Shrunken leather - leather shrinkage
Shrinkage of leather caused by heat
Leather can shrink and harden by excessive heat. Typical example of this is damaged car leather along the top of the rear backrest, the headrests and on dashboards. Leather also hardens because it dries out with time (without leather care). Leather fibres stiffen and become brittle.
This process is accelerated by the fact that the collagen of the fibres denatures when the water bound in them is released. Initially, small local thickenings are formed in the leather fibre structure, which subsequently expand and solidify. The density of the leather increases, but its dimensions decrease. This hardening is extremely stable. Once shrunken, leather is therefore irreparably damaged and can sometimes be softened a little with appropriate care products, but the loss of surface is permanent and cannot be recovered.
For wet leathers, even a little heat is sufficient to shrink it. Wet leather should never be allowed to dry in the sun or on a heater. The best temperature to dry leather is normal room temperature.
Video about the shrinkage of leather.
Well oiled and greased leather will never shrink as quickly as poorly maintained leather. The tanning method of leather directly influences its shrinkability. Traditionally vegetable-tanned leather begins to shrink at 70 degrees Celsius. This looks like a high temperature, but is reached quickly, when a convertible with wet, dark leather seats is left in the sun to dry. The leather is then "cooked" and shrinks. Modern tanning processes, such as chrome tanning, raise the threshold of leather shrinkage so that such leather starts shrinking between 85 to 100 degrees Celsius.
|Condition/tanning method||Shrinkage temperature|
|Rawhide||60°C - 65°C (140 - 149 °F)|
|After pickling before tanning||40°C - 60°C (104 - 140 °F)|
|Vegetable-tanned leather||70°C - 80°C (158 - 176 °F)|
|Chrome-tanned||85°C - 100°C (185 - 212 °F)|
|Synthetic tanned||75°C - 85°C (167 - 185 °F)|
|Chamois leather||60°C - 70°C (104 - 158 °F)|
(The measurements of the shrinkage are carried out according to standardized measuring methods.)
Chrome tanned leather shrinks later than chrome-free leather, but the effect of shrinkage is significantly stronger when chrome-tanned leather passes its threshold. Synthetic tanned leather starts shrinking at 75 degrees Celsius, but will not contract as strongly as chrome leather.
A problem of chrome-free (FOC) synthetic tanned leather is the sensitivity to moisture in the leather plus heat. Approx. 50% of all car leather is FOC. If a convertible is driven around in rain, or a wet towel or a wet swim suit remains too long on the leather, moisture migrates through the finish, the seam holes or the perforation holes. If sun shines on such moist leather, the matrix water (chemically bound in leather) starts boiling and the leather hardens and shrinks.
The moisture of leather has an affect on the shrinkage temperature. Dry leather is more stable than wet leather. Since leather absorbs the surrounding air humidity, the "moisture" parameters must be considered in case of critical applications of leather.
Shrunken headrest by sun. - Fire hole in leather furniture by fallen lamp.
Even during the tanning process, the temperature to which leather is exposed must be constantly checked. Rawhide should not be exposed to more than 30 degrees Celsius, since its stability against temperature influences is still low. During the tanning process, a temperature range from 50 to 70 degrees Celsius is generally maintained. Furthermore, the temperature has to be monitored during drying, finish and embossing of the leather.
The rear upholstery top of the BMW E30 convertible is particularly prone to heat damage.
Unexplained shrinkage of individual areas on the back seat of a Volvo. - Left picture: Only rear surface seat. - Right picture: Cracked leather in the back upholstery.
Left: halogen lamp. - Right: Expired hot liquid. Pulling of folds to the shrunken area is typical.
In case of cloth, the house would have burnt. Heat damage in the leather cause by a lamp.
Dashboard leather - Pre-shrunk leather
Automotive manufacturers and their suppliers are taking advantage of the shrinking behaviour of leather to trim dashboards. In order to achieve shape stability and to ensure that the leather remains stable even under the practical conditions in the car (sunshine and possibly high humidity), the leather is sometimes "pre-shrunken". This so-called "heat setting" is carried out by exposing the leather to short-term temperatures of more than 100 degrees Celsius. Long enough to achieve a lasting shape, but also short enough to prevent the leather becoming unusable.
Dashboard completely covered with leather.
The adhesive used also plays an important role. The adhesive must be stronger than the forces of shrinkage of the leather. This can prevent shrinkage damage.
Shrunken dashboard leathers of the Peugeot 307CC.
Shrinkage of leather in the tannery
Since leather is sold by surface, the tanner tries to get the largest possible hide during the tanning process. However, leathers are sometimes deliberately shrunken, so that the leather becomes thicker and the grain pattern is reinforced. Such treatments are common for cow neck leather and sheep leather. When shrinking sheep leather, it loses up to 20% of the surface and becomes thicker.
Shrunken sheep leather.