Here's the poop on linseed oil that I have been able to glean over a period of time searching for historical finishes.
Linseed oil (flaxseed oil) is a drying oil. It absolutely REQUIRES UV light from the sun to dry.
Raw linseed oil (unmodified) will dry, but it takes a LONG time to do so.
"Boiled" linseed oil, as bought today, is not boiled at all, but simply has a drying agent added (I think it is manganese based...)
Historical boiled linseed oil is actually boiled (generally with white lead, red lead, "roche alum" or some other drier added). This is often referred to as "black oil", as the boiling of the oil will turn it from the familiar yellow to a dark orange-brown. Boiling decreases the drying time, as, of course, does the addition of the dryers. I do NOT know if the "durability" of the oil is increased any with boiling. I don't really think it is much, if at all.
Traditional varnish was made by adding resins to the boiling oil, which gives it gloss and hardness.
Now, linseed oil, boiled or otherwise, is NOT waterproof. It is only somewhat water resistant. Water WILL pass through all types of linseed oil (and varnishes made with the oil), however, the water will also pass right back out again. The oil does seem to slow the water transferrence to a level which the wood can tolerate.
When fully dry, linseed oil seems to be almost indestructible. Water may pass through it, but it doesn't really seem to affect the oil at all. If you get a white water spot on your historically-correct-linseed oil-varnish, not to worry, as it will simply dry out and go away soon enough.
With any linseed oil finish, the key is to put it on Thin. Did I say "thin"? I mean THIN. If it is glopped on heavily, the outside may dry, but it will remain goo on the inside for a LONG time.
On drying time. First off, the purer and cleaner the oil is, the faster it will dry. Raw linseed oil, in THIN coats, could take MANY days to dry in the sun. Store bought "boiled" linseed oil can take several days in the sun to fully dry (take note of the word "fully"...). Historical real boiled linseed oil with driers added CAN dry in several hours to a few days in the sun. Again, Thin, thin, thin. The heavier the coat, the longer it will take.
I have been making my own varnish of linseed oil with rosin and mastic added along with lead carbonate (white lead/flake white). I only boiled it for about an hour and a half, and it dries fully in about 3 FULL days in good sun per coat. I kind of think, but have no empirical evidence for, that the oil alone would dry a bit quicker, and the resins slow down drying a bit...
For those who do not wish to boil oil, or make their own varnishes, "black oil" can be purchased from Kremer Pigmente, as can several different natural varnishes (none of which I have yet tried). Another option is to get Tried-and-True Varnish Oil. This is a raw, pure linseed oil that has been "pre-polymerized" by making it into "stand oil". As I understand it, stand oil is created by putting the oil in copper trays with glass lids in the sun. This "pre-polymerizes" the oil, while not allowing it to actually dry. This cuts down on the drying time without adding any heavy metal-based dryers. The "Varnish Oil" has rosin added, though I'm not sure how they do it... This is what makes it into a "varnish", and suitable for a top-coat. These are the only two ingredients, and it is utterly non toxic and food safe. I have tried it out on some scrap wood, and it dries fine in a few days of sun. I have had several people who use it on their guns tell me they just love it. It is available from Woodcraft, though they, for whatever reason, do not show it in their printed catalogs.
I used to be of the opinion that wood should be sealed up tight, and totally waterproof (something that is impossible anyway!). Recently, I have changed my position. It's kind of amazing how so many of these old guns have survived to this day with some of these "sub-standard" and "ineffective" finishes...or even with no finish left on them at all! :winking:
