Sunlight or UV light

[George F.]

I think I read somewhere that sunlight or UV light helps speed the drying time of linseed oil. Is this a question for the "Myth Busters"?   Fact or fiction? What da ya think?    ...Geo.

[Stophel]

It is a requirement.

Well, I suppose it will dry in the dark, but we're talking a LOOOOONG time.

[northmn]

DifIK for sure but the light also heats the stock, especially with a darker finish and seems to speed up evaporation time on liquids.  I have a bowyers hotbox I use that is made out of a heater vent and wired for 100 watt bulbs.  That combination dried out a stockblank for me in about 3 months with a little additional time here and there.  It works very well on finishing also. 

DP

[George F.]

I made a "Bivins" hot box years ago, using it for rusting barrels, but I don't think it's tall enough for a stock with a long barrel. ..Geo.

[rich pierce]

Fiction.  I tried UV light and I have access to powerful ones.  It does not speed drying of slow drying linseed oil-based finishes.

[Dphariss]

I can put on 2 fill coats day in bright sunlight warm weather. No sunlight it drops to about a coat a day with home cooked oil. I can fill porous American Walnut in about 3 days this way.
Sunlight works WONDERFULLY.
Now plain UV might or might not work I dunno. Might require full spectrum lights and a heat lamp to get the tempt to 100+???
Dan

[T*O*F]

Linseed does not "dry," it polymerizies. 
Heat speeds up the reaction process.

[northmn]

I do not know about Linseed but on the instructions for my Tung oil it states that if not drying within 12 hrs to put it under artificial light.

DP

[Dphariss]

Linseed does not "dry," it polymerizies. 
Heat speeds up the reaction process.

Yes but "dry" is easier to type than "polymerize".

Dan

[Loudy]

The "drying" or curing mechanism for tung oil, linseed oil and other traditional oil type finishes is an oxidation reaction.  Over time oxygen in the air reacts with the oil to form a durable coating.  Temperature can at as a catalyst for this reaction.  These oils are typically applied in multiple thin layers or coats to allow this reaction to occur.  If multiple coats are applied too quickly you risk cutting off the air supply needed for the so called "drying" reaction to occur.  In modern varnishes, catalysts such as soaps of various metals (cobalt, calcium, manganese, etc.) are used to initiate the oxidation reaction and provide even curing throughout the protective film.  We used to use a lot of nasty lead based compounds for this purpose also.  The oxidation reaction can continue to occur over several years (decades) in these oil base finishes.  This is why you see some very old finishes that are cracked and darkened due to excessive oxidation and the resulting embrittlement and discoloration.  Modern finishes are formulated to achieve the desired optimal durability properties quickly and to be resistant to degradation of these properties over time.  All that said, there are some very high tech coatings out there today that are catalyzed by exposure to UV light.  I recently had a crack in the windshield of my car repaired with a UV light catalyzed material.  I can't foresee ever using a product of this type on any recreation rifle. 

Mark

[Bill of the 45th]

Technical, Smechnical.  About two years ago Herr Stophel was working on varnish mixtures, and this subject came up.  I was working on finishing a gun with BLO,in the dead of winter, and discussion of UV lamps came up.  Up here in Northern Michigan we can have a lack of sunlight for days on end, not to mention rather chilly weather.  I tried a couple of UV grow lamps with no success, then had the idea to use a friends commercial tanning bed (read as lots of UV).  No Joy as the gun just didn't want to dry, and I was using artist grade BLO.  Finally a bright sunny 20 degree day, and in about six hours I had a dry, cured, polymerized stock, whatever you want to call it.  Solar radiation did it pure and simple.  That's all I know.  Anyone need a cheap grow light?

Bill

[Stophel]

I forget which it is, UVA or UVB that is the catalyst for drying linseed oil.  If the lamp doesn't do the right one, it won't do you any good.

Some time ago, I was fooling around with black lights to try to help things along.  They DO help....a little.  Mostly my problem with slow drying was with my oil/varnish, and not with light.  I'm still experimenting.

Drying time depends upon more than just light.  If you have raw linseed oil, for instance, you can put it on wood and leave it in a warm dry room, and it probably won't dry for years.  Sunlight, and lots of it, is an absolute requirement.  We're still talking several days, if not weeks, of full sunlight to really dry each thin coat.  Mark gave the reason to never "flood and soak" wood with linseed oil.  The oil will only dry on the very outside, and all the oil inside will not want to dry at all.  When you see people say "I hate linseed oil, why, I put linseed oil on a stock, and once I got out in the rain, and all the oil and the stain ran off", this is why. (it's also a good reason to stay away from aniline dyes!  )  They slopped the oil on and never let it dry.

