Introduction
The history of mould-blown glass is as old as the history of glass blowing itself. From the moment Roman craftsmen discovered that a gather of molten glass on the end of a blowpipe could be inflated inside a mould to produce a vessel of consistent shape and surface decoration, the mould became one of the most versatile and productive tools in the glassmaker's repertoire. Two thousand years later, mould-blown glass remains in production — and the forms and techniques developed in the Roman period, refined through the medieval and early modern periods, and transformed by the industrial revolution of the 19th century, are all represented in the collections of serious glass collectors today.
This guide traces the history of mould-blown glass from its ancient origins to the great period of English production in the 18th and 19th centuries, explains the different moulding techniques and what they produce, and provides practical guidance for the collector on identification and what to look for.
The Origins: Roman Mould-Blown Glass
The technique of mould blowing was developed in the eastern Mediterranean — probably in Syria or Palestine — in the 1st century BC, shortly after the invention of glass blowing itself. The principle is straightforward: a gather of molten glass is partially inflated on the blowpipe, then inserted into a mould — typically made of clay, wood, or metal — and further inflated until the glass fills the mould and takes its shape and surface decoration.
Roman mould-blown glass encompasses an extraordinary range of forms and decorative effects. The most celebrated are the head flasks — vessels moulded in the form of human heads, often with remarkable portrait-like detail — and the ribbed bowls produced by blowing into a ribbed mould and then rotating the vessel to produce a swirled rib pattern. Mould-blown vessels with inscriptions, mythological scenes, and geometric patterns are all found in the Roman period, demonstrating the versatility of the technique from its earliest application.
Roman mould-blown glass is among the most collected of all ancient glass, and fine examples — particularly head flasks and inscribed vessels in good condition — command very significant prices at specialist auction. The iridescence that develops on buried Roman glass — a rainbow-like play of colour produced by surface devitrification during burial — is one of the most beautiful characteristics of excavated examples and is entirely the product of age rather than original design.
Medieval and Early Modern Mould Blowing
The mould-blowing tradition continued through the medieval period, though the surviving material is less well documented than Roman glass. Medieval glassmakers used moulds primarily for producing consistent vessel forms — bottles, flasks, and utility vessels — rather than for decorative effect. The elaborate decorative mould blowing of the Roman period was not revived until the Renaissance.
In the 16th and 17th centuries, Venetian glassmakers developed pattern moulding — a technique in which a gather of glass is briefly inserted into a small mould with a pattern of ribs, diamonds, or other geometric forms impressed into its interior surface. The gather is then withdrawn from the mould and further inflated and worked, expanding and distorting the moulded pattern to produce a subtle surface texture on the finished vessel. Pattern moulding is not the same as full mould blowing — the vessel does not take the shape of the mould, only its surface texture — but it is closely related in principle and is one of the most characteristic decorative techniques of Venetian and façon de Venise glass.
Pattern Moulding in English Glass
Pattern moulding was adopted by English glassmakers in the 17th and 18th centuries and is found on a range of English glass forms, particularly drinking glasses and decanters. The most common pattern moulding on English glass produces:
- Vertical ribs — a series of vertical ridges running up the bowl or body of the vessel, produced by blowing into a ribbed mould and then inflating without rotation.
- Wrythen (twisted) ribs — vertical ribs that have been twisted by rotating the vessel after withdrawal from the mould, producing a spiral rib pattern. Wrythen ribbing is one of the most characteristic decorative effects on English ale glasses and rummers of the 18th century.
- Diamond moulding — a pattern of raised diamonds produced by blowing into a diamond-patterned mould. Diamond moulding is found on some English decanters and drinking glasses of the 18th century.
- Honeycomb moulding — a pattern of hexagonal cells, less common than rib or diamond moulding but found on some English examples.
Pattern moulding on English glass is typically subtle — the moulded texture is present but not dominant, and the overall form of the vessel is determined by the blower rather than the mould. It adds visual interest and tactile quality to the vessel without overwhelming its form.
The Silesian Stem: Moulding in the Stem
The most significant application of mould blowing in English 18th-century drinking glass is the Silesian stem — also known as the moulded pedestal stem or shouldered stem — which was introduced into English glass production around 1714 and discussed in detail in our companion guide on the evolution of the English drinking glass stem.
The Silesian stem is produced by moulding rather than free manipulation: the molten glass is pressed into a mould that imparts both the overall shouldered, pedestal form and any surface decoration — typically moulded panels on the sides of the stem — simultaneously. The earliest Silesian stems bear explicit Hanoverian political decoration — GR cyphers, crowns, and mottoes — moulded into the shoulder of the stem, demonstrating the capacity of the moulding technique to produce detailed surface decoration that would be impossible to achieve by free manipulation.
