Glass that cools too fast keeps a record of it. The outside stiffens first while the middle is still hot and shrinking, so the finished piece holds a quarrel inside itself, and some later knock or change in temperature can set it off. Glassworks prevent this with a lehr, a long tunnel oven the glass rides slowly through: held just warm enough for the stress to let go, then brought down a few degrees at a time. Nothing is added in there. The glass isn’t shaped or polished or inspected. It is only given time to cool evenly, and that is the step that decides whether it lasts.