Crown glass is thinner than cylinder glass and is thinner from the edge of the disc than from near the centre. Cylinder glass was the only way of making large sheets of glass, so all the glass for the great “Crystal Palace” London International exhibition in 1851 in Hyde Park, London, was made using this method. That was over 1,000,000 square feet of glass!
Which method was used varied over time, with many stained glass windows having glass produced using both methods. The Boppard glass would have been made using these techniques.
The glass tax of 1746, encouraged the use of crown glass as it was thinner and lighter. Crown glass cut from nearer the centre often shows curved ripples and cylinder glass shows parallel ripples. The ripples tend to be on one side only as it is where the outside surface of the original bubble has been slightly compressed while the inside surface has been stretched as the bubble or cylinder opens up into a flat sheet.
The bulls-eye in the centre of the crown glass disc was often returned to the crucible to be melted down again, but was sometimes used as a decorative feature in stained glass windows or small window panes.