How to clean pyrite safely
Salt is the most-suggested way to clean pyrite and the one that guarantees decay. Chloride plus pyrite makes hygroscopic, acidic ferric chloride on a crystal.
Brush it dry and stop. Do not use salt: the published conservation guidance records that chloride-based salts combine with pyrite to form hygroscopic, acidic ferric chloride, which appears as brown droplets and causes corrosion. Keep a pyrite crystal below 45% relative humidity and never above 60%, because above 60% it reacts with atmospheric water on its own.
In short
- The commonest suggested method is the one that destroys the specimen. How to clean pyrite with salt is among the top autocomplete suggestions for this question. The published conservation position is the opposite: chloride-based salts combine with pyrite to form hygroscopic, acidic ferric chloride.
- Hygroscopic means self-sustaining. Ferric chloride pulls water out of the air, so a salted specimen does not simply get damaged once — it acquires a mechanism that keeps pulling in the reagent it needs to carry on.
- Above 60% relative humidity pyrite reacts with atmospheric water unaided, forming hydrated iron sulphate and sulphur trioxide, which make a weak sulphuric acid. Below 45% RH the reaction is much slower. That is a storage instruction, not a cleaning one, and it does more good than any cleaning will.
- Pyrite is hardness 6 to 6.5, so it is not fragile to a brush — a stiff dry brush will not scratch it. What damages pyrite is chemistry and humidity, not handling.
- If it is already decaying, cleaning is the wrong question. Powdery yellow-white efflorescence, splitting along growth surfaces and a sulphurous smell mean the specimen needs stabilising and isolating, not washing. That is covered on why is my pyrite crumbling.
| Method | Verdict | What happens |
|---|---|---|
| Dry brushing with a soft or stiff brush | Do this | Removes loose dust and clay. At hardness 6 to 6.5 pyrite is harder than any bristle |
| Salt, brine, salt-and-lemon, or any chloride | Never | Chloride plus pyrite forms hygroscopic acidic ferric chloride, which corrodes and keeps going |
| Tap water soak | Avoid | Most tap water carries chloride, and any water raises the local humidity in the specimen's pores |
| Brief distilled-water rinse, then thorough drying | Only if necessary | Acceptable for surface clay on sound pyrite. The drying matters more than the rinse |
| Acid of any kind | Never | Pyrite oxidation is already acid-generating. Adding acid accelerates precisely the reaction that is destroying it |
| Ultrasonic bath | Never | Drives liquid into microcracks and shakes crystals off matrix. Loss is mechanical and immediate |
| Oiling, waxing or lacquering | Not a clean | Seals moisture in rather than out, hides the early warning signs, and is a disclosable alteration |
| Silica gel and a sealed box at 40 to 45% RH | The real answer | Slows the reaction that all the above are trying to treat the symptoms of |
The documented harm, in the words of the people who wrote it down
Most cleaning advice on the internet is somebody's habit. This one has a published source, and it says something specific enough to act on.
The Natural Sciences Collections Association's Care and Conservation of Geological Specimens states that chloride-based salts in minerals can combine with iron pyrite, forming hygroscopic (moisture absorbing) ferric chloride which appears as brown droplets on specimens; it is also acidic and can cause corrosion.
Read that as three separate problems stacked on one specimen. It is a reaction, so the pyrite is consumed. It is acidic, so it attacks anything else on the matrix — carbonates first, then the label if the two share a box. It is hygroscopic, so it pulls in the water it needs from the air, which means adding salt once installs a process rather than causing an event.
The same document gives the humidity thresholds: above 60% RH pyrite reacts with atmospheric water to form hydrated iron sulphate and sulphur trioxide, producing a weak sulphuric acid, and pyrite-bearing specimens should not be stored in conditions above 60% RH and preferably should be stored in conditions of 45% RH or lower.
So the harm is not hypothetical and the numbers are not ours. A collector who cleans pyrite with salt is combining the reagent the literature names with the humidity band the literature warns about.
What to do instead, in order
1. Do nothing first. Look at the specimen in good light and decide whether it is dirty or decaying. Dust and clay are cosmetic. A pale yellow-white powder, splitting, brown droplets or a sulphurous smell are not, and they change the whole plan — see why is my pyrite crumbling.
2. Dry-brush it. A stiff nylon brush, worked from the matrix outwards, under good light. Pyrite at hardness 6 to 6.5 outranks every bristle you will find. This removes most of what people reach for water to remove.
3. If clay remains, consider a short distilled-water rinse — and the drying is the operation. Rinse briefly, blot, and then dry thoroughly and slowly: warm air movement for hours, not a hairdryer for minutes. Water trapped in a vug is worse than the clay was. Clay removal in general is on how to remove clay from a mineral specimen.
4. Then control the humidity and leave it alone. A sealed clear box with conditioned silica gel at 40 to 45% RH, checked with a cheap hygrometer, does more for a pyrite than any amount of cleaning. The full storage argument, including the species that need the opposite conditions, is on how to store soluble mineral specimens.
Never: salt, brine, vinegar, lemon, hydrochloric or any other acid, ultrasonic cleaners, or oil and lacquer coatings. The first six accelerate the decay reaction; the last hides it.
Why so much bad advice attaches to this particular mineral
Three things collide here, and it is worth naming them because the pattern repeats elsewhere.
The suggestions are dominated by a different object. Autocomplete on how to clean pyrite returns bracelet, anklet, stone and crystal alongside specimen. Much of the advice in circulation is written for tumbled pieces and jewellery, where the owner's tolerance for surface loss is entirely different from a collector's.
Salt is the default folk remedy for everything. It is in the house, it is cheap, and it has a long association with cleansing and with crystal-healing practice. None of that is chemistry, and pyrite is the species where the mismatch is most expensive.
The damage is slow enough to break the feedback loop. A salted pyrite looks fine for weeks. By the time the brown droplets appear, the cleaning has been filed as a success and recommended to somebody else. A harm that takes six months to appear will be repeated indefinitely.
The same structure explains bad advice about several other species. Where it matters most is with material that is already unstable, and the general treatment is on how to clean mineral specimens. The rest of this section is indexed at collecting guides; we hold no stock and sell nothing, and the only commercial route here is the wanted list.
Marcasite, pyrrhotite and the other sulphides in the same box
Everything above applies with more force to marcasite. It is the same formula, FeS2, in a different structure, it forms under low-temperature acidic conditions in shales, limestones and coals, and it is the classic decaying specimen in museum collections. The conservation leaflet's cover image is pyrite decay on ammonites.
The comparison that matters for identification is on is marcasite the same as pyrite: identical Mohs hardness, densities 2.7% apart, and a Vickers hardness that differs by nearly 40%.
Pyrrhotite behaves similarly and is often worse. The Care and Conservation of Geological Specimens puts it generally: other sulphide mineral species can also deteriorate via similar reactions at high relative humidities. If one sulphide in a drawer is decaying, the acid it produces is in the air the rest of the drawer is breathing. Isolate a decaying specimen in its own sealed box the day you notice it, and keep carbonates and labels away from it.
None of this is conservation advice for an institutional collection. For a specimen that matters, the right next step is a conservator; the leaflet cited above is published by the Natural Sciences Collections Association for exactly that purpose.