How to determine if ther is a crack or imperfection
This is a ca 1910 fire grenade that has an imperfection that looks like a crack in the neck, but there is no evidence of a break on either surface. Is there such a thing as a "closed" crack?
Comments
I am neither an engineer or a physicist so I cannot speak for what is going on here with great precision but I can say that empirically, collectors see this type of flaw in glass frequently. You can run a fingernail across the area and feel not crack. You may even be able to run something sharper-edged across and feel nothing.
Nonetheless, you do have a crack here. If it refracts glass, then there is some type of fissure in the glass. It may extend part or most of the way through the wall of the bottle but it is there just the same.
Another common flaw are annealing lines in the neck of a bottle. These often only extend part way through the glass.
Hope this has been helpful.