Why Dark tea goes bad
Dark tea is post-fermented, which makes it the rare type where ageing well and going badly are separated by a fairly narrow margin. The tea is supposed to change: slow microbial activity and oxidation should soften the liquor, deepen the colour, and take the rough edges off a young pressing.
The failure modes sit on either side of that window. Too wet and you get mould, or the sour, cellar-like taint of bad wet storage. Too dry and the ageing simply stops, leaving you with a tea that is neither fresh nor matured. There is a third failure that is easier to miss: odour, because dark tea will carry a bad smell for years once it has taken one.
Moisture is the fastest killer
This is where dark tea genuinely differs from every other type. It needs some humidity to age (roughly 60–70% relative humidity) and it needs to breathe.
Sealing a pu-erh cake or a fu brick in an airtight bag is a common and well-meant mistake: it stalls the fermentation and can push the tea toward sourness. The opposite mistake is worse. ‘Wet storage’ at high humidity, used to force faster ageing, reliably produces mould and a musty, sometimes fishy taint that never comes out.
The target is stable, moderate humidity with air exchange: the cake in its wrapper, loosely enclosed rather than vacuum-sealed, in a clean cupboard. If you see coloured bloom, or the tea smells mouldy rather than earthy, it is not salvageable.
A practical way to check: press a cake gently with your thumb. Healthy stored dark tea feels firm and dry and smells clean. If it feels spongy, or the wrapper carries tide marks and the tea smells of a damp cellar rather than earth, the humidity has been too high. The liquor is the other reliable tell. Tea kept too wet usually brews cloudy and tastes flat and sour, while properly aged tea stays clear and tastes sweet even when the colour has gone very dark.
Odor travels
Dark tea is the most odour-absorbent tea on this list. Partly that is because it is stored for years, and partly because its own aroma is earthy and low-toned, so a foreign smell blends in rather than announcing itself as obviously wrong.
A brick kept near spices or in a smoky kitchen will carry that smell for as long as you own it. Store it in a clean, ventilated, odour-free space, and not alongside your scented or heavily roasted oolongs.
If a tea has already taken on a taint, extended airing in a clean, dry place can sometimes reduce a light one. A strong taint is permanent.
Heat and light cook it slowly
Dark tea wants stable, moderate warmth (roughly 20–30°C) not heat spikes. A steady room temperature beats a cupboard that bakes in summer and chills in winter, because that cycling drives condensation inside the cake, and condensation is how mould gets started.
Light matters less for compressed dark tea than for green, but it still fades the wrapper and dries the surface unevenly, so keep it dark anyway.
Do not refrigerate it. The cold stalls ageing, and the moisture risk when you take it back out is real. What ages dark tea properly is time at a stable, moderate temperature with clean air. Nothing more elaborate than that.
One more thing worth knowing: dark tea does not have to be expensive to age well, but it does have to be alive. A cake that was over-fired, or pressed from very poor leaf, will not develop much however carefully you store it, because there is little left for the microbes to work on. That is a buying decision rather than a storage one, and it is the reason good storage cannot rescue a badly made tea.
The habit that prevents all three
Check your dark tea a couple of times a year, and trust your nose more than the calendar. Healthy ageing smells clean and earthy (damp earth, old books, sometimes camphor in a sheng) and the liquor stays clear. Problem tea smells mouldy, sour, or like a wet basement, and the liquor can turn cloudy or taste sharply off.
Keep cakes in their wrapper, loosely enclosed, in a clean dark cupboard at a stable temperature and away from odours. Write the year on the wrapper.
And resist sealing everything airtight. For dark tea, some air exchange is part of the process rather than a compromise you are making.