Why is my specimen growing a white crust?
Four different faults produce a pale crust, and the humidity causing each is different. Below 35% dehydration, above 60% sulphide decay, above 65% mould.
Four different faults look alike. Below 35% relative humidity, hydrated species dehydrate to powder. Above 60%, sulphides oxidise to white, yellow or green efflorescence with a rotten-egg smell. Above 65%, mould grows. And soluble salts from mine material creep out at any humidity. The cure for each is different.
In short
- Smell it first. A sulphurous, rotten-egg smell alongside a white or yellow crystalline crust means sulphide decay, and that specimen needs isolating today. No smell moves you to the other three causes.
- A crust that appears in a dry room is usually dehydration, not decay. Hydrated species lose their water below about 35% relative humidity — deep blue copper sulphate crystals crumble to a pale blue powder in those conditions. Adding a desiccant makes this worse.
- Fuzzy and filamentous rather than crystalline means mould, which grows above 65% RH. It is usually feeding on the packing, the label or the adhesive rather than on the mineral.
- Brown droplets are a specific and serious signal. Chloride salts in a specimen can combine with pyrite to form hygroscopic ferric chloride, which appears as brown droplets, is acidic, and causes corrosion.
- The general store-room target is 45–60% RH at a stable 16–18 °C, but no single figure suits everything — which is the actual reason mixed collections develop crusts. Sulphides want it drier and hydrates want it wetter.
| What you see | Smell | Humidity that causes it | Cause | What to do |
|---|---|---|---|---|
| White, yellow or green crystalline efflorescence; hairline splitting; specimen crumbling | Sulphurous, rotten eggs | Above 60% RH | Sulphide oxidation — pyrite, marcasite, pyrrhotite and others reacting to hydrated sulphates and a weak sulphuric acid | Isolate today. Move to a sealed box with conditioned silica gel below 45% RH. Irreversible; you are arresting it, not repairing it |
| Pale powder replacing a formerly bright crystal, often on a soluble or hydrated species | None | Below 35% RH | Dehydration. The specimen has lost water of crystallisation | Raise the humidity, do not lower it. Remove any desiccant. Damage already done is not reversible |
| Fuzzy, filamentous, greyish or greenish growth, often on card, labels or packing | Musty | Above 65% RH | Mould | Dry the environment below 60%, remove and replace the packing, and check every neighbouring box |
| Brown droplets on the specimen surface, sometimes sticky | Faintly acrid | Damp, any warm still air | Chloride salts combining with pyrite to form hygroscopic ferric chloride — acidic and corrosive | Isolate immediately. This attacks the specimen, the label and anything metal nearby. Seek conservation advice |
| Fine white bloom on a carbonate near decaying sulphides | Sulphurous nearby | Above 60% RH in a shared drawer | Acid etching. The acid from a neighbouring specimen is attacking the carbonate | Separate the two. The fault is in the drawer, not in the carbonate |
| White powdery salts on rough mine or dump material | None | Any | Soluble salts left from the mine environment migrating to the surface as the specimen dries | Usually harmless and often removable with distilled water — but test on an inconspicuous area first |
Diagnose in this order: smell, then humidity, then texture
The four faults look similar in a photograph and are easy to tell apart in person, provided you check in the right order. Doing it the other way round — guessing from appearance and then reaching for a remedy — is how people add a desiccant to a dehydration problem and finish the specimen off.
First, smell it. Sulphide decay produces sulphur oxides, and the smell is unmistakable and specific. If a white or yellow crystalline crust comes with a rotten-egg smell, you have sulphide oxidation and nothing else on this page. That is a same-day problem because the acid it produces attacks the label, the packing and every carbonate in the same drawer.
Second, find out what the humidity actually is. Not what you assume it is — measure it. A cheap hygrometer settles the question in an hour and it eliminates two of the four causes immediately, because dehydration and mould sit at opposite ends of the scale. This is the step people skip, and it is the one that determines whether the correct action is to add moisture or remove it.
Third, look at the texture under magnification. Crystalline efflorescence has form — needles, blades, a sugary crust. Mould is filamentous and irregular. Dehydration produces a structureless powder in the shape of the crystal that used to be there. Salts from mine material tend to be patchy and concentrated in cavities and along fractures.
The rule: identify the humidity band before you choose the remedy, because two of these four faults have opposite cures.
