Body oil accumulation (and removal) from leather explained

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Body oil accumulation (and removal) from leather explained

Postby jmurray2112 » Fri Jun 19, 2020 8:31 pm

If you’ve been a tech for any length of time, you’ve seen it. On furniture head- or arm-rests, on steering wheels, on hand-bag handles, on leather jacket collars, oil accumulation is apparent wherever skin comes in repeated contact over time with leather upholstery. This is true of both people, as well as pets. Typically presenting as a darkening in those areas on unfinished, semi-aniline, pull-up, nubuck, or suede leathers (unprotected), it can also present as a lightening on base-and-print (also referred to as mottled or sauvage) finished (protected) leathers, where the darker print color has softened due to oil exposure and been removed during use, but the lighter base color underneath is intact. These differences in presentation depending on leather type can make identification difficult, unless you know what you are looking at.

In the case of unfinished leathers like the ones cited above, accumulation can happen quite quickly, given extended and repeated exposure (hours spent in the favorite chair every evening, for example). This is because there is no barrier to slow the migration of oil (or in the case of semi-aniline leather, very little barrier). Often these types of leathers are treated at the tannery with a fluoro-chemical to enhance stain-resistance (think Scotch-Guard), but this treatment is topical, and only resists water-based staining agents, not oil-based.

With finished or protected leathers, the process happens slower, but still happens, even though there is a barrier in place in the form of the color coating, and top-coat. The reason for this is that at a molecular level, oil is smaller in size than the gaps in the molecular chains of most water-based coatings used in leather finishing. Leather is an absorbent material, and so draws oil into itself, as the oil migrates into the coatings through those gaps. I’m not a chemist, and this has been explained to me by others, but decades of my personal experience bear this out.

If you are looking at a protected leather that the customer has described as “faded”, and the “fading” is predominantly in the seating area, you are probably looking at oil accumulation damage that has affected the darker print coat, exposing the lighter base coat. This can happen in any base-and-print color family, from dark brown or burgundy to lighter tans and taupes. Any leather using a base-and-print coloring strategy can be vulnerable. Advanced oil accumulation on finished or protected base-and-print leather can also present all three states, where the print color is missing, with an inner ring of the lighter base color, in turn surrounding a center dark stain where even the base color has been removed by the extent of the oil accumulation.

Once you have identified leather type and accumulation level, you need to determine if oil extraction prior to color restoration is viable. If the leather is still smooth, with no surface checking or cracking, then extraction is a go. Pulling oil out of unfinished leather is much easier and faster than finished leather, for the same reason that accumulation happened faster in the first place, no barrier. The inverse is true, however. Pulling oil through a finish is much, much slower, in the same way the accumulation was slower. The oil has to move through the barrier to exit at the same rate it moved in. So, to accelerate the removal, you need to either strip the surface finish, or extract from the back (suede) side of the leather. Obviously, the chemical resistance of the surface finish, and access to the suede side are determining factors in this.

If the affected area has very little surface checking or cracking, it still may be salvageable. If you can’t feel the surface damage if you pass your finger across it, that usually means the damage isn’t deep enough to have compromised the leather, and extraction and restoration are still possible.

If the surface damage in the affected area can be felt by passing your finger across it, then that means the damage has eroded into the epidermis (top, strongest surface fibers), and the strength of the material has been compromised. At that point, you need to either pass on restoration, or put a timer for your customer on the restoration longevity. The deeper the damage, the quicker the restored area fails.

This is a long post, and if you reached the end of it, well done you. As always, if you have questions, post them, or contact me directly.
Jason

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