1937 s 42 bluing
 

I purchased this pistol from a forum member years ago. All matching other than side plate. It has original finish, 80%+-, in indirect lighting looks like good bluing. In direct sunlight the finish looks like a rusty haze under the blue. What causes this? Thx

Blueing is an oxidation process. Whether it is rust blueing or hot salt blueing, it results in an oxidized surface. Most blued guns turn reddish brown over time. Some quicker than others.

The reddish oxidation like that is actually a tell for original finish. I know a few ways to bring it out with artificial lighting as well and use it often to determine originality of finish at shows or local shops.

I have a 1937 S 42 made mid-year. It exhibits a plum colored frame and extractor. Research indicated a change in the bluing process (as mentioned above rust to hot salt) caused this plum coloring in 1937.