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In the spell detect magic it states that there are things which can block the spell.

The spell can penetrate barriers, but 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt blocks it.

I don't know what know what glass in pathfinder is likely to be made of, but when I think of old glass (which is how I view most stuff from pathfinder) I think of lead glass. To be honest, lead glass is only about 400 years old, so its not the original glass, and I don't know what is or what it would be made of.

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So, what is lead glass? let's look up what it is...

Lead glass, commonly called "crystal", is a variety of glass in which lead replaces the calcium content of a typical potash glass. (wiki)

Is it realistic?

Now, this helps us to know what is mixed when making lead glass, but the common glass before 1700 AD north of the alps was forest glass. This Glass used Potash in mixture with Calciumoxide, often tinted green by iron. So much for lead glass being common. It is by far not unheard of, but not the most common type.

But we want lead glass!

But let's assume someone really wanted to make lead glass for they wanted to sculpt it into fancy shapes or because they are paranoid... What would we have in terms of lead content? Let's assume we take some common (more or less contemporary) glass types and just replace the calcium with lead. Let's look it up.

Glass
[O]rdinary glazing and container glass is formed from a specific type called soda-lime glass [...] (wiki)

Container glass: 10.5 % CaO
Flat glass: 9 % CaO (wiki)

Forest Glass:
Lime, CaO: 4/95 (15th century BC) 8/93 (1st century BC) 12/92 (13th century AD) 10/97 (14th century AD) 10/195 (modern) (wiki)

So, about 5% calcium content as a bare minimum for historical glasses, with something around 10% for more 'contemporary' medieval glasses and up to 13%. Just replacing that with lead allows us to estimate a 10% regular content. But there is also more!

Lead Glass:
Crystal can consist of up to 35% lead, at which point it has the most sparkle. (wiki)

So, how thick?!

Taking this into account, then a lead window would need to be between 3 to 20 times as thick as a lead sheet to block detect magic. "Thin lead sheet", however, is undefined, so I will assume 0.5 mm or less for a lead sheet, which is about the thickness used to line roofs.

Common window planes nowadays are 3.175mm (1/8th inch), but we'd need slightly thicker windows of 5mm to keep people from prying with an average 10% lead salts - which is by far not out of question for medieval stained glass windows.

With 5% lead content, we go towards the rare - but not unheard of - 10 mm windows.

For really high lead content (35%), 1.5 mm - which is about as much as watching through common glasses nowadays - would be enough.

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There's quite a lot of lead in lead glass, so my read would be that a pane of the stuff would be easily equivalent to "a thin sheet". Lead glass is a lot older than 400 years, but probably nobody would have made panes out of it. In the medieval-Europe period where most of the D&D lore comes from, glass panes would have been soda lime, and wouldn't have contained any lead.

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