How to store a mineral collection in a damp house
Air at 55% and 20 degrees is at 91.6% when it cools to 12 degrees overnight. That is why an afternoon hygrometer reading misses the damage a cold room does.
Relative humidity rises as air cools without any water being added. Air measured at 55 per cent and 20 degrees Celsius reads 91.6 per cent when the room falls to 12 degrees overnight. That is when sulphide specimens decay, labels mould, and hydrated copper crusts on a crystal spoil in the cabinet.
In short
- Measure at the coldest time, not the most convenient time. A hygrometer read at four in the afternoon in a heated room tells you almost nothing about what the collection experiences at four in the morning. The same air at a lower temperature is at a much higher relative humidity, and the calculation below shows by how much.
- Temperature stability beats dehumidification. A gently heated room that never falls below about 16 degrees solves most damp problems without any equipment, because it keeps the relative humidity down by keeping the air warm.
- Above 60 per cent, sulphides start to go; above 65 per cent, mould grows; below 35 per cent, hydrated copper sulphates crumble. Those three thresholds from the Natural Sciences Collections Association define the window, and the window is 45 to 60 per cent.
- A dehumidifier can cause the opposite fault. Drying a room to 30 per cent to protect the pyrite will destroy the hydrated copper species. The target is a band, not a direction.
- The garage, the cellar, the loft and the conservatory are all wrong, and they are wrong for the same reason: they are unheated, so they track the outside temperature and swing across the thresholds every night.
| Reading | What it means | What fails first | Action |
|---|---|---|---|
| Below 35% | Too dry | Hydrated copper sulphates — langite, chalcanthite, chalcophyllite | Turn the dehumidifier off or move the collection away from the radiator |
| 35–45% | Dry but tolerable | Nothing, if stable. Some clays and shales crack | Acceptable. Best band for a pyrite-heavy collection |
| 45–60% | The target | Nothing | Leave it alone and monitor monthly |
| 60–65% | Sulphides at risk | Pyrite, marcasite, then other sulphides | Heat the room by 3–4 degrees before buying anything |
| Above 65% | Mould growth as well | Labels, boxes, organic mounts, then the specimens | Move the collection out of that room now |
| Swinging more than 10 points in an hour | Unstable, whatever the average | Anything that has been stable for decades | Fix the cause — usually ventilation or intermittent heating |
The calculation that explains why your reading looks fine
Relative humidity is not a measure of how much water is in the air. It is the ratio of the water present to the maximum the air could hold at that temperature. Warm air holds far more, so cooling air raises its relative humidity without anything being added to it.
This is the single most misunderstood fact in collection storage, and it is worth doing the arithmetic once, because the numbers are larger than people expect.
The saturation vapour pressure of water is given by the Magnus formula, es(T) = 6.112 × exp(17.62T / (243.12 + T)) hectopascals, with T in degrees Celsius. Take a back bedroom that reads 55 per cent relative humidity at 20 degrees on a winter afternoon with the heating on. The actual vapour pressure is 0.55 × 23.34 = 12.83 hPa. That quantity of water does not change when the heating goes off. What changes is the temperature, and therefore the ratio:
20 °C → 55.0%. 18 °C → 62.3%. 16 °C → 70.7%. 14 °C → 80.4%. 12 °C → 91.6%. The dew point — the temperature at which that air is saturated and condensation forms on any colder surface — is 10.7 °C.
So a room that reads a reassuring 55 per cent at teatime and falls to 12 degrees by four in the morning is putting the collection above 90 per cent every night. It crosses the 60 per cent sulphide threshold at about 18.5 degrees and the 65 per cent mould threshold at about 17.5 degrees — which is to say, it fails as soon as the heating goes off.
Substitute your own figures; the method is the point. And note what it implies: the cheapest dehumidifier is a thermostat. Raising the overnight minimum by three or four degrees moves the collection back inside the window at no equipment cost, where a dehumidifier in an unheated room fights the temperature all night.
Diagnose the room before you buy anything
Work in this order. Each step is free or nearly so, and the expensive options come last on purpose.
