Evidence-based assessment of container suitability for alcohol-preserved natural history specimens

Specimens preserved in alcohol require ongoing monitoring to prevent deterioration, as the preservative is inherently unstable and susceptible to evaporation and concentration changes. The Natural History Museum, London, holds a collection of true flies (order Diptera) comprising 2,596 jars of alcohol-preserved material. Between April 2024 and January 2025, this collection was systematically surveyed to evaluate container performance in maintaining preservative integrity. Sufficient data were collected to compare three jar types: jam jars with metal screw-on lids, jars with glass lids and wire bail clip closures and jars with ground-glass stoppers. A composite comparison of performance, giving equal weight to the following indicators, seized lids, preservative loss, component failure, satisfactory preservation (full jars with 70–80% alcohol concentration) and unit cost. Jam jars with metal screw-on lids demonstrated the highest overall performance. Jars with ground-glass stoppers, although traditionally considered the gold standard, showed high rates of lid seizure and substantially greater cost. Jars with wire bail clip tops performed least effectively, exhibiting poor control of alcohol evaporation. These findings provide evidence-based guidance for selecting cost-effective and durable containers for alcohol-preserved collections.