Dan et al.-
That is a load of glass. Do you have any references of glass being
used to deter rats and insects? I think your "guess" is right; I
have an 1873 entry from a diary by William Wallace White, Vance
County, NC, saying "...brought 3 Bbl's [barrels] iron shavings, to
put between walls of house to make it rat proof." White's house
burned and he was rebuilding on the original foundation.
The only layers of glass I know of were from a bottle dump at Buffalo
Springs, VA. The company that bottled the water claimed they did not
reuse bottles for sanitary reasons but I suspect they didn't want
people rebottling water under their name.
Richard Kimmel