Re: varnishes, Gum and photogravures
I'm joining the thread a bit late, and maybe some of this has been addressed, but . . .
I believe the varnish that was used on the Strand portfolio was applied to produce a glossy or lustre look, not for archival qualities.
Most photogravure prints are considered archival, assuming you're using quality paper and inks (and considering the relatively short history of photogravure and photography in general). I suppose there is an official definition or definitions but the word archival is a bit fuzzy to me as it means different things to different people: some consider an inkjet print that doesn't fade for 80 years to be archival (or so says Epson), some people think in terms of hundreds of years, others in terms of thousands (consider pre-historic cave paintings). I would venture that once a print gets to be a certain age, its probably more environmental factors that cause it to age, and not necessarily the materials themselves. Consider many of the great church paintings that are being restore, not because they are not archival, but just that smog, water, smoke, fire have gotten to them.
I've been told that traditionally, etching inks included copal, a type of resin. I believe it was added to provide a certain lustre or sheen to the print. I'm not sure if there are any manufacturers who still include it in their inks.
Copal is the hardened excretion from different types of trees (not to be confused with sap, which flows through the tree). In Mexico, copal is used for incense. It is found in other parts of the world, including Africa and Asia--from a wide variety of different tree species. It is often dozens to hundreds of years old, but is not fossilized. Amber is the fossilized form of copal. It is much rarer and much more expensive (and often considered a gem or mineral), and is not often used for inks and varnishes. (Gums are related to resins and are also from trees, but they are usually, well, gummy, not hardened).
For a bit more detail, see: http://www.polymetaal.nl/beguin/mapg/gums_and_resins.htm
Just fun facts for your printmaking pleasure.