Storage and Humidity : Containers, Seals and Control
Storage decisions come down to three variables: what the container is made of, whether it seals properly, and whether humidity inside it is controlled. The third is the one most often ignored and the one that has the largest effect over weeks.
Container materials
Glass is chemically inert, transparent for inspection, and completely non-absorbent. Its only meaningful drawbacks are weight and fragility. Amber or opaque glass adds light protection, which matters because UV exposure degrades organic material steadily.
Metal tins are durable and light-proof, but the great majority use friction-fit lids rather than gasketed seals, and friction fit is not airtight. They are appropriate for short-term or travel storage and poor for anything longer.
Plastic is the weakest option. Many plastics are mildly permeable to air over time, some absorb aromatic compounds into the container walls, and static cling makes retrieval untidy. Where plastic is unavoidable, food-grade polypropylene marked with recycling code 5 is the most stable choice.
Seals
A genuine seal requires a compressible gasket — silicone or rubber — compressed by a clamp or a threaded lid. A screw lid alone with no gasket does not seal; it merely restricts. The straightforward test is to close the container and press the lid: a sealed container resists, because the trapped air has nowhere to go.
Checking a jar: a gasketed clamp jar should require noticeable effort to open the first time after sealing. If it opens freely, the gasket has hardened or is seated incorrectly. Gaskets are replaceable and inexpensive.
Humidity control
Two-way humidity packs contain a saturated salt solution in a permeable membrane. Rather than only absorbing moisture like silica gel, they both release and absorb to hold a container at a fixed relative humidity — commonly 58%, 62% or 65%. This bidirectional behaviour is what makes them useful for organic material, which degrades when it dries out and grows mould when it does not.
The packs are consumable. A pack that has gone rigid has exhausted its capacity and no longer regulates anything. In a well-sealed container of ordinary size, a pack typically lasts two to four months. Attempting to rehydrate a spent pack by adding water is unreliable and risks introducing free water into the container.
Light and temperature
Light and heat degrade organic material faster than air exposure does. A sealed opaque container in a cool cupboard preserves material considerably longer than a clear jar on a windowsill, regardless of how good the seal is. Stability matters too — repeated temperature cycling drives condensation inside the container, which is exactly the condition that causes mould.
Dana Whitlock
Dana Whitlock trained in materials science and spent six years in quality inspection for consumer metal goods. She writes about what accessories are made of and how those choices hold up over time.