According to the erecting card, #21’s boiler jacket was “Planished Iron” when she emerged from Baldwin’s shops. What colour that was is almost anyone’s guess, I think. I doubt it was true Russian iron, but as that was also planished, I figure the boiler was probably a similar colour – which is to say some sort of variable grey/black/blue/green, depending on the light. She could have been painted by the time she was 622; we are unlikely to ever know. However, since blue jackets are so much cooler than black, I have elected to stick with my formula for Russian iron.
Of course, it wouldn’t be one of my stories without a mishap along the way. This came in the form of a failed glue joint holding the left running board in place. Perhaps it was weakened by all the handling to re-do the tapered course. The boiler was still in primer, receiving its last bit of smoothing before painting, and the running board just simply wiggled off in my hand. I mounted it back on the chassis and clamped it down with the cab screws and a C-clamp while fresh epoxy flooded the joint. I have already redesigned this area to allow for a mechanical joint on subsequent engines.
My recipe for planished iron is simple: a base coat of bright silver (Polly Scale Stainless Steel), then a 20% blue glaze, followed by a 20% black glaze. The two glazes are painted by eye, and the true trick is in keeping them even. On #10, it yielded a subtle, yet pleasing variability under different lights.
Step 1: Stainless steel. This first coat does not forgive scratches on the boiler! 
Step 2: 20% (1:4) blue glaze. Hopefully that taper course will be improved by the boiler band! 
Step 3: 20% (1:4) black glaze.
Pembroke:87 
That’s really effective. Nice method!
Thanks Rob
As I understand it, planished iron was an attempt to replicate (ie, bypass) the secret pickling formula held by the Russians for Russia Iron. Cheaper to produce, and more available, it was a good substitute. As to color, well, I won’t wander into that swamp despite having seen samples and read lots from the experts (like Kyle Wyatt and Jim Wilke). I know you’re wise enough to make the right choice!
Thanks Dave, that matches my belief as well. Yes, stay out of the colour question as long as you can! At some point you have to give up on knowing what the colour really was, and transition to what colour people will believe.