Leather Properties and Care
What Is Leather Spew?
Leather spew is a white, waxy (or brown, oily) film of fats that rise out of leather and set on the surface. See where it comes from and if it harms leather.
Leather spew is a film of fats and oils that has moved out of the leather and set on its surface, usually white and waxy, sometimes brown and oily. It is also called spue, fatty spew, fat bloom or fatty bloom. Spew does not damage the leather in itself, but it tends to come back after it is wiped away.
This definition is part of our hub on leather properties and care.
Leather spew definition
A conservation dictionary defines spew in leather as part of the leather's oily content that comes to the surface of the grain. It shows up in two forms:
| Form | What you see | What it is | Fact |
|---|---|---|---|
| White spew | A white, powdery or frosty film that feels soapy or waxy; up close it looks like fluffy moss or small snowballs | Crystallized fatty acids | – |
| Dark spew | A brown, oily or gummy deposit | Fatty acids that have oxidized and darkened | – |
Where the fats come from
Spew can start at the tannery. A leather-chemistry review says a leather's free fatty acid content can be used to judge the risk of spew at any stage of production, and in crust and finished leather.
It can also come from oils added later. The National Park Service calls fatty acid spew the main problem created by leather dressings: free fatty acids migrate to the surface and go solid, much like olive oil in the cold. The same oils can also turn rancid, stain and encourage biological growth. Our page on what leather conditioner does covers this side of oiling.
What sets it off is often a change in climate, which makes loose fatty compounds migrate and crystallize.
Is spew bad for leather?
The Folger Shakespeare Library describes spew as a non-damaging reaction. The catch is that it can be reduced but never fully removed, and on one museum jacket it returned within five years.
Spew and mold can also turn up together, because many fungi feed on animal fats in humid conditions. Removal steps are a how-to topic, covered on hosob.com.
Why is my leather turning white?
Spew is only one cause. Fatty blooms from dressings, soap left by cleaners and salts from sweat can all look like mold. To tell it from mold, see leather spew vs mold.
A different white deposit, tannin bloom, comes from certain vegetable tanning materials rather than from fats. It is covered in leather bloom vs mold. If you suspect mold, read is mold on leather dangerous? first.
To keep conditions stable, Leather Collection's leather care instructions say to store leather in a well-ventilated, cool, dry place, away from hot attics and damp cellars.
FAQ
What is spew in leather?
Fats and oils from inside the leather that have moved to the surface and solidified, usually as a white, waxy film.
Is leather spew the same as mold?
No. Spew is a fatty deposit that melts under gentle heat; mold is a living growth that does not. The two can occur together.
HitKB Editorial Team
HitKB articles are researched from published sources, cited in the text, and reviewed before publication.