On Mon, 15 Jul 2019 13:09:40 +0100, Martin Brown
wrote:

Surface hydration damage. You can get irridescent surface layers in
glass which has been buried in soil for a while. You may be able to
polish it out but it isn't worth the effort.

Is this the same as Chris said: salts that have leached out?

I favour sliding them, but a thin blade will help lift them. It is
definitely something to wear stout gloves handling greenhouse glass.

Eye protection isn't a bad idea either - toughened glass can detonate
into copious 3mm glass grit when or if a scratch gets deep enough.

I have two pieces with a small crack in the glass in the corner; in
both cases there is a line about an inch long. I was wondering whether
to use them or whether to replace them. Is the worry that the crack
could get bigger at any time and the pane shatter?

You may remember I asked whether to glaze the roof or the sides first.
I did the sides first and then stopped because I didn't have enough
panes to do the roof! I have been and bought some more glass today. I
put two panes in the roof on the one side and they were perfectly
square. I went to do the other side only the glass will not sit square
in the frame. The bottom left hand corner of the glass goes in the
bottom left hand corner of the frame but the bottom right hand corner
of the glass is about half an inch above the bottom of the frame!

I'm puzzled by this. A spirit level says the greenhouse is perfectly
level, so it's not as if one corner has sunk. The back of the
greenhouse must be leaning out slightly, causing this section to not
be square. Perhaps the advice to do the roof first is the best advice!
But how do I pull the frame back in square? Why has it only affected
the one side?

Thanks,
Stephen