Hmmm...
I have been smithing for some time, and I have experienced some contradictions to the comments in this thread about plum coloration. Just yesterday, my wife and I blued a batch of rifle parts. This morning when I wiped them down, I saw what I have seen before randomly over the years. The bluing on the vast majority of parts/barrels/actions/bolts, etc., turned out quite nice...deep dark black. However, three parts plummed.
- All parts were in the same bluing process (in the tank at the same time for the same length of time).
- Temp and salt mix was consistent throughout.
- I do not use WD-40.
I will not claim to know the answer to the plum dilemma, otherwise I would have taken action to remedy it if possible. BUT, I will say that I have witnessed particular parts on particular firearms plum more than others. For example, Mauser extractors, front sight ramps on many older (military and civilian) rifles, H&R Topper receivers to name a few. While I understand if the bluing recipe is skewed or temp is off, the desired effect will likely not happen, but my experience has shown that consistency in bluing also produces the undesired plum effect. I put little solidarity in the WD-40 theory (if you believe it may have some effect, it certainly is not the only culprit). My belief now, as it has been, is a difference in the steel. Parts to a single gun do not come from a single piece of steel. Is Silica the culprit....is Nickel....is it something else? Smarter folks will be able to determine that. My degree is not in metallurgy, my intelligence in that subject is decent, but not exceptional. A second rebluing has, on occasion, has provided slightly better results with an adjustment to temp, but not always.
Cheers & Happy New Year!