What is the black mould in the basement?

Several things are black. Aspergillus, Stachybotrys (Toxic Black Mold), Cladosporium. Colour is the least reliable way to identify mould, so texture and location are better starting points.

Mould in the basement

Basements combine cool surfaces with humid air, which condenses. A dehumidifier holding relative humidity below 50% does more than any cleaning.

Start from the colour

Colour is unreliable on its own, but it is how people search — and each of these narrows to a short list.

Black mould in the basement Green mould in the basement White mould in the basement Grey mould in the basement Yellow mould in the basement Orange mould in the basement Brown mould in the basement Blue mould in the basement

What it is likely to be

Things that are not mould are listed first.

EfflorescenceNot mould

Dissolves in water. Mould does not — this is the definitive test.

AspergillusInvestigate

Powdery, granular surface that puffs when disturbed.

Dry RotInvestigate

Wood cracks into cube-shaped blocks across and along the grain.

Stachybotrys (Toxic Black Mold)Investigate

Wet, slimy appearance rather than the dusty or fuzzy look of common moulds.

CladosporiumRoutine

Velvety, suede-like texture that is dry rather than slimy.

PenicilliumRoutine

Blue-green colour with a white growing edge.

White MouldRoutine

Fuzzy or cottony with visible three-dimensional growth, not a flat crust.

The species matters less than you think

Public health guidance is to remove all indoor mould regardless of type. The CDC and EPA do not recommend routine species testing for ordinary households, because the response does not change: find the water, stop it, remove the growth.
“Toxic black mould” is a marketing phrase, not a category. Stachybotrys is real and does need soaked material to grow, so finding it tells you something useful about your building. But the health effects that are well established — allergy, asthma aggravation, irritation — are not specific to it.
Mould without water does not persist. Cleaning growth while leaving the moisture source is why it comes back. The leak, the condensation or the ventilation is the actual problem.

Call a professional for areas larger than about 10 square feet, for anything in HVAC ductwork, after sewage or flood water, or if anyone in the household is immunocompromised or has a lung condition.

Narrow it down

Already filtered to basement or cellar. Add the texture and surface.

1.Where is it?

Location narrows things faster than appearance, because each part of a house has its own moisture problem and its own usual suspects.

2.What does it feel and look like?

optional

Texture is far more diagnostic than colour. Slimy, powdery and crusty point in three completely different directions — and crusty usually means it is not mould at all.

3.What is it growing on?

optional

Material matters. Nothing grows on bare masonry that also grows on bread, and a white crust on concrete is almost certainly a mineral deposit.

4.What colour is it?

optional

Last, and deliberately so. Dozens of unrelated moulds are black, and colour changes as a colony matures — it is the least reliable thing about mould and the thing people lead with.

7 possibilities

from 1 of 4 answered

One of these is not mould at all

Things that are not mould are listed first, because that is the most useful answer to get and the one nobody offers.