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Identifying ancient glass

Iridescence, bubbles, and weathering.

Buried glass often develops a flaking, rainbow iridescence as surface layers weather — a good (though imitable) sign of age. That shimmer is the object’s own decayed surface, so never scrub or peel it off: with it goes the original face of the glass.

Early glass carries small bubbles and slight colour tints from impurities; perfectly clear, bubble-free glass suggests a later date. Strong colours were deliberate — cobalt for deep blue, copper for turquoise, manganese either to purple the glass or to decolourise it.

Note how it was made: free-blown, mold-blown, or cast. A pontil scar under the base, mold seams on the body, and the way the rim was finished are all datable features.

Weathered glass is fragile and hates swings of humidity. Photograph it before handling much, support it well in storage, and record the iridescence rather than “cleaning” it.

From the guide library inside Collectors Catalogues, a free app for cataloguing antiques and finds with honest, museum-grade records.

More on glass

Dating glass wine bottles — From onion to cylinder — bottle shape is a timeline.

Identifying trade and glass beads — Drawn, wound, and chevron beads that travelled the world.

Identifying Roman glass — Blue-green vessels, ribbed bowls, and iridescence earned underground.

Identifying window glass — Crown bullseyes, cylinder sheets, and painted church fragments.

Identifying vessel glass fragments — Wine glass stems, phials, and the anatomy of broken glassware.

Identifying inkwells, stoppers, and small glass objects — The little glass things that survive whole.

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