Leather Properties and Care
Leather Bloom vs. Mold: How to Tell the Difference
Bloom is a deposit: tannin bloom from tanning or fatty bloom from oils. Mold is a fuzzy, musty fungus that follows damp. A table and three checks.
Bloom is a deposit, and mold is a living fungus. On vegetable-tanned leather, tannin bloom is ellagic acid that forms in and on the leather during tanning. The other "bloom", fatty spew, comes from oils and does no harm in itself. Mold grows as fuzzy or spotty patches when relative humidity (RH) runs between 65% and 100%, and it often smells musty.
This answer is part of our hub on leather properties and care. Fatty spew has its own page: leather spew vs mold.
Bloom vs mold at a glance
| Column 1 | Tannin bloom | Fatty bloom (spew) | Mold |
|---|---|---|---|
| What it is | Ellagic acid deposited in and on the leather | Fatty acids from oils or dressings that solidify on the surface | A living fungal growth |
| Where it comes from | Tannins rich in ellagitannins, such as valonea and myrobalans | Dressings and oils applied over the years | Damp: RH 65 to 100%, warmth, still air |
| Leathers affected | Vegetable-tanned | Leathers dressed with fats or waxes | Any leather, especially with residues on it |
| Effect on the leather | Adds weight, firmness and water resistance; can look unsightly | Non-damaging in itself | Slowly destroys and discolors it |
| Facts | – | – | – |
What tannin bloom is
In the leather trade, bloom is a deposit of ellagic acid formed in and on leathers tanned with certain vegetable tannins. It is not chemically bonded to the fibers. Valonea, made from oak acorn cups, deposits a lot of bloom because it is rich in ellagitannins, and myrobalans also deposit it readily. It forms from the tannins themselves.
The same word is used for fatty bloom, also called spew or spue. Conservators add that fatty bloom, soap from cleaners and salt from sweat can all look like mold.
Signs that it is mold
- Color and spread: mold forms a low growth in various colors, either in spots or across a large area. It shows dark on light leather and white on dark leather.
- Texture: branched, soft or fuzzy, with hazy edges.
- Smell: a musty odor, often the first sign when several moldy items are stored together.
- History: mold follows damp. A water event or a rise in RH above 65% points to mold.
Three checks to tell them apart
- Recall the conditions. If there was no leak, spill or damp spell, a deposit such as spew is more likely than mold.
- Try dry air. Leave the item in a room below 50% RH. Active mold shrivels and goes inactive; a deposit stays as it is.
- Leave the solvent test to a conservator. Conservators dab a hidden spot with mineral spirits. If the white film disappears, it was probably wax, not mold.
Leather can carry mold and a fatty bloom at the same time, so one finding does not rule out the other.
If it is mold
Mold allergens can cause reactions when they are touched or breathed in, and dead mold still has to be removed. More in is mold on leather dangerous? and how to identify and prevent mold on leather. Removal steps are on hosob.com: how to remove mold from leather. To stop it coming back, Leather Collection's care instructions say to store leather somewhere cool, dry and well ventilated.
FAQ
How do you get rid of leather bloom?
That depends on which bloom it is. Fatty spew can be reduced on the surface, but it never fully goes away and tends to return. We found no reliable source on removing tannin bloom from finished goods.
Can moldy leather be saved?
Sometimes. Leather is porous, so a badly affected item may have to be thrown away. For valuable or sentimental pieces, the EPA suggests a restoration or conservation specialist.
HitKB Editorial Team
HitKB articles are researched from published sources, cited in the text, and reviewed before publication.