Glass from used plates is thinner than picture frame glass or single strangth glass. Single strength glass is 1/16th inch thick. Photographic plate glass is thinner than that. It is not half as thick, because two plates are thicker than single strength glass. Not much thicker, but thicker.
Single strength glass will fit into plate holders with a little space left over. There is a lot more space between the glass and the slide if plate glass is used, photographic plates that is.
Where can I get that thinner glass?
It is about 1mm thick. hmmmmmmmmm