[Salamander]

Window panes were made by spinning a blob of molten glass on a rod.
I watched a Williamsburg reenacter do it on a TV show once and he said up until very recent times, it was not possible to get more than about a 2 foot circle of "window pane".
Out of that circular more-or-less flat piece of glass, small square or rectangular panes were cut.
That why medieval and ancient windows had so many panes of glass of in them.
It wasn't for "looks"; it was simply because bigger panes of glass weren't possible yet.
My house is almost 300 years old and there are still original panes left in most of the windows.
You can see pontil marks and "bull's eyes" where the glassblower cut it off of the rod.
The glass in the old windows is all "wavy" to some extent and the thickness of each pane varies wildly, even within itself.

I can't imagine where they found a man-sized sheet of glass back then.
[let alone knew how to make photo-negative images]

[mtbopfuyn]

They wouldn't need a man-sized piece of glass. They'd only need to place several end to end.

Using the sun is a simple process which I've done many times to safely bleach out antique fabric. A solution of salt or lemon juice accelerates the process so the 10 period isn't needed.

[A.J.Armitage]

[let alone knew how to make photo-negative images]

That's the part the article explains.