It was SOP to cover chalk pastels with non-glare glass and tape the edges back in the day. The artists were also using pulp wood illustration board, and corrugated cardboard supports.
Doesn't mean it was the best choice of substrates or the finest in preservation framing. We have been learning from our mistakes.
Metal and glass are true vapor barriers, hence non-porous.
Residue from acid etching doesn't prove porosity, it just means the residue has accumulated on the irregular surface of the glass.
In order to be porous, the liquid or gas has to actually penetrate the surface and get into and through the physical structure of the subject material.
Acid residue is superficial and can be fairly easily removed from etched glass.
Back to the question of the afterimage on the inside of the glass.
I usually see this when the support or mount is a pulpwood board.
Like most of the damaging causes in framing, I think this boils down to oxidation and how the chemicals in pigments and how variably they are applied to the substrate, the composition of the matting and support materials, and a source of energy and oxygen are present.
I also think that this is not a single defined process, and can involve a number of variables in differing quantities. The physical shape of the monochromatic embossed paper could differentially filter the acidic gasses and cause the image transfer. Think of it as incredibly slow photography with the art being the negative.
