How to tell blown glass from pressed glass
Updated: 9 hours ago
Blown since the first century BC · pressed by machine since the 1820s
How do you tell blown glass from pressed glass? Two vases can look alike on a shelf, and perhaps only one of them was blown by hand. The difference lies in how the hot glass took its shape.
Blown glass is inflated. The glassmaker gathers molten glass on the end of a pipe, blows into it and works the bubble into shape with tools - or blows it into a mould, where it takes the mould’s form and pattern.
Pressed glass is pushed into shape. A blob of molten glass goes into a metal mould, and a metal plunger presses it out against the walls. The mould makes the outside, the plunger the inside.
Neither is the better glass. Pressing was first done by machine in the United States in the 1820s, and in the words of Encyclopaedia Britannica it became “the single most important factor in making glassware affordable for everyday use”. It also brought patterns of its own: the lacy pressed glass of the late 1820s, covered in dots, leaves and garlands, could not be made by cutting or engraving. Still, how a piece was made is the first fact about it. The search for its maker and its period starts there.
The hand-blown vase with green handles from FIND DROP [01]: an air bubble in the body, a dark speck The base of the small pulegoso vase from FIND DROP [01]. The ring-shaped scar in the centre is the p A cobalt coupe by Clairey from FIND DROP [01]. It is pressed: the fine line across the foot is a sea
01. Feel the inside
In pressed glass the outside and the inside are made by different tools, so they need not match: a pattern can stand out sharply on the outside while the inside stays smooth. In glass blown into a mould, the inside follows the outside. Run a finger inside a ribbed vase. If the ribs come through as hollows, it was blown. If the inside is smooth, it was most likely pressed - or cut, which the edges of the pattern will show.
02. Look for seams
A mould that gives a piece its final form opens in two or more parts, so the piece can be taken out. Where the parts meet, a fine raised line can be left on the glass. Seams are one of the usual signs of pressed glass, although polishing, or reheating the surface, often removes them.
But a seam proves only that there was a mould. Glass blown into a mould of several parts carries seams too. And some blown glass has none at all. In a turn mould the glassmaker keeps turning the pipe while blowing, so the glass does not stick to the mould or take its marks. The piece comes out seamless - and always round. At Blenko, in West Virginia, about eight pieces in ten are shaped this way, in hand-carved wooden moulds.
03. Turn it over
While a blown piece is finished, it is held by its base on a metal rod, the pontil. When the rod is broken off, it leaves a scar on the base, irregular or ring-shaped: the pontil mark. It is the best-known sign of hand-blown glass. Blenko’s own catalogue of 1992 points to it as proof of handwork. Two vases of FIND DROP [01] carry it: a threaded vase attributed to Fulvio Bianconi for Venini, and a small vase of pulegoso, glass full of bubbles.
But there are exceptions both ways. A missing pontil mark does not rule out blowing: many glassblowers grind the scar into a smooth hollow or grind the whole base flat, and glass blown in a mould may never have needed the rod. Nor does a pontil mark prove blowing. Some pressed pieces were put on a pontil to finish the rim. And some early American “pressed” glass, made before the machine press, was in fact blown into the mould, and carries the mark too.
04. Look at the edges of the pattern
Pressed glass and cut glass are easily confused: both have facets. Cutting removes glass with turning wheels of stone, wood and cork, and the surface is then polished. Pressed facets are blunter at the edges, and the pattern is more precise and regular than a cutter’s. Crisp, sharp edges point to cutting; softer edges, perfect regularity and perhaps a seam, to pressing.
05. Do not rely on one sign
Bubbles in the glass are often read as proof of handwork. On their own they prove little: bubbles form in the melt, and they turn up in machine-made glass as well.
“Blown” does not always mean blown by hand, either. Holmegaard’s present owner sells both mouth-blown and machine-blown glass under the name, and marks the mouth-blown pieces with a swan.
And words can mislead. Crystal is not the opposite of pressed glass: it is a popular name for clear lead glass, and in its first decades pressed glass was made of good lead glass, then called flint glass. Carnival glass is inexpensive pressed glass with an iridescent sheen, made in America in the early twentieth century. Holmegaard’s Carneval series of 1970 has nothing to do with it.
Several signs together
No single sign settles it. A smooth inside under a sharp outside pattern, seams, a base without a pontil mark, soft and perfectly regular facets: pressed. An inside that follows the outside, a pontil mark or a ground base: blown. When the signs disagree, the honest word is “probably”.
The glass itself keeps the record of how it was made.
More on two makers of blown glass: Blenko Glass and Holmegaard.Sources: Corning Museum of Glass, Glass Dictionary; Encyclopaedia Britannica, “pressed glass”; The Metropolitan Museum of Art, “Roman Mold-Blown Glass”, 2003; Society for Historical Archaeology, Historic Glass Bottle Identification; Lenore Wheeler Williams, Sandwich Glass, 1922; Bard Graduate Center; Blenko Glass Company, 1992 catalogue; Rosendahl; Wikipedia.
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