A sink, a buffer and a brake
The accessories sold for tea storage do three different jobs and are sold in one vocabulary. A desiccant is a sink: it takes water out of the enclosure and does not give it back until it is regenerated, so it drives the environment in one direction until it is saturated and then does nothing. A two-way pack is a buffer: it holds a target humidity and exchanges in both directions, absorbing when the air is damper than its set point and releasing when it is drier. Unglazed clay is a brake: it neither dries nor humidifies, it simply slows the rate at which the interior follows the room, because vapour has to move through the wall of the vessel. Choosing between them is not a matter of quality. It is a matter of which of the three behaviours the tea wants, and the three do not substitute for one another. The mistake this page exists to head off is the assumption that anything sold as humidity control is a good thing to add. A sink in a store meant to age tea is a slow way to stop the ageing; a buffer set to an ageing humidity in a green tea tin is a slow way to spoil it; and a brake in a room that is itself wrong merely arrives at the wrong answer later.
How a desiccant behaves over its life
Silica gel and the clay desiccants take up water vapour until their capacity is used, and their behaviour changes as they fill. A fresh sachet drives the enclosure dry quickly; a partly loaded one does so slowly; a saturated one does nothing at all and is indistinguishable from a small stone. That last state is the practical problem, because a sachet that has been in a tin for two years looks exactly as it did on the first day and has been doing nothing for most of the period. Indicating gels that change colour address this and are worth preferring for that reason alone. A desiccant is correct for the preservation categories, where the aim is to hold the leaf as far from the moisture threshold as it can be got, and it is most useful for the specific case of a fragile tea being put away untouched. It is close to useless for a container opened daily, since each opening replaces the air and the sachet spends its capacity on the room rather than on the leaf. It is straightforwardly wrong for anything being aged, where over-drying stalls development and produces the quiet failure Sprint 31 describes: no smell, no visible sign, and a tea that has lost aromatic intensity and gained nothing.
Two-way packs, and the argument about them
A two-way humidity pack contains a solution or a gel formulated to sit at a fixed vapour pressure, so the air around it is driven towards a particular humidity from either side. They were developed for other products and have been adopted by some pu-erh storers, and they are the only member of this family with a plausible role on the ageing side of the rule. The case for them is that a small enclosure holding an expensive collection is easier to hold steady with a buffer inside it than with a room, and that a stable figure is worth more than an ideal one. The case against is made by collectors rather than by chemists and is worth reporting rather than adjudicating: it is that a manufactured material sitting among ventilated cakes for a decade is a source of something as well as a regulator of humidity, and that a store which is deliberately inert should stay inert. TeaHQ does not have evidence either way about transfer, and states the disagreement as a disagreement. What is not contested is the practical requirement that follows from using one: a pack has a finite life, a pack that has dried out is a desiccant, and a store with a buffer in it still needs a hygrometer to show whether the buffer is still working.
Unglazed clay, and what a brake is good for
A porous clay jar does not hold a humidity. It slows the exchange between what is inside it and the room, and it holds a small amount of water in its own wall which it trades with both. That makes it a poor container for a green tea, since a slow leak is still a leak and the tea has nothing to gain from exchange. It makes it a traditional and sensible container for loose dark tea, where a slow exchange is exactly the brief and where the room is already approximately right. What clay cannot do is correct a room. A jar in a dry northern flat will arrive at the dryness of that flat, more slowly than a cardboard box would; a jar in a damp cellar will arrive at the cellar. Collectors who use clay well are using it in a climate where the ambient conditions are already tolerable, and using the vessel to smooth the daily and seasonal swings rather than to establish a level. The same reasoning applies to wooden boxes, bamboo and thick cardboard, which are all brakes of varying strength and none of which is a control. Where a store needs to be moved to a different humidity than the room, the correct instrument is an enclosure plus a buffer plus an instrument, and no material choice substitutes for that.
The saturated salt method, and why it is worth knowing about
The reason a two-way pack works at all is a piece of physical chemistry that predates the product: a saturated solution of a given salt, in contact with undissolved crystals, holds the air above it at a characteristic humidity for that salt at that temperature. This is the basis of the reference standards used to check hygrometers, and it is the most useful thing a domestic storer can borrow from it — a hygrometer that has never been checked is an instrument of unknown accuracy, and cheap ones are frequently several points out, which is more than the difference between the storage schools people argue about. Checking one against a known reference before trusting it is a small piece of diligence with an unusually good return. TeaHQ does not give a procedure or a figure for any particular salt here, because the numbers are temperature-dependent and the method has to be followed properly to mean anything, and a half-remembered version produces a confident wrong answer. The point worth taking is narrower and holds regardless: the humidity readings that storage arguments are conducted in come from instruments, instruments drift, and two collectors comparing conditions may be comparing two different errors rather than two different cupboards.
Choosing, by what the tea is for
For a preservation tea in daily use: nothing. A small container the tea nearly fills, kept closed, in a dull cupboard, outperforms any accessory, and the accessory cannot keep up with the openings. For a preservation tea being put away for months: a desiccant, preferably indicating, inside a sealed pack, and the decision made when the tea arrives rather than when it starts to fade. For an ageing tea in a temperate climate that is too dry: an enclosure, a hygrometer inside it, and a buffer if the enclosure alone does not hold. For an ageing tea in a climate that is already suitable: a brake — clay, carton, wrapper — and nothing else. For any store at all: the instrument before the intervention, because a store whose humidity is unknown is a shelf with opinions attached. The general test is the one that runs through every storage page in this catalogue. Name the vector that dominates for the tea in question, then ask whether the accessory addresses that vector or a different one. Most storage purchases fail that test, and the ones that pass are cheap.