1. Put a hygrometer where the collection is, and read it at the coldest hour. A cheap digital hygrometer with a min/max memory costs very little and settles the question in a week. Read the minimum temperature and the maximum humidity, not the current figures. If you can only take one reading, take it first thing on a cold morning.
2. Check for a cold surface. Put the back of your hand on the external wall behind the cabinet, on the window reveal, and on the floor. A surface below the dew point — 10.7 degrees in the example above — is condensing, and a cabinet against it sits in a local microclimate far wetter than the room average.
3. Look for the source. Rising damp, penetrating damp and condensation are three different problems with three different fixes, and only the third is about the collection. Black spotting on the ceiling and window reveals is condensation. A tide-mark low on the wall with salting is rising damp, and that is a building problem — see a surveyor, not a mineral dealer.
4. Check whether it swings. The Natural Sciences Collections Association's guidance notes that specimens can split when relative humidity fluctuates by more than ten points in an hour. A stable 62 per cent is kinder than an average of 52 that swings between 40 and 70. Stability is the property to buy first.
The fixes, in order of what they cost and what they achieve
Move the collection. Free, and usually the answer. An interior room in the heated part of the house beats every piece of equipment. A cupboard on an internal wall in a room that is lived in is better storage than a purpose-built cabinet in a garage. If the house has one reliably heated room, that is where the collection goes, and everything else is negotiable.
Raise the overnight minimum. Cheap, and the highest-leverage change. The calculation above shows why: three or four degrees of overnight heat is worth more than any amount of dehumidification, because it attacks the mechanism rather than the symptom. A low thermostat setting overnight, or a small tubular heater in a cupboard, does it.
Move the cabinet off the external wall. Free. Even a few centimetres of air gap behind a cabinet lets the wall surface warm and stops the local pocket of cold, damp air forming.
Seal the specimens rather than the room. Moderate cost, very effective. Clear lidded polystyrene boxes — the storage the Natural Sciences Collections Association recommends — with a small quantity of conditioned silica gel buffer the specimen's own microclimate. This is the right approach for a handful of sensitive pieces in a room you cannot fix. Condition the gel to the middle of the band rather than using it dry, or you will create the below-35-per-cent fault instead.
Dehumidify the room. Highest running cost, and the option with a trap. A dehumidifier in an unheated room will run continuously and still lose to the overnight temperature drop. And it is the direct cause of the opposite failure: a collection dried to 30 per cent loses its hydrated copper sulphates. If you use one, put a hygrometer in the room and set the target at 50, not as low as it will go. The failure mode is described on why a specimen grows a white crust.
Which specimens to rescue first if you cannot fix the room
If the room is bad and cannot be changed this month, triage. Not everything is at equal risk, and the order is not obvious.
First: iron sulphides. Pyrite and marcasite above 60 per cent relative humidity react with atmospheric water to form hydrated iron sulphate and sulphur trioxide, which forms a weak sulphuric acid. The specimen crumbles to dust, the acid attacks everything stored with it, and the process accelerates. Marcasite is worse than pyrite. This is the only failure on this page that spreads. See why pyrite crumbles.
Second: anything with chloride salts present. The Natural Sciences Collections Association records that chloride-based salts in minerals can combine with pyrite to form hygroscopic ferric chloride, appearing as brown droplets, acidic and corrosive. Brown weeping droplets on a specimen are an emergency.
Third: the labels and the boxes. Above 65 per cent, mould grows, and it grows on paper and card long before it troubles a mineral. A collection can survive a damp winter with its specimens intact and its documentation ruined, which is the worse outcome of the two.
Fourth: other sulphides and the soft hydrated species. Chalcopyrite is more robust than pyrite but is still a sulphide. Langite and the hydrated copper sulphates sit at the other end of the band and are at risk from the fix rather than the fault.
Last, and genuinely fine: the hard silicates and oxides. Kyanite, prehnite, hematite, quartz and the tourmalines are indifferent to humidity and can wait. Do not spend the good storage on them. Our handling and storage page covers the general regime, display covers where a cabinet should go, and material we are looking for is on the wanted list.