skin exist. Not all of the options may be performed depending on the final product the hide is intended for. Preparatory stages may include: Preservation, Soaking, Liming, Fleshing, Splitting, Bating, Degreasing, Frizing, Bleaching, Pickling and many more. As starters, we have chosen a few slightly more interesting ones to discuss.
A chemist would understand this word instantly. This whole process does not have to do with the fruit lime. Instead, a white powder (aka Lime) is used to maintain the pH levels of the leather if too much acid is used to lower the pH levels. Phenolphthalein is used in conjunction with ammonium chloride as a buffer. This process is just to increase the leather's durability and speed up the whole process. It is estimated to be about 2 hours or so in the beam house.
The ultimate goal for this step is to make hides pliable (flexible) and prepare them for the tanning process.
In the bating process, proteases are used to remove scud and unwanted proteins. The process also "deswells" swollen pelts. The result of bating with the use of enzymes is a clean,smooth and soft hide surface. Bating with enzymes achieves the best possible quality of leather and cannot be substituted by chemicals.
Though there have been methods previously devised like using pigeon or hen manure, those methods have been thrown out of the window due to the unpleasant smell and the extended period of time required to get one piece of hide completed.
Also known as Friezing, is the process of removing the grain surface involving severe liming for not less than a month, during which the elastin structure of the grain layer is destroyed. Liming is the process where a white powder is placed directly on the hide so as to remove specific fat layers and preserve the hide for further processing in the future.
Basically, this process is to remove the fat layer (elastin structure) directly under skin which usually cannot be removed by hand or physically.
Bleaching is a common process that everyone must know. However, not everybody knows that this process hardly uses any chlorine.
Although it is true that bleaching has come a long way, with advanced technology, new "Bleaching Agents" surface the markets. There are mainly 2 types of bleaching agents; oxidising bleaching agent and reducing bleaching agent.
An oxidizing bleach works by breaking the chemical bonds that make up the chromophore. This changes the molecule into a different substance that either does not contain a chromophore, or contains a chromophore that does not absorb visible light. This is the mechanism of bleaches based on chlorine.
A reducing bleach works by converting double bonds in the chromophore into single bonds. This eliminates the ability of the chromophore to absorb visible light. This is the mechanism of bleaches based on sulfur dioxide.
Fun fact: The sun is also an active bleaching agent (chemically speaking). Notice when you were to sun your bedsheets out for an extended period of time over a few years, the colours of your sheets tend to turn less vibrant?
Depickling (or the opposite - pickling) is the raising or lowering of the pH out of the acidic region to assist with penetration of certain tanning agents after numerous preparation stages. Since tanning agents only work best in a certain range, manufacturers then added this sub-process to increase productivity through increasing cost.
The whole process can be said as placing the hide into a huge vat of brine(salt water, not necessarily sodium chloride) which the salts will then adjust the pH of the Leather into a range suitable for tanning. Upon removing from the vat after a few days, the leather is usually dipped in oil so as to rehydrate the hide for tanning.
This would end our preparatory stages.
2nd step: Tanning
Tanning is a process of treating skins and hides of animals to produce leather, which is more durable and less susceptible to decomposition. From the picture, it shows a huge barrel which is also known as a drum and the place where leather is "tanned" is also known as Tannery.
Tanning hide into leather involves a process which permanently alters the protein structure of skin. The prepared leather in stage one will then go on with this process and turn into another type of leather also known as "wet blue". There are mainly 2 types of tanning methods which we will be elaborating more later.
1. Vegetal tanning
Vegetal tanning uses tannins (a type of chemical), which occur naturally in the bark and leaves of many plants. Tannins bind to the collagen proteins in the hide and coat them, causing them to become less water-soluble and more resistant to bacterial attack. The process also causes the hide to become more flexible to a limited extent. The primary barks processed in bark mills and used in modern times are chestnut, oak, redoul, tanoak, hemlock, quebracho, mangrove, wattle, and myrobalans from Terminalia spp., such as Terminalia chebula. Hides are stretched on frames and immersed for several weeks in vats of increasing concentrations of tannin. Vegetal-tanned hide(compared to Chrome tanning) is not very flexible and is used for luggage, furniture, footwear, belts but rarely for garments.
2. Chrome tanning
A more commonly used method is Chrome Tanning. As mentioned, it uses Chrome for this process. However, there is a prerequisite for before this method can be used. The pH level of the leather must be very low (or acidic) when the chromium is introduced so as to ensure that the chromium complexes are small enough to fit between the fibres and residues of the collagen. Once the desired level of penetration of chrome is achieved, the pH level of the hide is then raised again to facilitate the next step; basification. In simple terms, a base is added till precipitation point.
Many ask why would tanneries use chromium instead of aluminium? Although both have similar physical properties (like having to precipitate upon adding bases like NaOH), the final product using chrome would have a higher shrinking temperature.
Shrinking temperature is the maximum temperature that an uncompleted leather can take. Following up with the previous post, Aluminum Tanning will produce an intermediate product of “wet white” (something like”Wet Blue” but a different form) with a shrinking temperature of about 70 to 85 degrees. On the other hand, “Wet Blue” has a shrinking temperature of 95 to 100 degrees. Imagine pouring hot water on your sofa and realise your leather starts to shrink till it breaks.
Final step: Crusting
This is the part where the hide/skin is thinned, re-tanned and lubricated. Often a coloring operation is included in the crusting sub-process. The chemicals added during crusting have to be fixed in place. The culmination of the crusting sub-process is the drying, dying and softening operations. I have chosen a few of the many processes: sammying, neutralisation, filling, softening, buffing and surface-coating.
Sammying is usually done after rehydrating the leather achieving not so “wet blue”. It is estimated that 45-55% of the water contained in the hide is then squeezed out by the leather. There is nothing much to discuss about the process. It is basically the process of removing excess water from the leather for the next step. The process can be also justified by saying that the removal of water can also reduce the overall weight when transporting from machine to machine, saving both time and money.
Neutralisation is the addition of an acid, base or buffer to achieve the pH level of approximately 4.5 to 6.5. There is no specific acid or base added to the leather to achieve the desired pH level. However, the wasted by-products are usually insoluble in water and will be rinsed off with water.
Next up, splitting and filling. These two processes usually go together or there would be a few intermediate steps in between. Splitting can be referred to as the physical softening of the leather by separating the leather fibres. The original thick hide will first be split into different horizontal layers then sent for further processing like adding of a chemical reagent to make the layers stay “strong”.
Moving on to the addition of dense chemicals that make leather harder and heavier which is also known as filling. So that whole process will then make the leather thick, durable, and slightly more elastic.
Dyeing and Buffing. Dyeing is very self explanatory, the dyes that were used in the market long ago are known as “azo dyes”, which water was usually used as a medium to dissolve the dyes. However, it is said that “azo dyes” produce an amine which is very harmful to the environment. As such, anionic dyestuff was invented as a replacement. There are mainly 7 different types of dyes used in the market today.
Lastly, Buffing is the abrasion of the surfaces of the leather to reduce nap or grain defects. The whole process will cleanse the fabric and restore its luster, keeping the leather looking its best. Furthermore, buffing the leather will remove minor surface scratches without paying an expensive leather restorer. You can use commercial cleaners or homemade solutions to buff your leather clean.
These are all the steps into making leather. Isn't that interesting? View our collection of sofas here!