I am not very knowledgeable about commerce in the 1700s. I was imagining a smith working in a small settlement, not a big city. He was making varnish out of linseed, copal, rosin, lead oxides, and turpentine.
One other thought. pH needs water to be a thing. Is has to do with ion content in an aqueous solution. IF the wood is @ 10% water is that enough to make pH relevant?
An other thing I have done. For filling the grain I have used wood dust and paste fillers. Over that lacquer. Lacquer flashes off super fast. The carrier is a non polar solvent, like the varnish top coat. I would expect those two to bond together better than shellac. The carrier in shellac is alcohol and is not very soluble in the solvents used in varnish.
Not to sure about sealing the wood. You can not really seal off the wood unless you use modern finishes that encase the wood in plastic. Fullerplast, epoxy, and such........
French polish is a finish that uses linseed and shellac together. IT works fine. Is it the shellac that builds up or a combo of both? They are clearly compatible. Shellac can turn white if exposed to water.
Anyway, don't have fun wrong. : )
I don't know if the residual moisture in the stock is enough to be relevant, but practical experience with homemade aquafortis over 30+ years has proven to me that *something* will cause it to go very dark if not neutralized. Many older rifles finished with this as a stain are almost black. Probably something worthy of a real scientific study, because I'm just guessing based upon observation and experience.
Why would the solvent utilized to dissolve the shellac matter? I should preface this with stating that I do definitely believe it better to use straight grain alcohol as opposed to hardware store 'denatured' mystery mix, but the solvent evaporates, correct? I'd assume that it isn't measurably adding any water content to the stock. So, when you speak of 'lacquer' vs shellac, are you talking about nitrocellulose? I.e., the 'in' retro finish for guitars nowadays. That stuff tends to check/crack fairly easily and quickly and I don't believe it was available in the 18th century. At that time, it's my understanding that the term 'lacquer' would have indicated a base of various resins dissolved primarily in alcohol - just like shellac - or sometimes in turp. Of course a wide variety of pigments were added as well for color but I don't believe this was ever applied to a firearm, at least not an American firearm.
I agree the wood is not being "sealed" as if encased in plastic but the idea behind the sealing process if the term 'seal' is used somewhat loosely is to provide a hard and relatively impermeable (immediately, not necessarily long term) base so as to be able to quickly apply a layer or two of an oil/oil varnish as a weatherproofing agent. Without sealing whether via shellac or various dissolved resins, or the much longer process of simply using thinned oil/oil varnish, a single or two coatings of the top coat will end up looking very blotchy and uneven because particular areas or raw/stained wood will suck up finish like a sponge regardless of whether thick or thin, while other areas will not do so to the same degree. So, I always viewed the 'sealing' process as something akin to gesso or "canvas" preparation to ensure that the top coating appears evenly all over the piece in question. I've used shellac or seedlac as a sealer for probably close to 30 years now and never had any problems with it, and I have seen pieces I've done close to that age with no negative effect.
It's unfortunate that historically, pretty much every period text dealing with wood finishing is directly applicable to virtually every other woodworking trade but gunstocking!