Sulphide decay — the one that spreads
This is the serious one and it is worth recognising quickly. Above 60% relative humidity, iron pyrite reacts with atmospheric water to form hydrated iron sulphate and sulphur trioxide, which forms a weak sulphuric acid. Other sulphide species deteriorate by similar reactions at high humidity. The damage is described in the conservation literature exactly as collectors see it: white, yellow or green crystalline efflorescence, a rotten-egg smell, and the occasional collapse of specimens.
What makes it urgent is not the specimen, it is the neighbours. The acid attacks paper, so labels go. The efflorescent sulphates migrate within a closed drawer and settle on whatever else is in there, and any carbonate they land on — calcite, cerussite, smithsonite, malachite — will etch. A single decaying pyrite in a shared cabinet is a slow chemical accident affecting everything around it.
The remedy is arrest, not repair. Manual removal of the decay products combined with chemical treatment by a professionally qualified conservator is possible where a specimen has been damaged, but reconstruction of the original specimen may not be. What a collector can do is drop the humidity: storing specimens below 45% RH slows the decay markedly.
The practical method is a microclimate rather than a dehumidified room. Seal the specimen in a rigid box with a pierced bag of oven-dried silica gel, packed in a Plastazote cut-out. That keeps the sulphides dry without imposing a damaging low humidity on the rest of the collection. Our page on why pyrite crumbles covers the chemistry and the humidity bands in full.
The opposite fault: crusts caused by air that is too dry
Every piece of advice about mineral storage tells you to keep specimens dry, and for sulphides that is right. Applied to a whole collection it causes the second fault on this list.
Hydrated minerals carry water in their crystal structure, and below roughly 35% relative humidity they start to give it up. The documented example in the conservation guidance is stark: deep blue copper sulphate crystals lose water and crumble to a pale blue powder below 35% RH. Chalcanthite is the obvious species, but the principle covers a long list of hydrates — the uranium micas, borax, mirabilite, melanterite, many of the secondary sulphates and some zeolite-adjacent species.
The signature is that the crust is the specimen. In sulphide decay, a new material grows on and out of the crystal. In dehydration, the crystal itself turns to powder in place, keeping roughly its own outline. Colour usually pales rather than yellows.
This is where a collector does real damage with good intentions. Discovering a crust, concluding it is decay and adding desiccant accelerates a dehydration problem sharply. It is also why a whole-room dehumidifier is a blunt instrument for a mixed collection: shale dries and becomes brittle at low RH, and sub-fossil material should not go below 45%.
There is no single humidity that is correct for a mixed collection. Keep the room in the 45–60% band, then create dry microclimates for the sulphides individually. That resolves a genuine conflict rather than trading one fault for the other. See handling and storage.
Mould, salts and the crusts that are not a mineral problem at all
Two of the four causes are not really about the mineral, and both are commonly misread as decay.
Mould grows above 65% RH. It is filamentous and fuzzy rather than crystalline, it usually appears first on the organic material in the box — the card tray, the label, the cotton wool, the adhesive — and it smells musty rather than sulphurous. It is a straightforward environmental problem: bring the humidity below 60%, discard and replace the affected packing, and inspect neighbouring boxes, because if one box is above 65% the others probably are too. The mineral itself is usually undamaged.
Soluble salts are the most benign case and the most often panicked over. Material collected from mine workings, dumps and coastal exposures carries salts from its environment, and as the specimen equilibrates indoors those salts migrate to the surface and crystallise as a white bloom, typically in cavities and along fractures. There is no smell and no ongoing reaction. On a robust specimen these often wash off with distilled water, but test on an inconspicuous area first, because the underlying mineral may be soluble, friable or water-sensitive. Our cleaning guide sets out where that line falls.
The exception in this group is not benign at all. Brown droplets mean chloride salts have combined with pyrite to form hygroscopic ferric chloride, which absorbs moisture from the air, is acidic and causes corrosion. That is a self-sustaining reaction and it needs isolating at once and, realistically, professional advice.
Finally, worth saying plainly: if a crust appears on a specimen you have just acquired, the fault is very often the change of environment rather than the specimen. A piece that has spent thirty years in a stable cabinet and a fortnight in your porch has been asked to re-equilibrate. When you are chasing species with known conservation requirements, saying so up front is genuinely useful — it is the sort of constraint worth recording on a wanted list, because it changes which specimens are suitable rather than merely which are attractive.