Humidity, temperature and silica gel: ideal conditions for a coin collection

Where to keep coins at home, what humidity is safe and how to use silica gel properly

BulgarianCoins editorial team 19 October 20263 min read

BNB silver proof coins – “200 Years Since the Birth of Nayden Gerov” and “Dryanovo Monastery”

Most damage to coins in home collections does not come from drops or scratches, but from the air around them. Moisture accelerates the tarnishing of silver, causes spots on proof coins and feeds bronze disease on copper. The good news is that the right conditions are easy and cheap to achieve.

Target values

Parameter	Recommendation	Why
Relative humidity	35–45%, never persistently above 50%	Above 50–60% corrosion and spots accelerate
Temperature	Stable, 16–22 °C	Stability matters more than the exact value
Temperature changes	Minimal	Sudden cooling leads to condensation on the metal
Light	Dark	Light warms and accelerates reactions in the packaging
Air	Clean, without fumes	Paints, adhesives, cleaning products and sulphur attack the metal

Where at home – and where not

Good places:

a wardrobe or cupboard in an interior room;
a drawer away from outside walls and windows;
a home safe in a heated room (with a moisture absorber inside).

Places to avoid:

the basement and the attic – large swings in temperature and humidity;
the bathroom and the kitchen – steam and grease;
next to radiators, air conditioners and stoves;
next to outside walls, where condensation forms in winter;
the garage – fumes from fuels and chemicals.

Silica gel: how to use it properly

Silica gel absorbs moisture from an enclosed space. For it to work:

Use it in a closed box or case. In an open cupboard it simply becomes saturated within days.
Prefer indicating silica gel. It changes colour when saturated – for example from orange to green.
Regenerate it. Most types are dried in an oven at a low temperature (usually around 100–120 °C). Follow the manufacturer’s instructions.
Do not put it in direct contact with the coins. The sachet should be next to them, not on them.
The quantity depends on the volume. For a standard coin box, a few sachets of 10–50 g are usually enough if the box is opened rarely.

Hygrometer

A small digital hygrometer costs about as much as an ordinary coin and tells you what is happening in the box or safe. Put it inside for a week and check the readings. If the humidity stays above 50%, add silica gel or change the location.

Condensation: the silent enemy

When a cold coin enters a warm room – for example brought from a cold car or taken out of a bank safe in winter – moisture condenses on it. Let the coins warm up in their closed packaging before you open or examine them.

BNB proof coins

BNB commemorative coins are sold in a capsule and box, with a certificate. The capsule protects them well, but is not airtight. In a humid environment, white or dark spots may appear on silver proof coins over the years. So:

keep the boxes in a dry, dark place;
do not keep them in a damp basement or near the kitchen;
do not open the capsule unnecessarily. See also why you should keep the certificate and the packaging.