Beaker with Chalcedony Glass and Aventurine

Chalcedony, the Venetian glass that imitates the natural stone chalcedony, was made in the 18th century, as we see here, and around 1500, as we see here. Chalcedony is a glass made with silver.  It’s poorly mixed; it’s an inhomogeneous glass.  On the left and right, you see crucibles that were used to melt chalcedony.  Here they’ve been cross-sectioned, and it’s very easy to see how very inhomogeneous the glass is. Aventurine often accompanies chalcedony in 18th-century objects.  Aventurine is a glass made with copper, and as the glass cools, the copper crystalizes, forming these sparkly features. Here, a small piece of aventurine has been melted to the end of a blowpipe, and it’s being inflated slowly.  The bubble is pulled free of the blowpipe.  Cold water causes the glass to crack, and the aventurine glass is broken into small pieces. Chalcedony glass is gathered from a small crucible.  To get enough glass for the vessel, the glass is gathered twice.  The gather is elongated and marvered to make it perfectly round and concentric with the blowpipe.  As the glass cools, the crystals form, and the glass becomes cloudy. Here, the mass is rolled over fragments of aventurine.  The fragments are reheated, marvered to smooth the surface. A bubble is blown, held from the end, pulled, and elongated.  A large gather is added to the tip of the bubble and tooled to become a foot.  This glass cools very quickly, and the foot will be a very different color than the vessel. After transfer to the punty, the object is given its final profile. Chalcedony is a glass very sensitive to its thermal history.  Different parts are different colors because they cool at different rates.  The interior of the vessel on the left was thinly lined with clear glass, and it has a very different appearance than the one on the right, which is bare chalcedony glass.  Chalcedony is also very sensitive to the atmosphere in which it is reheated.