The pressed glass technique was invented in the United States some time in the 1830s and various firms took out a number of patents covering the technical aspects of producing this.
This is an extract from Apsley Pellatt’s book “Curiosities of glass making with details of the processes and productions of ancient and modern ornamental glass manufacture” 1849
AMERICAN PRESSED GLASS.
Pressing is a mechanical operation, unknown to the
ancients or the Venetians.* It may be thus described:—a die being prepared, secured by the ring and handle, A, metal is gathered and dropped into it, B, and the matrix, or plunger, c, operated upon by the lever, &c D, presses the metal into the required form of the article. If an overplus of metal be gathered, it thickens the article throughout; but if too little, it fails to fill up the mould, and is spoiled. This is a rapid mode of reproduction, but great practice is required to gather the exact quantity of metal. The chief condition of success, in getting a polished surface on pressed Glass, depends upon the moulds being kept at a regular temperature, a little short of red heat. The effect is not so good as pillar-moulding, nor does it appeal so well; but it is much less expensive. The interior plungers and the outer die will adhere to the Glass if too hot; and if not at a proper temperature, will fail in producing a clear transparent surface.
In the illustration we can see the man on the right who might be the “gaffer” or experienced glass manipulator, with his runner on the left providing a gather of glass. The gaffer is seen cutting of the required amount of molten glass before bringing the plunger down and forming the article. The glass article looks like a fairly simple small bowl from an open perhaps one piece mould but much more complex items were made in three or four part moulds.
The text mentions the need to keep the mould and perhaps the plunger at close to the temperature of molten glass in order to prevent a poor surface on the article. The illustration however shows a very primitive set up where this would likely be impossible unless the mould was used repeatedly to keep it hot. Perhaps this was what was done until machine pressing was developed and the metalwork kept hot by other means.
Before the advent of pressed glass the gaffer sat in his seat and his team rotated around him, providing new glass, taking away finished articles and doing whatever he wanted. He was the skilled glass blower and finisher and the ability of these skilled men was challenged by the new machinery of production. The English glass houses had very poor industrial relations and frequent disputes and stoppages which were a contributory factor in their steady decline.