Leather production is done from animal hides. There are differences in the individual processes depending on whether chemical or natural methods are used, and whether traditional or machine-based methods are applied. A number of individual steps are carried out. The tanning process is the most well-known and important step.
Individual Steps of Leather Production
The most common individual steps, as they occur in chrome tanning, will be briefly outlined here.
1. Raw Hide
2. Measuring and Selecting
3. Softening
4. Liming / Hair Removal
5. Flesh Removal
6. Splitting
7. Pickling
8. De-liming
9. Bating
10. Tanning
11. Neutralization
12. Wilting / Draining
13. Re-tanning
14. Sorting
15. Graining / Thickness Regulation
16. Wet Finishing
Bleaching
Dyeing
Fatting / Oiling
17. Softening (Strolling, Milling, Tanning)
18. Drying
Hanging Drying
Vacuum Drying
Tension Drying
Pasting Method
High Frequency Drying
19. Surface Treatment
1. Raw Hide
At the beginning of the leather production process is the raw hide, which is completely untreated. If a raw hide cannot be processed into leather immediately after acquisition, it must be stored. This is only possible by preserving the hide, as bacteria will begin to decompose the hide in a very short time.
2. Measuring and Selecting
The leather processing always begins with inspecting the raw material. Usually, the measurements of the hides are taken by hand, but sometimes machines are also used. The thickness is determined, and possibly the density and strength are tested. In any case, the hides are also inspected for defects.
A check like this will be required repeatedly throughout the manufacturing process to ensure that the final product meets all requirements for further processing.
3. Softening
After the hides have been stored, they must be returned to their original state. This begins by soaking the hides.
4. Liming / Hair Removal
The purpose of liming is to remove the excess hair from a hide and reduce the fat content. This step is only necessary if the hide is to be used without hair. For pelts and furs, this step is omitted.
Liming is an important step in preparation for the most well-known tanning process. In the past, as the name suggests, ash was used for this process. Today, calcium hydroxide and sodium sulfide are often used. These substances, however, create wastewater that must be treated and cannot be released into the environment.
Liming is the step that largely determines the properties of the final leather. The longer the hides stay in the liming solution, the softer the leather will be. To create firm, durable leather, the liming time is shortened.
After liming, the hair on the epidermis is loosened, and the flesh and fat residues are loosened enough to be removed, leaving the hide free of unnecessary proteins and fats. Additionally, the hide has swollen and is now further prepared for tanning.
Through liming, the hair has now been loosened, so that in the hair removal process, the hairs are completely removed.
5. Flesh Removal
As previously described, the hide is already prepared for flesh removal.
6. Splitting
A hide consists of various layers, known as splits. In the splitting process, the hides are separated into thinner splits. The common division is into flesh splits, grain splits, and middle splits.
7. Pickling
During pickling, the leather is softened, and the fibers in the leather become more flexible. Enzymes are used to break down various proteins.
8. De-liming
This process, also known as de-liming, is self-explanatory by its name.
9. Bating
Acid and salt lower the pH of the hides and make the hide receptive to the tanning agent.
10. Tanning
Tanning is the term used to describe the actual transformation of the hide into leather. Previously, there was a hide that contained native proteins and was still organic, rapidly decomposing material. Tanning fixes and cross-links the proteins, stabilizing the leather and making it less prone to decomposition.
The actual tanning process in chrome tanning is done by adding chromium salt (III-valent). Other tanning methods use various substances, which can be natural, synthetic, or chemical.
11. Neutralization
In neutralization, the pH of the hides is adjusted for the following steps.
12. Wilting / Draining
Wilting aims to remove water from the hides. The hides are pressed between two rollers to achieve this.
13. Re-tanning
Re-tanning refers to the process that subjects the hides to another tanning treatment. The goal is to optimize the hides, especially in terms of appearance and tactile properties. Depending on the intended use, the hides may be prepared for special finishing treatments. The softness, visibility of the grain structure, light fastness, color, weight, stability, and dyeability can all be influenced during re-tanning.
14. Sorting
The seemingly simple step of sorting the hides is similar to the first step described here—one that must be repeated constantly and is of great importance. The hides are inspected for cracks, size, scars, strength, parasite infestations, etc. Afterward, the leather can be sorted by quality.
15. Graining / Thickness Regulation
If the hides do not meet the required thickness or are uneven, they are passed through a machine designed to smooth them. The shaved leather remnants are often used as inexpensive padding leather for resale.
16. Wet Finishing
Wet finishing refers to the steps of bleaching, dyeing, and fatting.
Bleaching
During bleaching, undesirable discolorations or uneven colorations are adjusted.
Dyeing
Next, the leather can be dyed in the desired color. It is possible to dye using natural or chemical methods, depending on the technique.
Fatting / Oiling
Fatting leather is used as a way to make the leather softer.
17. Softening (Strolling, Milling, Tanning)
18. Drying
Hanging Drying
Vacuum Drying
Tension Drying
Pasting Method
High Frequency Drying
19. Surface Treatments
Additional sources: https://www.leder-info.de, accessed on: 15.06.2018, https://www.leder-info.de/index.php/Gerbung, accessed on: 02.07.2018.