The Silesian stem is the most historically significant mould-blown element in English 18th-century drinking glass, and the earliest examples — with their explicit Hanoverian moulding — are among the rarest and most historically important objects in the English glass collecting field.
Full Mould Blowing: Decanters and Bottles
Full mould blowing — in which the vessel takes the complete shape of the mould — was used in English glass production primarily for bottles and decanters rather than for drinking glasses. The characteristic three-piece mould decanter of the late 18th and early 19th centuries is the most commonly encountered full mould-blown object in English glass collecting.
Three-piece mould decanters are produced by blowing the gather into a mould made of three sections — typically a base section and two side sections — that together define the complete form of the vessel. The mould seam lines — the faint ridges left where the mould sections meet — are visible on the finished vessel and are one of the primary indicators of mould-blown production. On fine examples, the seam lines are carefully finished and may be difficult to detect; on more utilitarian pieces, they are clearly visible.
Three-piece mould decanters were produced in large quantities in England, Ireland, and America in the late 18th and early 19th centuries, and they are among the most widely collected objects in the antique glass field. The range of moulded patterns is considerable — geometric, floral, and pictorial designs are all found — and the collector who specialises in this area has a very large field to explore.
Press Moulding: The Industrial Revolution in Glass
The most significant development in mould-blown glass in the 19th century was the invention of press moulding — a technique in which molten glass is pressed into a mould by a mechanical plunger rather than inflated by blowing. Press moulding was developed in America in the 1820s and adopted in England from the 1830s onwards, and it transformed the economics of glass production by making elaborately decorated glass available at very low cost.
Press-moulded glass is immediately distinguishable from mould-blown glass by several characteristics:
- Interior surface — press-moulded glass has a smooth, polished interior surface produced by the plunger; mould-blown glass has an interior surface that follows the exterior form, with the same texture as the outside.
- Sharpness of moulded detail — press moulding produces sharper, more mechanical-looking moulded detail than mould blowing, reflecting the greater pressure applied by the mechanical plunger.
- Thickness uniformity — press-moulded glass tends to be more uniform in thickness than mould-blown glass, which varies in thickness as the blower inflates the gather.
- Weight — press-moulded glass is typically heavier than mould-blown glass of the same size, reflecting the greater volume of glass required to fill the mould under pressure.
Press-moulded glass was produced in enormous quantities in Victorian England — the characteristic slag glass (also called purple slag or marble glass), Vaseline glass, and the pressed glass tableware of the major Midlands and North-East manufacturers are all press-moulded. It is a distinct collecting field from blown glass, with its own specialist literature and collector community.
Identifying Mould-Blown Glass: What to Look For
- Mould seam lines — the most reliable indicator of mould-blown production. Look for faint ridges running vertically up the vessel where the mould sections meet. On fine examples these may be polished away; on more utilitarian pieces they are clearly visible.
- Surface texture consistency — moulded surface decoration — ribs, diamonds, panels — is perfectly consistent around the vessel, reflecting the precision of the mould. Free-blown decoration is slightly variable.
- Interior surface — in mould-blown glass, the interior surface follows the exterior form. In press-moulded glass, the interior is smooth regardless of the exterior decoration.
- Pattern regularity — moulded patterns are perfectly regular; hand-applied decoration is slightly variable. A pattern that is absolutely consistent in every repeat is almost certainly moulded.
- Pontil mark — mould-blown glass, like free-blown glass, is finished on a pontil rod and will show a pontil mark on the base. Press-moulded glass may or may not show a pontil mark depending on the finishing process.
Collecting Mould-Blown Glass
Mould-blown glass offers the collector an unusually wide range of entry points — from Roman head flasks at the top of the market to Victorian three-piece mould decanters at very accessible prices. The key collecting areas within the broader field include:
- Roman mould-blown glass — the most historically significant and the most expensive. Head flasks, inscribed vessels, and ribbed bowls in good condition are the primary collecting targets.
- English pattern-moulded drinking glass — wrythen-ribbed ale glasses and rummers of the 18th century are among the most accessible and attractive collecting targets in this area.
- Silesian stem glasses — particularly the earliest examples with Hanoverian moulding; rare and historically significant.
- Three-piece mould decanters — a large and varied field with examples at all price points; Irish examples are particularly sought after.
- Victorian press-moulded glass — a distinct collecting field with its own specialist community; slag glass and Vaseline glass are the most actively collected categories.
The Wonder Room occasionally offers mould-blown antique glass across several periods and traditions. Browse our Georgian glass collection or explore further guides in Collector's Insights.