White Powder on a Leather Bag: Mold or Bloom?

It could be either. The Canadian Conservation Institute says mould appears as a thin grey, green or white powdery deposit or small black dots, while fatty blooms from dressings and soap from cleaners tend to be slightly waxy, and salts from perspiration are crystalline. The conditions matter too.

What the CCI says

• Mould on leather appears as a thin grey, green or white powdery deposit, or as small black dots.
• Fatty blooms from dressings, soap residues from cleaners and salts from perspiration may resemble mould.
• The first two tend to be slightly waxy, and the third is crystalline.
• Mould needs a relative humidity of more than 65 per cent to grow, so damp storage makes it more likely.
• If you see mould, CCI says to lower humidity, isolate the object and dry it slowly with air circulation, and to clean it following its separate note on removing mould.

How to tell the difference

1. Look at the conditions: was the bag stored damp or in a closed space?
2. Look at the deposit: powdery and patchy, waxy or crystalline?
3. Check whether you applied a conditioner or soap recently.
4. Note any musty smell.
5. If in doubt, treat it as mould and follow the safe steps.

What not to do

Do not scrub or wet the area to test it. Rubbing can spread spores and damage the surface.

One more clue is where the film sits. Deposits that follow the folds, the seams or the places where the bag touched a damp wall may point to moisture, while a film on a handle that is touched every day may be residue from skin or from a product you applied. Neither clue is certain, so combine it with the other checks above.

Where this answer stops

The CCI notes are written for museum objects, so they describe appearances in general terms. Identifying a deposit with certainty needs expert help.