Turpentine is actually carcinogenic, I'm told.
The big reference book for a lot of this stuff is Meyers 'Artists Handbook of Materials and Methods'. In the section on varnishes he talks a lot about the issues of quality control, basically coming to the conclusion that it's impossible to make cooked oil-resin varnish with consistent qualities in batches of less than 200 gallons. The properties of the final finish depend not only on the ingredients and proportions, but on how long you cook it, how hot you get it, and how long it takes to heat it up and cool it down.
I did make one batch of 'Fulton' varnish once. The ingredients are raw cold-pressed linseed oil and pure spirits of gum turpentine, along with a few drops of dryer. You heat and aerate a gallon of turps with the dryer in it for about a month, until it turns the consistency of honey and half of it cooks off. Then you carefullycook the remainder down to a terpene resin: it can explode, and gives off volumes of nasty smoke, so you do this outdoors. Then add in the oil and cook it into a co-polymer. Thin the resulting goo with thin turps, and you've got varnish. It works, but I probably won't do it again.
Many violin makers use 'spirit varnish', which is basically some mix of resins dissolved in alcohol. They do this in part because of the QC issues with oil varnishes in the old days, and in part because many oil varnishes can take a while to cure, particularly if they have not been cooked right. Again, QC.
Not all oil-resin varnishes are wonderful, either, and some have long-term durability issues as bad as nitro. Rosin based varnish, which used to be popular, tends to break down over time because the rosin is acidic; crazing terribly. Wood ash is said to neutralize that. 'Run Copal' varnish was popular about 150 years ago. They found that copal resin, which is not normally soluble in oil, could be used if it was 'run'; heated up enough to drive off some fraction. The only problem was that the initially clear and beautiful varnish turned black with age.
Martin Schleske's research into varnishes found that there is no significant difference in the film properties between 'violin' varnish and good furniture finishes. The varnish I use now is actually a floor finish, and not cheap, at about $50/qt. That's still a lot less than violin varnish. The main thing is to use a thin film.
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