Leather is produced from animal hides. There are differences in the individual processes, depending on whether chemical or natural methods are used and whether the work is done traditionally or by machine. Overall, numerous individual steps are carried out. Tanning is the best-known and most important of these individual processes.
Individual Steps in Leather Production
The most common individual steps, as they occur in chrome tanning, are briefly described here.
1. Raw hide
2. Measuring and selecting
3. Soaking
4. Scouring/dehairing
5. Deboning
6. Splitting
7. Pickling
8. Descaling
9. Pickling
10. Tanning
11. Neutralization
12. Wilting/Draining
13. Retanning
14. Sorting
15. Folding/Thickness control
16. Wet finishing
Bleaching
Dyeing
Greasing/Licking
17. Softening (beating, rolling, fulling)
18. Drying
Hang-drying
Vacuum drying
Tension drying
Pasting process
High-frequency drying
19. Surface treatment
1. Raw Hide
The leather manufacturing process begins with the raw hide, which is completely untreated. If a raw hide cannot be processed into leather immediately after harvesting, it must be stored. This is only possible by preserving the hide, as bacteria begin to decompose it within a very short time.
2. Measuring and Selecting
Leather processing always begins with an inspection of the raw material. The dimensions of the hides are measured, usually by hand but also using machines. Thickness is determined, and in some cases, water resistance and strength are also tested. In any case
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the hides are also examined for defects.
Inspections of this kind are necessary throughout the entire manufacturing process to ensure that the final product meets all requirements for further processing.
3. Soaking
After the hides have been stored, they must be restored to their original condition. This begins with soaking the hides.
4. Scouring/Dehairing
The purpose of liming is to remove excess hair from a hide and reduce its fat content. This step is, of course, only necessary if the hide is to be used without hair. For furs and pelts this step is therefore omitted.
This makes liming an important step in preparing for the most well-known stage of tanning. In the past, as the name suggests, this process was carried out using ash. Today, calcium hydroxide and sodium sulfide are frequently used for this purpose. However, these substances produce wastewater that must subsequently be treated and must not be released into the environment.
Liming is the step that decisively determines the properties of the resulting leather. The longer the hides remain in the liming vat, the softer the leather becomes. Therefore, if firm, robust leather is desired, the liming time is shortened.
After liming, the hair on the epidermis is loosened, and the flesh and fat residues are loose enough to be removed and freed of unnecessary proteins and fats. Furthermore, the skin is further prepared for tanning through the swelling of the leather fibers and the breakdown of the skin.
Through liming, the hair has now been loosened so that during the dehairing process, the hair is completely removed.
5. Fleshing
As previously described, the skin is already prepared for fleshing.
6. Splitting
A hide consists of various layers, which are referred to as splits. During the splitting process, the hides are separated into flatter splits. It is common to separate the flesh split, grain split, and middle split.
7. Tanning
Tanning softens the leather and makes the fibers more pliable. This process uses enzymes that break down various proteins.
8. Deliming
This step is also called decalcification, and its name essentially explains the process.
9. Pickling
Acid and salt lower the pH of the hides, making the leather receptive to the tanning agent.
10. Tanning
Tanning is the term that describes the actual transformation of the hide into leather. Up to this point, the hide contained native proteins and was therefore still an organic, rapidly decomposing material. Through tanning, the proteins are now fixed and cross-linked, resulting in stabilization that makes the leather less susceptible to decay.
In the widely used chrome tanning process, the actual tanning takes place through the addition of chromium salt (trivalent). In other tanning methods, a wide variety of other substances are used, which can be natural, synthetic, or chemical.
11. Neutralization
During neutralization, the pH value of the hides is adjusted to prepare them for the subsequent steps.
12. Dewetting/Dehydration
The purpose of dewatering is to remove water from the hides. To do this, the hides are pressed between two rollers.
13. Retanning
Retanning refers to the process of subjecting the hide to a tanning treatment once again. The goal is to optimize the hides, particularly in terms of appearance and feel. Depending on their intended use, retanning can prepare the hides for specific finishes. For example, factors such as softness, the visibility of the grain, lightfastness, color, weight, stability, and dyeability can be influenced.
14. Sorting
The seemingly simple step of sorting the hides is, like the first step described here, a process that must be repeated constantly and is of great importance. The hides are inspected for cracks, size, grain defects, strength, parasite infestation, etc. The leather can then be sorted by quality.
15. Planing/Thickness Adjustment
If, after inspection, the hides do not meet the thickness requirements or are uneven, they are planed smooth by feeding them into a machine designed for this purpose. The planed-off leather scraps are often pieced together and resold as inexpensive upholstery leather.
16. Wet finishing
Wet finishing refers to the steps of bleaching, dyeing, and fattening.
Bleaching
During the bleaching of the leather, unwanted discoloration or uneven coloring is corrected.
Dyeing
The leather can then be dyed in the desired color. Depending on the method, it is possible to dye using natural or chemical agents.
Greasing/Conditioning
Greasing is used to soften the leather.
17. Softening (Stollen, Millen, Fulling)
18. Drying
Hang-drying
Vacuum drying
Tension drying
Pasting process
High-frequency drying
19. Surface Treatments
Additional sources: https://www.leder-info.de, accessed on: June 15, 2018, https://www.leder-info.de/index.php/Gerbung, accessed on: July 2, 2018.