Start adding drying agents, and boiling the oil, and you start getting it to dry more quickly, without quite so much necessity of sunlight (though it still helps a LOT!).  I'm not certain yet, but I believe that boiling the oil does more to aid drying than the addition of drying agents.  I need to test that out.  As I understand it, boiling the oil oxidizes it, and basically "pre dries" it, so you won't have to wait so long after you put it on the wood for it to dry.

For my linseed oil (in-the-wood) finish, I have stuff that is boiled for about an hour or so, and is THICK.  Lead carbonate and burnt Umber (for the manganese) added.  Dries like you wouldn't believe.  It will film over in the jar in just a day....inside, no sun.  For an oil finish, this is great.  I don't need a smooth finish, and am not worried about the oil leveling on the surface, as I'm gonna cut it all off with a coarse cloth later anyway.

For varnish, though, I've found I can't really easily use a concoction this thick.  It's just too thick and sticky.  No matter how much turpentine I cut it back with.  Oh, it works, but it's mighty hard to get a smooth surface, as it gets tacky immediately.  Can't smooth it out.  Here's where I really need to work on my "perfected" formula...  If I don't boil it very long, it stays smooth and light, and it flows and levels BEAUTIFULLY...but it takes FOREVER to dry.  I mean forever.  I have to find the happy medium...  For my next batch of varnish, I think I'm going to boil the oil for about 45 minutes, stop, and let it cool enough to where I can tell how thick it's going to be.  If I think I can go a little longer without it getting too thick, I'll turn 'er back on again and boil for a few minutes longer and add my resins.  I'm going to try this time to not use any added drying agents.  We'll see how it does.  Maybe I can get the "perfect" varnish that stays fluid and levels well, but still dries in a reasonable amount of time.

[Dphariss]

Oxidation is part of the process.
For oil that is in the wood and "cut off from the air" aged turpentine (exposed to the air for 2-3 days in a shallow dish) can be mixed in to carry oxygen in with the oil. This will promote oxidation in oil that penetrates the stock deeply. But it still "dries" 5 times faster in direct sunlight.

Cracked finishes. Cracking and checking are the result of resins in the oil making a hard varnish that lacks the *elasticity* (flexible is not the same thing) to stretch and contract with the wood in temperature and moisture changes. If the varnish is too hard and brittle, from using a hard brittle resin the finish will tend to fail. Remington used a synthetic coating on guns at one time and it would actually snap, crackl,.pop if brought in from the cold. Varnishes that are soft, high linseed oil content, and made with smaller amounts of resin do not do this.
The "brown varnish" in the "Bridger" Hawken in Helena is worn, its dented and it has been cut through by sharp dents and abrasion but I could see no instances of checking cracking or flaking and I specifically looked (though the glass). I have seen the same behavior on BL rifles from the 1870s and 80s. The soft varnish will often not even break the surface film with dented with a blunt object. Viewed under 10x or so magnification shows the film conforms to the dent. Further more many of these rifles were not varnished with a brush but by hand according to a friend who has looked closer than I have. In this case 19th century Ballard rifles.

As modern factory made furniture finishes (most of which were unsuitable for gunstock use) became common and the custom gunmaker was forced out by factories starting well before our Civil War. The knowledge of shop made oils and varnishes started to die out and by WW-II pr perhaps by the 50s American gunmakers were clueless about linseed oil and how it was prepared and used on gunstocks. They associated linseed oil with the unboiled oil used by the military (apparently) and the "boiled oil" sold in hardware stores for thinning paint and brushing on board fences. This stuff is very poor on gunstocks.
The oils sold for use by artists is not the same as used by gunsmiths and it was made differently in the past. It is not stock finish.
In the context of the Kentucky Rifle anything that dried with a shine was a varnish or a spirit varnish. This a very soft oil finish that when applied to a gunstock would dry with a shine was a varnish even though its not what "moderns" would call a varnish today. So when you see an old document about stocks being "well varnished" it was extremely unlikely that its was anything remotely like store bought varnishes of today, or Permalyn etc etc. It was likely linseed oil, made up in the shop from low priced resins with enough driers to make it dry fast enough to shine.
If anyone will do the research they will find this is the case.
This is all based on someone else's research which he has sent to me piece by piece over the past 25-30 years. For which I am VERY thankful. He has saved me a LOT of work.

Dan