What I have is one pint of oil with (if I recall correctly, I don't have it in front of me) 7 oz. of dark rosin and one ounce of mastic. About a teaspoon of lead carbonate, and probably the same amount of burnt umber...it's been so long since I made it, I don't recall. Boiled for about a half hour is all, then add the resins. Boiled much longer than that, and it gets too thick for varnish purposes (but as oil alone, it is fine, and dries rather faster). It is of very smooth easy-to-spread consistency with no turpentine added at all....check that, I MAY have some aged turpentine added to this...again, it's been a while since I made it up...
Oh, and I will NOT add turpentine to the oil and boil it. No way, no how. I did that once, and nearly burnt the house down. When a pot of linseed oil catches fire, there's about a foot or less of flames which are easily extinguished with the pot lid. If a pot of linseed oil and turpentine catches fire...six foot raging flames of perilous death. Generally, I don't use paint thinner for anything but cleaning my hands (though turpentine is nicer...it smells better!) or other non-varnish-application-related tasks.
This is a simple "plain brown varnish", and so far, it seems to work fine for me (after LOTS of fiddlin' and experimenting). I honestly have never cared to try the resins that have to be "run" prior to adding to the oil. One more step of complexity I would prefer to avoid. Rosin and mastic both dissolve quite readily into the hot oil. Resins like Sandarac and Copal need to be "run" first.
For just linseed oil, I have a THICK concoction of dark oil. Boiled for probably an hour and a half, with the same lead carbonate and umber added for driers. I can't do anything with it as it is, and quite a bit of turpentine has to be added to get it to be workable. I know I have some aged turpentine added to it as it sits in the jar, but that is thick too, and has no thinning properties. Dries quite fast, and a good application is dry in a day of good sunlight.
Like I said, this particular varnish has no real amount of turpentine in it, it is smooth enough to spread as-is. I know turpentine is resinous and varnish-like in and of itself, and varnish can be made with turpentine as the carrier...I might like to investigate and try that one day... I've also wanted to give a try at making my own "aged turpentine". I figure I could simply use a fish tank bubbler to run air through it...
I did add some prepared Japan drier (just a drop on my fingertips as I was smearing the varnish on the stock) because I couldn't trust the weather and I need all the help I can get with drying right now. Sunny today, but tomorrow..... And by golly, it was durn near completely dry just from sitting overnight in front of my black lights! I have it in the sun today, and will put on the second coat tonight.
Oh, and the varnish is RED!