Many people believe that because hides are sold in square feet or meters tanneries stretch the leathers to maximize the saleable area. Yes, there is a real commercial incentive to maximize saleable area, but do not assume that reputable tanneries deliberately leave leather dimensionally unstable just to invoice more square meters. In the long run, that strategy can backfire badly.
A tannery can obtain more apparent area through stretching/tension during processing.
The hide first undergoes wet processing, during which it is treated and prepared for further processing. It is then stretched or set to increase and stabilize its dimensions. After this, it is dried under tension, which helps maintain the enlarged area. The leather may therefore have a relatively larger measured area when it is assessed and sold.
Later, when exposed to moisture or heat, or as a result of ageing and natural relaxation, the leather, as a natural product, can lose some of the dimensional increase created during processing. Its structure relaxes, causing the measured area to decrease.
This doesn’t necessarily mean the tannery intentionally cheated anyone. Stretching and setting are legitimate parts of leather manufacture, and they are necessary to achieve the desired handle, grain and area yield.
And while the yield is measured after stretching the leathers and before relaxation and area decrease, the tannery works within a prescribed range of dimensional loss which is monitored and controlled throughout the whole process. A responsible tannery committed to producing quality leathers and certified for every step of the production will not fail to keep the shrinkage within the specified range that will make the leathers suitable for quality leather garments.
How the tannery does it
Tanneries have established procedures and can control this through a combination of:
1. Controlled drying
The leather is dried under controlled temperature, humidity and tension.
This is one of the most important stages for dimensional stability.
2. Setting
Mechanical setting removes excess water and lays the fiber structure in a controlled configuration.
It can reduce later relaxation.
3. Vacuum drying / toggle drying
These methods control how the leather is dried and how much it is held or stretched during drying.
Excessive tension can actually be counterproductive because the leather may later relax back.
4. Staking and conditioning
Mechanical softening and controlled moisture conditioning help stabilize the fiber network.
5. Controlled heat/humidity treatment
The tannery can deliberately expose the leather to controlled conditions that encourage dimensional relaxation before the leather is made into the jacket.
6. Tanning and retanning chemistry
The tanning system itself has a major influence on dimensional stability. Properly stabilized collagen is much less prone to subsequent dimensional change.
7. Testing
The application of the above treatments combined with the tannery’s routine chemical and mechanical tests enables the tannery to and ensure that any dimensional change or shrinkage remains within the predetermined, acceptable limits.
++
This means that the hides are be dimensionally stabilized before shipment, so that the majority of post-tanning dimensional relaxation has already occurred before cutting of the jacket panels and construction of the jackets takes place.
Otherwise, if customers repeatedly discover that a tannery’s leather loses 3–5% of its dimensions after production, the tannery will eventually pay for it through claims, rejected lots, lost customers and reputation. The short-term extra m² isn’t worth much compared with losing its valuable garment customers.