The chemical makeup of glass affects what happens when different colors or elements are melted together. The results are somewhat predictable with experience. But sometimes other factors result in unexpected reactions. This sets the stage for the thrill of the hunt! What will happen if I try this? These plates have a border of silver foil on the edges. When fused in the kiln at 1460 degrees the silver reacts with sulfur in the glass and the resulting fumes color the glass in a halo emanating from the silver.