[SEL] Saw Outfit Blade finish/now linseed oil

[John Culp]

> I have a really dumb question. What really is "boiled" linseed oil?

"Boiled Linseed Oil" is stuff that's sold under that name. There really 
are two kinds and no easy way to tell what you've got. One is basically 
raw linseed oil with Japan drier added to it to make it dry faster. The 
makers used to use lead and copper soaps or naphthenates, but in more 
recent years it has been usually the less toxic cobalt and manganese 
naphthenates. (Naphthenic acids are petroleum derivatives, kind of like 
fatty acids, but instead of straight chains they have saturated ring 
hydrocarbon structures attached to the carboxylic groups. So these 
salts are basically detergents, like soaps.) The metal ions promote 
free radical formation stimulating the oxidative polymerization of the 
oil. The same way metals from your babbitt bearings promote your engine 
oil sludging up with heat and oxygen.

The other kind is simply raw linseed oil that's heated modestly, to 
about 150-200°C, and air is blown through it till it reaches a certain 
point of beginning oxidation without getting too thick and gummy for 
use. Since linseed oil's oxidation is self-catalyzing once started, 
this stuff dries much faster than raw linseed oil, which takes forever 
and a week. This "boiled linseed oil" is essentially the same as the 
"stand oil" used by artists for centuries, which is raw linseed oil 
left standing for months in a clear glass jar exposed to sunlight, with 
some air exposure.

I recently did a search for MSDSs on boiled linseed oil and looked for 
any mention of metallic ingredients. My search wasn't exhaustive, but I 
looked at a bunch of them and couldn't find any. So I think most of the 
BLO on today's market is the preoxidized sort without metallic 
additives. That fits with my observation that it initially dries pretty 
quickly but stays in the gummy stage a lot longer than the leaded stuff 
used to.

There is yet another product generically termed "polymerized linseed 
oil," sold under brand names like Tru-Oil and Lin-Speed. (Some of the 
"Danish Oils" fall into this category, too, while some of them are thin 
wiping varnishes.) It's been cooked at a higher temperature that 
directly breaks some of the double bonds in the fatty acids, which 
re-link with each other, forming polymers without the oxidation. These 
oils are very fast drying and dry to a hard, glossy shine like varnish. 
They're mostly sold in small quantities for gunstock finishing, but are 
NOT the stuff to use to get an old-fashioned oil finish on a stock.

John Culp
Bristol, Tennessee, USA