Fatty spew

Fatty spew

Avoid fatty spew on leather

On processed leathers it is sometimes observed that, over time, an undesired, white, milky film appears on top of the leather. This white layer often gives rise to debates on its origin and causes and, in some cases, leads to a claim for damages. This phenomenon is called ‘fatty spew’. The possible sources of fatty spew may be of diverse nature, they are similar in one respect. They always originate from high melting point substances.

What is fatty spew?
All fats have a tendency to migrate through leather when they are not irreversibly fixed to the matrix. Free saturated fatty acids, as all other substances, are sensitive to the influence of temperature. One of temperature’s effects is that it can change a substance's appearance. The reason for the white layer on the grain is the high melting point, which is about 60°C to 70°C for all of the substances causing fatty spew. Below this temperature they begin to solidify, but remain undetectable to the eye. 

Since they are not part of a larger structure and thus free, they tend to cling to one another.  With decreasing temperatures, the tendency to form larger structures and change their appearances, increases. Like water turning into ice, the fatty acid crystalizes when its temperature is lowered beyond a certain point. This point indicates the temperature where the fatty acid changes its structure to crystalline, becoming opaque and milky to the eye.   

The most common origin of fatty spew is the migration of natural fat.  At the surface, the fat reaches the end of its migration, settles, and crystallizes, resulting in a white layer. This mechanism does not act instantly, but represents a gradual process requiring a certain length of time. A similar phenomenon can be observed on chocolate after longer storage and/or temperature changes. This mechanism also applies to all other substances, apart from natural grease, which have a high melting point.

A quick way to determine if the white film is fatty spew and not dirt, salt, or fungus, is to hold a flame of a lighter close to the affected area for a short time. In case of fatty spew, it will easily melt and disappear. In all other cases, it remains visible.

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