Ravenscroft was likely the head and financier of his glassworks but not actively involved in the physical process of making the glass – that role was likely performed by one or more craftsmen in his employ such as Italians Signor da Costa or Vincenzo Pompeio or his English assistant Hawley Bishopp, who set up his own glassworks in the Savoy after Ravenscroft’s death. Ravenscroft’s glassworks produced mainly drinking glasses but also made some bowls and posset pots.
At this point the circumstances concerning Ravenscroft’s role in lead crystal manufacture becomes less clear, partly because records from the mid-17th century are incomplete but largely because Ravenscroft was secretive about his ingredients and processes to prevent competitors from copying him and to seal a deal with the London Glass Sellers’ Company, to which he gave exclusive rights to buy his creations at pre-determined prices.
There is some debate over how, when, and why Ravenscroft got the idea to use lead in the production of glass. Some believe that he accidentally discovered that adding lead to the glass mixture lent the final product special qualities while others believe that he learned the technique while living in Venice. Whatever the origin of the idea, Ravenscroft believed that he had a unique product to offer the English market, so he applied for a patent in 1674 to establish his right to be sole manufacturer of lead crystal glass in England. He only produced lead crystal glass for a period of five years, disintegrating the business in 1679. His patent expired in 1681.
Ravenscroft’s glass works were set up in two locations, the primary facility being established in Savoy, London in 1673 and a secondary, temporary facility set up between 1674 and 1675 in Henley-on-Thames.
Early Ravenscroft glass (1674-1676) developed crizzling (gradual, unstoppable deterioration characterized by numerous cracks, making the glass look cloudy) quickly (within 1-2 years) because of some fault in the type or components of the glass-making mixture; excessive alkaline salts or insufficient amounts of lime, which acts as a stabilizer, have been suggested as possible causes. No early pieces are known to exist today.
The crizzling resulted in damage to the reputation of the company, and Ravenscroft and his team worked to fix the problem. Ravenscroft announced in 1676 that the crizzling problem had been resolved and that the new, improved glass vessels would bear a raven’s head seal to distinguish them from earlier, faulty pieces. A small number of glass vessels bearing the raven’s head seal exist today, some of which have crizzled and some of which have not.
More pieces created by Ravenscroft may exist, but in the absence of the raven’s head seal, which he stopped using in about 1677, or any descriptions or drawings of his designs it is difficult to positively attribute particular pieces to him. Some pieces thought to strongly resemble Ravenscroft’s work bear an “S” seal; some have suggested that the “S” stands for “Savoy,” Ravenscroft’s main production facility, while others believe that the “S” stands for “Southwark,” indicating the South London glassworks of John Bowles and William Lillington.
The addition of lead oxide to the raw ingredients of glass resulted in a melted mixture that had a lower viscosity than ordinary glass, which had the advantage of being less likely to contain air bubbles but made it difficult to blow and made it particularly suitable for blowing into moulds. Lead glass also has a higher refractive index, making it appear sparkling, bright, and brilliant in light, and it “rings” when struck.
|Description||Date of Manufacture||Location||Condition|
|Bowl||1676-1677||Victoria & Albert Museum, London, UK||Crizzled|
|Bowl with Stand||1676-1677||Fitzwilliam Museum, Cambridge, UK||Crizzled|
|Roemer||1676-1677||Victoria & Albert Museum, London, UK||Crizzled|
|Roemer||1676-1677||Corning Museum of Glass, Corning, NY, USA||Crizzled|
|Roemer||1677-1678||Muzeum Narodowe, Warsaw, Poland||Not crizzled|
|Bottle||1676-1677||British Museum, London, UK||Slightly crizzled|
|Jug||1676-1677||Cecil Higgins Museum, Bedford, UK||Crizzled|
|Tankard||1676-1677||Victoria & Albert Museum, London, UK||Crizzled|
|Posset pot||Unknown||Toledo Museum of Art, Toledo, OH, USA||Not crizzled|
|Posset pot||1677-1678||Fitzwilliam Museum, Cambridge, UK||Unknown|
|Data from table above taken from ,, and .|