In March 1674, the English glassmaker George Ravenscroft applied for a patent to make colorless lead glass. Unfortunately, this glass was prone to crizzling, a chemical instability that results in an attack by atmospheric moisture, producing a network of cracks in the surface. Ravenscroft revised his formula, adding more and more lead oxide to the batch. By mid-1676, he announced that his improved glasses were to be marked with a seal, a custom known from several glasshouses that made lead glass. His stamp featured a raven's head, taken from his family's coat of arms. Corning's rare Ravenscroft goblet was made with the new lead formula. It is decorated with mold-blown ribbing that is pinched to form a mesh design on the bowl. This pattern is called "nipt-diamond-waies." One of the prunts at the bottom of the stem is stamped with a raven's head.