Optic-blown glass

Optic-blown glass has a pattern formed while the glass is hot, using a patterned mould followed by further blowing. Ribs and undulating structures are examples. Here, “optic” describes a decorative forming technique.

Pattern before final shape

The Corning Museum of Glass explains that an optic mould or dip mould imparts a pattern to the hot glass gather. The gather is withdrawn, then further blown and tooled to the desired size and shape. The pattern mould therefore need not determine the complete final shape.

Ribs can also be twisted before further inflation, producing a spiral. The Toledo Museum of Art describes a vase given ribs in one mould, subsequently blown in a plain mould and twisted. This is a documented process, not a formula applicable to every ribbed glass object.

A smooth exterior can conceal an optic pattern

Hadeland Glassverk describes its current Optikk crystal lighting globes as formed using upright blades followed by a smooth graphite mould. The result is a smooth exterior and an undulating interior. This product description documents the method for those lighting components, not every older piece of Hadeland glass.

Distinguish this technique from pressed glass, where a plunger presses glass into shape. Ribs alone do not identify a maker or date.

Practical suggestion: Photograph through the glass against a plain background, as well as the exterior, base and any marks. Explore glass and crystal at OBA, or send photographs for assessment and possible purchase.