Why heated indoor air is so dry
Cold outdoor air holds very little water. When it is brought indoors and warmed, its capacity rises sharply while the amount of water in it does not, so the relative humidity falls — which is why a heated house in winter can be drier indoors than the same house in a summer heatwave, and drier than many arid outdoor climates. The effect is largest where winters are coldest and heating is most continuous, which covers a great deal of northern Europe, Canada, the northern United States, Russia and northern China, and it recurs every year for months. Genuinely arid climates produce the same indoor result by a different route. For the preservation half of the catalogue this is a gift, and it is worth saying so plainly: a sealed green tea in a dry, stable, cool room is in about the best domestic circumstances available, and a household in such a climate can be more relaxed about tins and less relaxed about heat. The problem lies entirely with the categories that were bought because they change. Those are being held in conditions where the change slows to something close to nothing, and unlike damp the condition announces itself with no smell, no visible sign and no moment at which anybody notices.
What over-drying does to an ageing tea
Sprint 31 names it as the quiet failure and the description is worth expanding, because it is the outcome most Western pu-erh collections are actually experiencing. In a very dry store the slow processes that turn a young sheng into an old one proceed at a reduced rate, while the loss of aromatics — which depends on volatility rather than on humidity — continues unabated. The result over years is a cake that has lost the bright, sharp, aromatic character it had when young and has not gained the depth, sweetness and thickness it was supposed to gain in exchange. It is not spoiled and it is not musty. It is thin, quiet and slightly dusty, and the owner has usually attributed the change to the tea rather than to the room. The asymmetry matters because it is not a pause. A tea held very dry for ten years and then moved to good conditions does not resume where it left off with ten years still to spend; it has spent ten years losing the fraction that was going to be part of the result. That is the strongest practical argument for measuring a store before committing anything expensive to it, and for treating a first cake as an experiment in a known enclosure rather than as the beginning of a collection.
The enclosure, and why it does more than a humidifier
The instrument that fixes a dry store is not a device that adds water to a room but an enclosure that holds a small volume of air the tea can bring into balance with itself. A cabinet, a sealed food-grade box with the cakes inside their cartons, a converted wine cooler: the principle in every case is that a small space surrounded by tea reaches its own equilibrium and stays there, where a room cannot. Sprint 31 records the practice and the argument about it, and the point to add for a dry climate specifically is that the enclosure is doing most of the work before any humidity source is introduced. A quantity of tea in a closed box will raise the humidity inside that box on its own, because the leaf gives up water until the small atmosphere balances it, and in a very dry room that alone can move the interior into a usable range. The corollary is that quantity helps: a single cake in a large box will be dragged towards the room, and a full box of cakes will hold its own conditions much better. This is the mechanism behind the collector’s advice to buy a tong rather than a cake, restated for the climate where it matters most.
Adding humidity without adding a hazard
Where an enclosure alone does not hold, storers add a source. The options in circulation are a two-way pack, a dish or vessel of water, damp material such as a sponge or a clay bead in a container, and in the more elaborate arrangements a small humidifier under a controller. They differ mainly in how sharply they can overshoot. A buffer holds a level and cannot exceed it; an open water source is limited only by the enclosure and the season, and is the arrangement most likely to produce mould in a corner nobody is looking at. Whatever is used, three rules apply and none of them is optional. Nothing wet touches tea, ever, and a source is separated from the leaf by distance and by a barrier. A hygrometer sits inside the enclosure and is actually read, because an enclosure fails in ways a room does not. And changes are made slowly, since a store driven quickly from one condition to another is wetting leaf that was dry rather than establishing a new balance. A dry-climate storer’s commonest error is impatience: having discovered that the collection has been too dry for three years, they correct it in a week.
The categories that are fine, and the two that are not
In a dry house, black tea, roasted oolong, hojicha, genmaicha and every bagged tea are in good conditions and need nothing beyond a closed container away from the cooker. Green tea, matcha and unroasted oolong are in the best domestic environment they are likely to get, and the remaining risks for them are heat and light rather than moisture, which shifts attention to where a tin is kept rather than what it is. Two categories need the intervention. Anything being aged deliberately — sheng and shou pu-erh, hei cha, pressed white tea — needs an enclosure. And traditionally roasted oolong being held for maturity is the interesting middle case: it wants to be dry, so a dry climate suits it, but it still needs the periodic re-firing the tradition includes, and a very dry room does not remove that requirement so much as lengthen the interval. A household in a dry climate that owns nothing from the first group needs no equipment at all and should spend the money on tea. That is worth stating because storage advice generalised from collectors tends to prescribe an apparatus to people who have no use for one.
The seasonal swing, which is the real enemy
A continental house is not simply dry; it is dry for part of the year and much less so for the rest, and it crosses between the two twice. The swing is worse for stored tea than either end would be on its own, because the leaf is being driven to give up water and then to take it back on an annual cycle, and any container that is not a real barrier is passing the swing through. Two responses follow. For preservation teas, the swing barely matters if the tea is genuinely sealed, which is one more argument for keeping the laminate pouch. For ageing teas the swing is exactly what the enclosure exists to damp, and the season to watch is the start of the heating period, when indoor humidity falls fastest and an enclosure that held all summer can drop away within a fortnight. A store checked twice a year, at the two turns of the season, catches almost everything that goes wrong in this climate. That is a much lighter regime than the weekly inspection enthusiasm produces, and it is better, because the other principle running through every storage page here is that a store which is opened often is a worse store than an identical one left shut.