Found onion bottle glass with "layers"

untbunny

A blogger posted a found onion bottle bottom that leaves me with a question. Knowing most onion bottles are blown, what would cause the layers or striations shown? Is this from adding material during manufacture? Thank you for helping.

Harry_Pristis

I think this sort of delamination occurs when the glass components are imperfectly blended in the glass batch. Some components dissolve faster than others, so if the batch has bands of say soda ash, the band may be transferred to the gather. The bottle blown from the gather may then have a surface vulnerable to chemical attack.
Or, more commonly, I think, if the batch contains slightly too much soda ash, the surface of the bottle may decompose ("sick glass") under attack by salt water or by humic acids. Any other hypothoses out there?

Chris

Thanks so much for posting this - it is a great observation and a great question.

I have seen this layered appearance on many bottle with etching. To me, it is as @Harry_Pristis says, a result of the decomposition of glass.

The link below is a rather technical explanation of what is going on and one fo the best resources I have found on the subject of glass composition, etching, etc. To quote,

"In water, especially salt water, the Na and K carbonates in unstable glass may leach out, leaving only a fragile, porous hydrated silica (SiO2) network. This causes the glass to craze, crack, flake, and pit, and gives the surface of the glass a frosty appearance. In some cases, there is an actual separation of layers of glass from the body."

http://nautarch.tamu.edu/CRL/conservationmanual/File5.htm