Hi sirrith. As you have now found out, it is very easy to overheat the (very thin) tin plating on copper ware. After a bubble forms, it will eventually burst, leaving a bare spot on the copper. Small tinned copper items such as you have were never intended to baked with, more for things like mousses, rice puddings, ice cream molds, and the like. They are suitable for baking in a water bath, like creme caramel (baked caramel custard) though.