Mouth-Blown or Pressed Glass? The Marks That Give It Away

Mouth-blown or pressed glass is recognised by the marks left by manufacture. A mouth-blown glass often has a pontil mark under the foot where the pontil was attached, small irregularities and air bubbles and, if it is free-blown, no mould seams. Pressed glass has thin, raised seams where the parts of the mould met, an even pattern with rounded edges on the outside and a smooth inside, because a plunger pressed the glass against the mould.

Facts

Two ways of shaping glass

The glassblower gathers a lump of molten glass on the blowpipe, an iron tube, at about 1,100 °C and blows it into a hollow bulb. The bulb is shaped with tools, in a mould of wet wood or freely in the air, and reheated between stages. When the blowpipe is knocked off, the glass sits on an iron rod, the pontil, while the mouth is shaped. This is how glass has been made since antiquity, from the Venetian goblet to today’s art glass.

Pressed glass is made in a quite different way. A set quantity of glass falls into an iron mould, and a plunger is pressed down and forces the glass out. The mould gives the pattern on the outside and the plunger the form of the inside. The technique was developed in the USA in the 1820s and made patterned glass cheap, in the age of industrialisation. The glass diagram further down shows both methods step by step.

The marks of the blowpipe

The surest sign of a mouth-blown glass is the pontil mark under the foot, where the pontil was broken off. The mark may be an uneven, sharp break, or a ground, polished hollow if it has been ground away. A ground hollow is thus also a sign of mouth-blowing. Older glass often has small air bubbles and a slight tint in the glass, like the English wine glass with an air twist.

Free-blown glass has no mould seams, and two glasses from the same series are never quite identical: height, mouth and foot vary slightly. The thickness of the walls may vary, and the foot is seldom completely flat. Art glass is almost always mouth-blown, like Orrefors’s Graal glass, where the pattern lies inside the glass.

The marks of the mould

Pressed glass is recognised by its mould seams: thin, raised lines along the sides where the parts of the mould met. The inside is smooth, because it was shaped by the plunger, whereas the outside has a sharp but even pattern. The pattern often imitated cut glass, but the edges are rounded and soft instead of sharp, and the pattern is exactly the same on every piece. The surface may be slightly dull or have small folds where the glass cooled against the mould.

The early pressed glass from Sandwich in Massachusetts was given a dense stippled pattern that glittered and hid the irregularities. The technique spread quickly to Europe, and Swedish glassworks such as Reijmyre and Kosta made bowls, decanters and dishes in great quantities, like the pressed-glass bowl. The moulds were used for decades, so the same pattern may have been made many years apart.

Mould-blown and machine-blown

Between free-blown and pressed there is mould-blown glass, which is blown into a mould. It has seams like pressed glass, but because it is blown the inside follows the pattern of the outside: where the outside bulges out, the inside bulges too. The Finnish Riihimäki made colourful mould-blown glass in the 1960s and 70s. Lalique’s glass too was press-moulded or mould-blown, like the vase Bacchantes; see the guide to Lalique.

In the 20th century machines took over utility glass. Machine-blown bottles have a seam that runs right over the mouth, and industrial utility glass has even wall thickness and a machine-pressed foot. Green and brown bottle glass with air bubbles and uneven walls, on the other hand, is older utility glass.

What the technique says about age

The technique gives a time frame. Free-blown glass has been made for two thousand years and can date from any period. Mould-blown glass existed in antiquity but became common again in the mass production of the 19th and 20th centuries. Pressed glass cannot be older than the 1820s, and machine-blown utility glass belongs to the 20th century.

The decoration narrows the date further. Deep cutting in lead glass became common towards the end of the 18th century, acid etching in the 19th century and was used in Art Nouveau cased glass, and Graal and Ariel are Orrefors techniques from 1916 and 1937. The combination of technique and decoration, together with the signature, usually gives a safe dating.

Cut, pressed or blown?

Three questions will take you a long way. Are there seams? Then the glass is pressed or mould-blown. Is the inside smooth while the outside has a pattern? Then it is pressed. Does the pattern have sharp, polished edges that cut against the finger? Then it is hand-cut, often in lead glass, and it is probably mouth-blown first; the guide to crystal and glass explains the cutting.

Pressed glass is no worse than mouth-blown glass, just made for a different market, and early pressed glass is collected in its own right. But if you want to date art glass, the signature is the next step: see the guide to glass signatures. The page on glass has the history of the techniques, and the terms are in the glossary.

How to recognise it

Exercise

A bowl has thin, raised lines along the sides, an even pattern with rounded edges on the outside and a completely smooth inside. How is it made?