How to display a mineral collection
An open shelf exposes a specimen to roughly 4,380 lit hours a year; a cabinet opened weekly, about 9. That 500-to-1 ratio decides what may go on display.
Display is a light budget, not a decision. A crystal specimen on an open shelf receives roughly 4,380 lit hours a year; the same specimen in a closed cabinet opened weekly gets about 9. That 500-to-1 ratio decides which species can be displayed — realgar and amethyst cannot, quartz and hematite can.
In short
- Treat display as an exposure budget you are spending, not a state the specimen is in. The arithmetic below turns “is this safe on a shelf?” into a number, and the number is usually the answer.
- Three species groups should never be on open display. The arsenic sulphides — realgar above all — are destroyed irreversibly by light. Amethyst and green fluorite fade. Hydrated copper sulphates crumble in air that is too dry, which a heated display room often is.
- Put the cabinet on an internal wall. External walls are colder, and relative humidity rises as air cools. The same air that reads 55 per cent at 20 degrees is above 90 per cent at 12 degrees.
- Nothing heavy above head height, and nothing on the wall that shares a stud with a door. Vibration and gravity between them cost more specimens than chemistry does.
- The label goes in the case with the specimen, visible. A display where the labels are in a drawer is a display that will eventually orphan them, and an orphaned label cannot be reunited.
| Species group | Examples | Open shelf? | Glazed cabinet? | Why |
|---|---|---|---|---|
| Arsenic sulphides | Realgar, orpiment | Never | Never — light-proof box only | Light converts realgar irreversibly to a powder |
| Colour-centre species | Amethyst, green and blue fluorite | No | Yes, away from daylight | The colour fades in strong light and does not return |
| Hydrated copper sulphates | Langite, chalcanthite, chalcophyllite | No | Yes, if humidity stays above 35% | They lose water and crumble in dry air |
| Sulphides | Pyrite, marcasite, chalcopyrite | Yes, if the room is heated | Yes | They decay above 60% relative humidity |
| Hard silicates and oxides | Quartz, kyanite, prehnite, hematite | Yes | Yes | Stable to light and humidity; only physical risk |
| Toxic arsenates and lead species | Clinoclase, mimetite, cerussite | Closed box only | Yes, lidded | Handling and dust control, not deterioration |
The light budget: the calculation that makes the decision for you
“Keep light-sensitive specimens out of the light” is advice everyone has read and nobody can act on, because it does not say how much light is too much. Converting it into hours does.
Open display, in a room people use. Assume the room is lit — by daylight, by lamps, or both — for twelve hours a day. Over a year that is 12 × 365 = 4,380 lit hours. A specimen on an open shelf receives all of them.
A closed cabinet with solid doors, opened once a week for ten minutes. That is 52 × 10 minutes = 520 minutes, or about 8.7 hours a year.
The ratio is 4,380 to 8.7, or roughly 500 to 1.
Now the decision is arithmetic rather than judgement. A specimen that would visibly fade in two years on an open shelf would take a thousand years in the cabinet. A specimen safe for twenty years on the shelf is safe anywhere. And the middle case — a glazed cabinet in a room with curtains, lit only when the room is in use, say three hours a day — comes to about 1,095 hours, a quarter of the open-shelf figure. That is the position most collections are actually in.
These are our figures, computed from stated assumptions on this page; substitute your own hours and the method still works. The point is not the precision, it is that the difference between display options is two or three orders of magnitude, not a matter of degree, and that is large enough that no estimate error changes the conclusion.
Which species care is a separate question, and it is answered on which minerals fade in light. This page assumes you have read that and are deciding where things go.
The sequence, from an empty wall to a finished case
Day one, before you buy anything: measure the room. A cheap hygrometer left in the intended spot for a week, read morning and evening, tells you more than any amount of planning. You are looking for a stable reading between 45 and 60 per cent relative humidity, which is the range the Natural Sciences Collections Association gives for store rooms holding mixed geological material, at a temperature between 16 and 18 degrees Celsius. If the reading swings by more than about ten points between afternoon and morning, the room is the problem and no cabinet will fix it.
Step two: pick the wall. Internal walls, not external ones. External walls are colder, and cold surfaces raise local relative humidity. Away from a radiator, which does the opposite and dries the air below the 35 per cent floor that hydrated copper sulphates need. Away from a door that slams and away from the wall a washing machine backs onto.
Step three: choose glazing over openness, and solid doors over glass for the sensitive material. A glazed cabinet cuts dust to almost nothing, which removes the only routine reason to handle a specimen — and handling is where damage happens.
Step four: light the case, carefully. Use LED, positioned outside the case or in its top with an air gap, and put it on a switch rather than leaving it on. Avoid halogen: it produces heat, and heat inside a sealed case drives the humidity down. The rule that follows from the budget calculation is simple — light the case when you are looking at it and not otherwise, which costs you nothing and cuts the annual exposure by an order of magnitude.
Step five: place the specimens by species group, using the table above, and put the labels in the case, readable, with their specimens.
Gravity, vibration and dust: the three things that actually break specimens
Conservation writing concentrates on chemistry because chemistry is interesting. In practice most specimens are lost to three duller causes.
Gravity. Nothing heavy goes above head height. A hematite or cassiterite specimen at 5 to 7 grams per cubic centimetre is far heavier than it looks, and a top shelf that flexes is a top shelf that eventually tips. Deep shelves with the heavy material at the bottom, always.
Vibration. Doors, stairs, washing machines, traffic and floorboards move specimens slowly across a shelf over months. A crystal group standing on three points walks until it reaches an edge. The fix is trivial: a cut Plastazote or felt bed under each specimen, or a museum-wax dot on a flat-bottomed piece. Do not wax a specimen you might want to move — removing it risks the base.
Dust. Dust on a display specimen is why people clean display specimens, and cleaning is where the damage happens. A glazed case makes the question go away. In an open display, the honest position is that you will be dusting, so only put species on open display that tolerate a soft brush — quartz, hematite, kyanite, prehnite — and never a crystallised secondary species. Our page on cleaning specimens explains why the risk is asymmetric.
The quotable rule: a display cabinet is a device for not touching things. Everything else it does is secondary.
What a good display actually shows
A collection arranged well is more interesting than the same specimens arranged badly, and the arrangement is free.
Group by something. By locality is the strongest for a British collection — a Cornwall shelf, a Caldbeck Fells shelf, a Weardale shelf — because the specimens then explain each other, and a visitor who knows nothing learns something from the grouping alone. By species is the alternative, and it suits a collection built around one mineral. By colour is the arrangement that tells the viewer nothing, though it does look the best in a photograph.
Show one thing properly rather than six things badly. Space between specimens is what makes a case read as a collection rather than as a shop window. A shelf with four well-lit pieces and room around them is more impressive than the same shelf with twelve.
Turn the labels outward and make them readable. Species, locality, and the collection number. A visitor reading a locality they recognise is the entire social pleasure of the hobby, and the number is what ties the specimen to the catalogue — see how to label and catalogue a collection for the five-field minimum.
Leave room to grow, and keep a reserve. Most collections are better shown in rotation, with a third of the material in the case and the rest in a cabinet. It protects the sensitive species, it lets you change what is on show, and it stops the display becoming a storage solution. The rest of our collecting guides cover storage and condition, and specimens we are looking for are listed on the wanted list.