steve cook wrote:
>
> Hello - Does anyone have any recent or "decent" references on window glass
> identification?  I am trying to learn about apparent differences between
> 18th and 19th century  window glass when inventorying historic sites.  I am
> curious about the importance and reasons for color variation, and if
> thickness is truly a indicater of Crown vs. Cylinder.
>
> Thank you
>
> Roberta Charpentier
 
Hello
For what it is worth I offer my own limited experience.About 30 years
ago I renovated our early Victorian sash windows. The panes are 30
inches by 24 inches and from memory about 5mm thick. Had I realised at
the time, greater care would have been taken when removing old putty and
replacing the glazing sprigs. Glass made in 1840 scratches very easily!.
Glass known to be 20th century and given the same treatment does not
scratch.
   Our  Georgian sash windows with small 11in by 7in glass were also
glazed in 1840 and most of the original panes were wafer thin and
readily distinguished from the replacements by simple tapping. These
thin panes did not usually survive attempts to remove old paint and
loose putty. At least not my attempts.
    While hardness,  variable thickness and optical distortion can no
doubt separate 19th and 20th century window glass, but did the
technology change significantly in the preceeding century? I assume
glazing practice in 18th century America would be the same as that in
the UK.
 
George Twigg