Underground storage

In the UK there are archives in salt mines. The Winsford salt mine in Cheshire UK is also home to 'Deepstore' an archiving company.

The temperature is constant at 12°C, the RH about 50%, maintained by dehumidification of ventilation air. The ventilation is to avoid 'pockets of stagnant air' (see the reference below), a mythical container which, even if it could be found, has no significance in a salt mine at a uniform temperature.

http://www.bl.uk/npo/pdf/conf2004.pdf (page 30...)

The Wieliczka salt mine in Poland also has to be dehumidified in summer:

http://heritage.xtd.pl/aboutus/Climate_Control.pdf

The Kansas salt mines are operated as a vault by 'Underground Vaults and Storage':

http://www.undergroundvaults.com/offerings/securestoragefacilities/undergroundstorage.html

The temperature is 20°C, and the average relative humidity 35%-40%. I have asked the management how they achieve this RH, well below the deliquescence point of salt. I await an answer.

Not all nations are blessed with salt mines with stagnant pockets of air.

Here are two suggestions for underground storage in a sand dune, which is common in Denmark (though there are also salt deposits at considerable depth). The first model assumes considerable unavoidable exchange of air with the outside, possibly combined with saturated air permeating the sand. The second, simpler model, assumes slow air exchange around a mass of humidity buffer to give a half life for humidity change of about a year.

The first illustration shows an archive surrounded by a humidity controlling void containing wagons of calcium chloride, which is a cheap industrial material, used to stop dusting on tennis courts, for example.

Anhydrous calcium chloride enforces a RH near zero %. It changes to a solid hydrate which absorbs water at 33% RH to form a solution, which drains away, reasonably harmlessly.

Because of this two stage dehydration of the atmosphere, the calcium chloride has to be separated from the archive materials by a buffering layer to even out the RH within the archive.

An alternative solution is to dispense with the CaCl₂ wagon and rely only on the buffering by the unfired brick walls. In winter, outside air is pumped in. The air will warm to the underground temperature and fall in RH, drying the brick. In summer the air flow is cut off completely. The underground air must hold its purity by keeping visits to a minimum. Also the walls must be made impermeable.