Two different problems that produce the same complaint
A tea that has stopped tasting like itself has usually suffered one of two things, and the remedies do not overlap. It has lost material — the volatile aromatic fraction departs during storage and nothing brings it back — or it has gained material, in the form of reabsorbed atmospheric water or an odour taken up from its surroundings. The first is subtraction and is permanent. The second is addition and is at least addressable in principle, because water can be driven off and some absorbed volatiles go with it. Almost all of the disappointment around domestic re-drying comes from applying the second remedy to the first problem: a stale tea gently baked is a warm stale tea, and if the heat was heavy enough to change anything it is now a roasted stale tea. The diagnostic is available before any intervention. Warm a little of the dry leaf in a preheated empty vessel and smell it. Leaf with something left to give will release it under gentle warmth; leaf that has lost its aromatics will smell of very little no matter how warm it gets. Leaf that smells damp, dull or faintly sour has the second problem, and it is the only case where heat has anything to offer.
What re-firing does, in the tradition that owns it
Re-firing is a real craft operation belonging to the roasted oolong traditions of Wuyi, Anxi and Taiwan, and Sprint 31 describes it: a light, low, slow bake applied to a finished tea every few years to drive off reabsorbed water, at a temperature chosen so that the tea’s existing roast character is not pushed further. It is normally done by the roaster or the seller rather than the drinker, it is judged by what the tea is doing rather than by a schedule, and over-firing is irreversible. Three effects follow and are worth separating. It removes moisture, which is the point, and moisture in a roasted tea produces exactly the dull, faintly sour cup that prompts the intervention. It removes some absorbed odour along with the moisture, which is a secondary benefit for a tea stored imperfectly. And it inevitably advances the roast a little, because heat applied to roasted leaf continues the reactions the roast began. A tea re-fired often enough accumulates that advance, which is why a repeatedly maintained aged oolong is a different product from a heavily roasted one that has simply been kept — a distinction buyers rarely make and sellers who do the work can answer.
Why a domestic oven is a poor instrument
The requirement is a low, even, controllable heat held for a long period, and most domestic ovens fail on all three counts. Thermostats cycle, so the actual temperature swings around the setting by a margin that is small for a casserole and large for this; the lowest setting on many ovens is above where a light re-fire wants to be; heat is uneven across a shelf; and a fan pushes air across the leaf, driving off aromatics along with the water. Beyond the equipment, the judgement is the hard part: a roaster is watching and smelling a tea through the process and stopping at a point that cannot be specified in advance, and a household following a written temperature and time is running an open-loop process on material it cannot replace. The honest position is that the operation is worth having done and generally not worth attempting, and that the sellers who do it are one of the things a buyer of aged oolong is actually paying for. Where somebody attempts it anyway, the sensible constraints are to use a small trial quantity rather than the whole tin, to work at the lowest heat the equipment will hold, and to accept that the trial is the experiment rather than the rescue.
The categories where it is simply wrong
Green tea, white tea and unroasted oolong have no roast character to preserve and their entire value sits in the fraction heat removes, so warming them trades a small gain in dryness for a larger loss of aroma. Matcha cannot be re-dried in any meaningful sense: a powder that has taken up water has clumped and taken it across its whole surface at once, and heat applied to it changes the material rather than restoring it. Anything being aged should not be dried at all, since moisture is a participant in the change rather than a contaminant. Black tea gains little because its clock is slow and its problems are rarely moisture. That leaves roasted oolong as the category the practice belongs to, hojicha and other roasted teas as adjacent cases, and everything else outside it. The pattern is the one that runs through the storage set: an intervention is worth making when it addresses the vector that actually dominates for the tea in question, and heat addresses moisture. For a tea whose problem is that its aromatics left, no vector is being addressed and the intervention is a way of feeling active about something that has already happened.
What to do instead with a tea that cannot be saved
Change what it is for rather than trying to make it what it was. A green tea that has flattened is often still pleasant cold-brewed, where the low temperature collects a narrower and sweeter fraction and the missing top notes are less exposed. Iced and with citrus, dilution and acidity cover a great deal. In cooking and baking, where sugar, dairy and heat dominate, a tired tea and a fresh one are much closer than they are in a cup, which is exactly why a secondary market for lower grades exists. And a written-off tea is the ideal material for practising brewing, because nobody experiments freely with leaf they are afraid of ruining, which is why most people never find out what their variables actually do. None of this is a reason to be casual about storage and all of it is better than persisting with a declining tin out of thrift. The corrective action for staleness is always at the next purchase rather than in the cupboard: buy less, more often, in the categories where the clock is short. That is the conclusion every storage page in this catalogue arrives at from a different direction, and it is the only one that acts on the problem rather than on its consequences.
The one case where drying genuinely rescues something
A tea that has been in a damp place for a short period, is still short of the point at which anything biological establishes itself, and has softened rather than acquired a musty smell. Here the material is intact, the problem is water, and moving the tea to a dry sealed container with a fresh desiccant will over some weeks bring it back towards where it was without any heat at all. That is the intervention worth trying first in every case, because it is slow, gentle, reversible and requires no judgement. It works because the leaf is hygroscopic in both directions: given a small volume of dry air it will give water up as readily as it took it. What it will not do is remove an odour, and it will not address a tea that already smells musty on the dry leaf — that is a storage casualty and the useful question becomes whether the rest of the cupboard was in the same conditions. Isolate first, assess afterwards, and regard a musty tea as evidence about the store rather than as a problem with one packet.