Topic: Glass optic phenomenon question

[Lapimano]

Hi,

In this glass i wanted to demonstrate an optic phenomenon, where when held at a certain angle, you can see vertical, or spiral lines inside the glass, when otherwise the surface of the glass is not specifically ribbed, so this should come from internally.
I have seen this phenomenon many times, and I always considered it to be the sign of some kind of modern mass-producing manufacturing technique.
However this glass was advertised as old (but not specified), and has what looks like a polished pontil mark.
Im not sure what is causing this, and importantly it would be nice to use this to get extra information about the piece, like possible era and or location.

I suspect that this topic came here before, but I gave up after a short search, because the keywords I used led to misleading results.

[Ekimp]

Hi, they look like striations or tooling marks from (handmade) manufacture. They are probably very slight imperfections on the surface that are difficult to feel, being small and smooth.

If you search Georgian glass tooling marks or striations, you should come up with an explanation. For example, at the end here: https://www.theworldismadeofglass.co.uk/indentification/18th-century-glass-indentification/manufacturing-process/

Looks quite a nice little glass, c 1810 maybe?

[Paul S.]

it's possible there are two separate effects seen here  -  the tooling marks mentioned by Ekimp and the other being something to do with the shape of this bucket bowl and how we see light through that shape - these other rings being caused possibly by the thickness and how light shines through the glass.     I'm likely not correct - it's just that the very noticeable rings look far too prominent to be caused by the tooling marks.            Don't know the size of the piece, but looks like a bucket shaped rummer with a bladed knop, and quite likely C19.               Let's see  what others think.

[Ekimp]

Yes, those bright parallel lines towards the bottom of the bowl on the side nearest the camera in the second photograph are just to do with how light passes through the glass or bounces around inside it. The lines are probably multiple reflections of the rim or base of the bowl. I forget the physics, refraction, critical angles etc.

[Keith Mick]

Striations on the bucket shaped bowl, with a bladed knop and polished pontil. Difficult to judge size but looks like a gin or dram glass. Early 19th Century around 1820-1830.
Nice find

[Lapimano]

Thank you all.
So its more like an optic effect coming from the shape of the bowl rather than something to do with the inner stirations created by the manufacturing process if I understand it right. That would explain the almost perfectly symmetric lines.
Unfortunately I dont have the more modern pieces where I experienced this pehnomenon first.

[Paul S.]

sounds about right.     You wouldn't see this 'multiple ring' appearance through flat glass, so obviously it's an effect produced by the curvature of the glass and the shape of this bowl.     We can see the tooling marks and they don't appear to produce anything like the pronounced multiple rings, which as Ekimp suggests are a reflection of the rim.