I'm just curious about something. Monk has given us an excellent explanation of the formation of lead white and its action in the preparation of BLO. It would seem that the formation of lead acetate on lead from vinegar fumes and subsquent conversion to lead carbonate when exposed to air creates another carbonate like CaCO3 that reduces acidity in the oil when boiled. Wouldn't this give rise to a lead oxide. As such, wouldn't it be an opacifier as used in lead based paints? If so, why don't other opacifiers like TiO2 work? I don't quite understand the function of an opacifier in BLO. I didn't do too well in chemistry or mineralogy, but I do know that Dupont mines a heck of a lot of titanium bearing mineral sands here and around the world for TiO2 in their paints.
Just wondering.
The use of lead, manganese and cobalt as "dryer metals" in boiled linseed oil is based on the fact that some metals will cause the oil to polymerize during the "drying" process. Some metals will not act as a dryer. Others will actually slow or prevent the oil from drying. Again, this is not actually a "drying" process but is a polymerization process.
When the oil is "boiled" any excess dryer metal compound will be unchanged. That will either be filtered out of the oil or settle out during storage by itself. If one used a gross excess of dryer metal compound it would yield a boiled oil with oil-insolubles that would then act as a filler or pigment.
Tribasic lead carbonate had been used as a pigment in linseed oil paints. Beyond a certain amount it would have no speeding up effect on the "drying" of the oil.
According to paint industry sources the maximum drying effect of lead is at about 1%, by weight of metal, in the finished oil.
According to some old information published by C.K. Williams, the maximum drying rate of a boiled oil is gained with 0.5% lead and 0.5% of manganese in a boiled oil. This also gives what they considered to be the optimum oil film protection properties.
Lead dryers yield an oil film described as leathery in texture. Straight manganese gives a hard surfaced film on drying. That means that lead based oil films are a bit softer but more elastic compared to manganese films.
A soft oil film as a gunstock finish is not overly resistant to abrasion found in the normal handling of the gun. Factory guns were often finished with a varnish that was "long on oil" and "short on resin". You did not want a varnish that was too hard and inelastic because it would crack and chip off with rough use.
I worked for over 30 years in a plant that made synthetic resins for use as industrial coatings, flooring, siding, etc. We had a solution resin testing lab and tech group. I had once asked them about the best film properties for a gunstock finish. They in turn commented that such an application is one of the most rigorous applications. When the gun industry first fell in love with polyurethane and epoxy finishes there were problems where a shooter would take a gun out of a heated hunting cabin out into air below zero and have the finish crack while they were holding the gun. There were also times when the stock wood would expand and contract with rapid changes in humidity and the finish would crack with the grain as a result.
There really is no "perfect" finish for a gunstock. You are trading off various finish film properties to give acceptable abrasion resistance, chipping resistance, etc.
E. Ogre
