On 15/07/2019 20:39, Stephen wrote:
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?

Glass is amorphous so it is a bit ambiguous whether salts have leached
out or the water has hydrated some of the weak spots in the network. The
result is ultimately the same glass that stays wet for extended periods
will get a surface bloom of some sort.

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?

I wouldn't use any glass that has a visible crack in it. YMMV

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 can't quite visualise what you are describing. Are you saying that the
rectangular pieces of glass are too tight a fit at the bottom? You may
need to get a friend to lean gently on the frame to tweak it square.

Once you start (on a calm still day) then you should aim to complete the
job as quickly as possible in one session if you can. The whole thing is
uniquely vulnerable to wind damage when it is partially erected.

It is also the case that a greenhouse with a broken pane facing into the
wind can be very rapidly stripped of glass. I keep a piece of riot
shield with a baton on the back for instant repairs to storm damage. It
didn't protect me from the super storm of Xmas 1998 but that was gusting
to 100mph and crippled the aluminium frame. Glass doesn't bend

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?

Best bet is measure up to find where the discrepancy lies and use some
assistance to square it up while you get the first few pieces in to the
awkward bit. I suspect once it has some more glass in it will be OK.

--
Regards,
Martin Brown