On Tue, 18 Jun 2019 13:38:35 +0100, Jeff Layman
wrote:

My understanding is that it is due to old glass having many micro-cracks
and scratches which weaken it and assist in crack propagation throughout
the glass once it starts. Modern glass (and more expensive glass than
the "agricultural" grade glass often used for greenhouses) has a surface
treatment to inhibit the production of these microcracks and scratches
over time. So toughened or tempered glass is much less likely to become
brittle.

Hello,

So to update: the galvanised clips arrived but looked a lot bigger. I
measured the "legs" but both the stainless steel and galvanised ones
are 30mm or 1.25" long. The difference is that the legs are at a 45
degree angle on the stainless ones but at a smaller angle on the
galvanised ones, so the galvanised ones have more depth. The
galvanised ones hold the glass perfectly because the legs are "longer"
and under more tension that the stainless ones.

The glass was stored against my garage wall and it has got wet in the
rain. I've noticed that the glass has a white discolouration in places
that I can't seem to clean off. I'm not sure what this is? Perhaps it
is the micro-cracks you talked about, either refracting light or
perhaps something has got into them?