Why does mould stain my leather?

by Super User

When mould grows into leather, usually it does not cause major damage.  The mould soaks up its food from the parts of the leather that do not contribute to strength. In fact, until the mould starts to grow new spores, it is effectively invisible – you cannot tell it is there.

The biggest problem with moulds in leather is caused by a change in color - staining. Often they stain the leather in bright colors, and leave spots and grain marks that will make finished leather look inferior.  This causes downgrading of the finished product, and can cost a lot of money.

Once moulds are inside the leather, simply cleaning the surface will not solve the problem, since the moulds will grow right out again.

Moulds can also consume some important additives used in leather making, such as fat liquors, tannins and so on.

How to slow down mould growth?

If we understand the things moulds need to grow, we can change these things, and slow down growth.

1. Water.  Mould will not grow on dry materials.  If finished leather is dried and kept dry, there will be no mould.  However, allowing leather to get wet will encourage mould growth.  Leather in the wet blue state is very prone to mould growth.

2. Acidity.   Mould grows best when it is in a slightly acid environment.  Pickled pelts are too acid for fast mould growth.  Very alkaline conditions do not support much mould growth.

3. Warmth.  Mould likes it best between 25 and 30 deg. C.  That is why it is worse in summer, or in the tropics.  Keeping leather stored in cool conditions will not stop mould, but will slow its growth a little.

4. Oxygen.  Without oxygen, moulds cannot grow.  If wet blues or pickled pelts are baled and pressed into a tightly packed state, mould will only grow on the outside.  There is too little oxygen inside the pack.

5. Nutrients.  Moulds soak up the soluble parts of leather as food.  Unfortunately, we cannot remove these to control mould growth.  Some leathers (like vegetable tanned leathers) have more nutrients, and support especially vigorous mould growth.