Why were the backs of the cap iron originally treated

First a quick recap.
I mentioned a few times that the back on some of my cap irons looked like they had originally some sort of coatings on them.  Black on the older English irons then some sort of blue on Stanley cap irons until roughly Type 12, which saw the introduction of the high knob.
I have no idea what that coating was or is, let alone if it is an additional coating or the results of the thermal treatment of the blade thru its manufacture.  After all you can buy steel today to make handsaw which still has a blueish coating on one side, for the user to remove, via abrasion.

My thinking was, this extra step was to mitigate rust formation between both metal plates (irons), but then they stopped that practice around the great depression 1929-30s.
Extra costly step??

So one of my reader, Steve and I have been discussing back and forth trying to solve that mystery.
I am of the opinion it must be a coating, he is more incline to think its a result of the manufacturing steps (EG heat treatment).

Here are the two blades I picked., both Stanley.

You can definitively see the remnants of something blueish.

But what is it??.

It really spike my curiosity, so in a  effort to see if we could narrow it down or establish what it probably is, I took out my USB microscope and had a closer look.

My elaborate and scientific method of doing stuff.

In this pic, I am looking at the dressed edge of the cap iron

There are many version of this microscope, resolution, both in optics and chip size etc.
But you can buy a cheap one like mine for about $30 delivered to your door straight from China (just do a quick google search).  AMscope, call them kids microscope simply because they can also sell you much higher quality and prices.  But don't let the low price fool you, they can be very handy for taking a closer look at practically anything.  I use it when sharpening to inspect problem edges.

.

Mine is like the one on the left.

Its only drawback is the very finicky focusing,

the stand being flimsy.

You have to hold your breath when taking the pic :-)

What do you want for $20? 

So have a look at what I found.
If anyone can help us identify it, please leave a comment.
First up, the infamous Blueish coating from Stanley

Near the top of the cap iron

Where the cap iron meet the blade.

Often the inside of the curvature is clear steel (bright).

but this one definitively shows signs of it till the edge

Then lets look at the plain steel one on a more modern Stanley Type.

The edge that meet the iron.

Can you see shavings jamming in there? Yes

Around the big hole to clear screw that tighten every thing together on the frog.

You can clearly see evidences of stamping to cut the hole

The inside threads cut into the cap iron to attach the cutting iron

 And again here they are together with and without the flash.

No flash

Flash on

The top sides.

Flash on

I moved them around in an effort to show the change in curvature at the edge.

The oldest one (blue) has a more pronounced curvature and hump.

The newer one is mostly bent at the edge

Finally lets look at the English black coated ones
Hummm on closer look, they sure look very similar to the blueish Stanley...

The blades in question

AUBURN TOOLS CO

THISTLE
TRADE MARK

WARRANTED

English maker?  It got a 

I SORBY

SHEFFIELD cap iron

The back no flash

The back with the flash on

To give you an idea of the magnification I'm using,

this is what the smooth grey looks like on my paper

Near the start of the curvature on the cap iron.

looks a lot like Stanley

The inside threads cut on the brass nut

How the brass nut is swaged (tight compressed fit)

Near the top of the cap iron

The brass nut seen from the other side, the show side

Next one up

THOMAS TURNER

WARRANTED
CAST STEEL

The back flash on

Yes we have pockets of rust. 

Barely visible to the naked eye

See how the scratch lines seems to skip?

It is because you are looking at a crater of a pitted area.

Strangely this rust is only attached to the blue coating??

It sacrificed itself to protect the steel?? :-)

The show side of the bras nut

Finally lets have a quick look at one of my prior working edge blade, Hint the Thomas Turner

In its defence that edge has been thru the wire wheel

The bevel side

The back side.

I had to turn down the LEDs illumination it was too white out (shiny)

Yap, nicks

The back side.

Notice the hollow in the back, near the edge

Hard to see the bevel.

Notice the remnants of an hollow edge

The other blade Thistle, has not been sharpened yet

One thing I did not looked at is how a blued edge from heat  or a gun bluing treatment would look like under the microscope, having none of the above.  If you do have access to those and one of those microscope, would love to see how it looks abraded

And just because, lets have a look at some wood.
Can you identify the species?

OK there you go.
If that did not convinced you how useful this "kid" toys is, I gave it my best shot :-)
Seriously, it is a very handy tool to have. Oh and the kids and grand kids can have fun with it too.
There are of course versions who plug directly in your phone (IOS and Android), making them more portable.  Mine is attached to my PC via USB.

Bob, the grand kid at heart.  Never cease to explore your world.