Turn a Boveda pack over and the label describes its contents as a saturated salt solution. The phrase sounds like something from a chemistry set, and for most people it stops there: the pack works, the cigars are fine, the mechanism is somebody else's concern. That is a perfectly reasonable position. But the principle behind the pack is simple, more than a century old, and worth understanding, because once it is understood a good deal of humidor folklore falls away and a handful of practical habits become obvious.
What follows is the science with the mathematics left out. No formula is required to see why a small pouch of brine can hold a box of cigars at a fixed number for months, why that number never drifts as the pouch is used up, and why nothing about the arrangement can be adjusted, improved or hurried.
What relative humidity actually measures
Air holds water as vapour, and the amount it can hold has a ceiling that rises with temperature. Relative humidity is simply how full the air is, expressed as a percentage of that ceiling. At 100 per cent the air can take no more and water condenses out; at 50 per cent it is half full and will readily accept more from anything damp nearby.
The key word is relative. Warm the air in a sealed box without adding water and the ceiling rises, so the same amount of vapour becomes a smaller percentage and the reading falls. Cool it and the reading rises. This is why a humidor moved from a cool hallway to a sunny desk can change its reading without a drop of water entering or leaving. It also explains why a device that holds a fixed relative humidity, rather than a fixed quantity of water, is the only kind worth having.
Water, salt and a settled argument
Place a dish of pure water in a sealed container and the air above it will climb toward 100 per cent relative humidity. Water molecules leave the surface, the air fills, and eventually as many return to the liquid as leave it. That balance is called equilibrium, and for pure water it sits at saturation.
Now dissolve salt in that water. The dissolved ions hold on to the water molecules around them and make it harder for those molecules to escape into the air. Fewer leave the surface, so the balance point drops. The air above a salt solution settles at a relative humidity below 100 per cent, and the more salt is dissolved, the lower that point falls.
Keep adding salt and there comes a moment when the water can dissolve no more. Extra crystals simply sit undissolved at the bottom. The solution is now saturated, and something useful happens: its equilibrium humidity stops changing. It has reached a fixed value that depends on which salt was used and on almost nothing else.
The salt does not aim at a number; it is one
This is the fact the whole system is built on. Every salt has its own characteristic equilibrium humidity when saturated. Ordinary table salt, to take the textbook example, holds the air above it close to 75 per cent at room temperature, and it has done so in laboratory calibration jars for well over a hundred years. Other salts and blends settle at other figures. Choose the right one and you have a material that does not try to reach a humidity; it simply is that humidity, in the same way that melting ice is zero degrees whether there is a cube of it or a bucket.
The comparison with ice is exact in one important respect. A bucket of ice water does not become warmer as the ice melts; it stays at zero until the last piece is gone. A saturated solution behaves the same way. As it gives water up to a dry humidor the volume of liquid shrinks, more salt crystallises out, and the solution that remains is still saturated. Its equilibrium humidity has not moved. This is why a fresh pack and a half-spent pack hold exactly the same number, and why there is no slow decline to watch for. The pack holds its value to the end and then, when the last of the free water has gone, it stops.
Why the ratio matters and the quantity does not
It follows that the amount of solution in a pack has nothing to do with the humidity it produces and everything to do with how long it can keep producing it. A small pack and a large pack of the same rating hold the same number. The large one simply has more water to give before it is spent, and more capacity to absorb before it is full.
It also follows that nothing can be done to change a pack's number. It cannot be topped up, diluted or adjusted, and adding water to a spent pack does not restore it: it dilutes the salt below saturation and turns a calibrated instrument into an ordinary damp sponge that pushes toward 100 per cent. A pack that has gone stiff is finished and should be replaced with a new one of the same rating.
The membrane does the rest
Chemistry alone would leave you with a dish of brine, which is neither tidy nor safe in a cigar box. The second half of the design is the envelope. Boveda seals the solution inside a membrane that is permeable to water vapour and impermeable to liquid. Individual water molecules pass through it in either direction; the salt, the liquid and everything dissolved in it stay put.
The consequence is that the pack behaves like a saturated solution that has been made solid and portable. When the air in the humidor is drier than the pack's rating, vapour leaves the solution, crosses the membrane and enters the box. When the air is wetter, the traffic reverses and the solution absorbs the surplus. Both movements are driven by nothing more than the difference between the air and the pack, and both stop of their own accord when that difference reaches zero.
This is what two-way means in practice. There is no sensor deciding when to act, no reservoir that can be over-filled, no medium that only adds. The exchange runs in both directions, continuously, with no adjustment and no maintenance, because the physics does not permit anything else.
What it cannot do
The same physics sets the limits, and they are worth stating plainly. A pack cannot raise a humidor above its rating, however many are used; once the air reaches the number, the flow stops. It cannot hold that number in a box that leaks faster than the membrane can pass vapour, which is why a poorly sealed humidor drains packs quickly and reads low despite them. And it cannot control temperature. The pack holds relative humidity at the temperature it finds itself in; keeping the room cool and steady is a separate task.
One caution follows directly from the equilibrium principle. Two packs of different ratings in one box do not average out. Each pulls the air toward its own number, the higher one giving water and the lower one absorbing it, and both exhaust faster while the humidor sits somewhere between. One rating per space is the rule.
From the bench to the box
Saturated salt solutions have been used to calibrate hygrometers since long before anyone thought of sealing one in a pouch. The insight was not the chemistry but the packaging: a membrane that lets the solution work on a whole enclosure without spilling, leaking or drying out in the wrong direction. The result is an instrument that requires no reading, because it cannot give a wrong one, and no adjustment, because it was set at the factory by the choice of salt.
For the collector, the practical lessons are few. Trust the number on the label; it does not drift. Judge a pack by its stiffness, not its age, and replace it when it goes rigid. Use one rating per humidor and enough packs for the volume, particularly if the seal is uncertain. Never wet or refill a spent pack. El Laguito is an authorized Boveda retailer and stocks the common cigar ratings at 62, 65, 69, 72 and 75 per cent; the shop is happy to help choose between them, but the physics inside the pack will take care of everything after that.