Fatty Bloom of Spue/Spew on your leather goods

I have in the past had spew/spue on leather notebook covers, boots, and a wallet. I remove it by applying heat from a blow-drier and wiping it off. It does come back though, and I have heard that unless you use a spew remover it will never be permanently removed. Other shoemakers and leatherworkers say that spew remover does not work.

Many spew removers work with aniline leather, which (according to Wikipedia) is a ‘type of leather dyed exclusively with soluble dyes without covering the surface with a topcoat paint or insoluble pigments. The resulting product retains the hide’s natural surface with the ‘grain’, i.e. visible pores, scars etc. of the complete original animal’s skin structure’.

Fatty spue periodically appears on leathers and leather goods. It is well known that the fatty spue is caused by free fatty acids, mainly palmitic and stearic, which have a high melting temperature. This can cause their precipitation onto the leather surface where they form a grey whitish patina.

How to remove leather spew

From What’s That White Stuff? The most common white stuff we have seen on Alaskan leather items is white bloom resulting from fats, oils and waxes and may be referred to in the literature as ‘fatty bloom,’ ‘fat bloom,’ or ‘fatty spue (or spew)’. These terms all refer to the migration of fats/oils through the leather material that crystallize on the surface in the presence of air. When seeing fuzzy white growth on an object, people’s initial assumption is often that it is a mold or mildew. But this is not always the case. Blooms can sometimes have a feathery or matted fibrous look similar to mold, but microscopic examination and solubility tests can confirm the presence (or absence) of bloom.

Bloom can be considered as being Primary or Secondary.   Primary bloom results from fats used during the tanning process and can be considered as an inherent vice of the material. Manufacturing flaws contribute to Primary bloom and can cause mineral salts to exude or fat bloom to develop through insufficient degreasing methods during production. Secondary bloom is caused by the application of fats and oils to the surface of the leather. At one time, it was believed that applying leather dressing or other kinds of soaps and oils to a leather surface would extend the life of a leather object. Now it is known that this is not the case and often the application of such substances can do quite a bit of damage (my underlining as I have never believed in applying dressings to leather and I can never understand why other people do it. 

There are a number of hypotheses regarding the exact mechanism of the formation of these blooms. Some attribute it to free fatty acids migrating through the leather (Ordonez and Twilley 1998, 3-4). Analysis by Scott R. Williams (1988, 65-84) found bloom on objects to be primarily composed of a variety of fatty acids including palmitic, stearic, myristic and dicarboxylic acids (such as azelaic). These were present individually or occasionally as mixtures; however palmitic and stearic were the most commonly found (Williams 1988, 68-69).

Others have cited lactic acid, produced from the presence of potassium lactate in leather dressings, as the principle component of white efflorescence on leather (Gottlieb 1982, 39). In general, however, it is believed that temperature and humidity levels play important factors in the migration and crystallization of whatever is moving through and out of the leather.

Bloom can look powdery or gummy in appearance. Powdery bloom can be caused by either the natural fat of the hide or fatty materials applied to the leather. A number of variables are implicated in the formation of powdery bloom. These include: temperature, humidity, acidity of the leather, or materials used during the tanning process. Sticky or gummy bloom is believed to be caused by oils that are highly oxidizable, such as fish oils. If these kinds of oils were used during processing (and incompletely removed) or applied later, then they may cause sticky white bloom. High temperatures and humid environments, as well as exposure to air and light can accelerate these formations.

Fat bloom is often primarily found on areas of an object exposed to air. For example, on a leather-bound book the spine of the book (if it faces outward) may have the heaviest bloom. In some instances, it has been found that items closer to an air conditioning vent had a higher occurrence of bloom (Gottlieb 1982, 37) indicating that air circulation, temperature, and humidity play an important role.