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.

The double iron blade

Or a cap iron screwed unto a blade.  I am not going to debate the merit or not of such an addition , simply point out some details I came across while cleaning a bunch of them.

 In the beginning, our planes had only one blade.  Usually tapered in thickness, being thicker at the cutting edge. Said cutting edge was a hardened piece of steel (tool steel) forged welded to a softer iron blade.  Similar to how axes are bited.

Everything worked fine, but the addition of the second blade (cap iron, chip breaker) around 1760 accomplished two things:

- It helped curled up the shavings faster near the cutting action, and
- It trapped water condensation between both blades.  A perfect recipe for rust to take hold.

I'm guessing they recognized that problem early on because for a while they were covering the back of the cap iron (chipbreaker) with some sort of blueish coating.

Take a look at the two chip breaker back.

The older Stanley one (second from left) has a blueish coating.

The newer one to its right does not anymore, they long stop doing that.

But that practice long preceded Stanley.  I do not know if it was the same process (coating) but you can often see remnants of it on older cap irons.

You can clearly see some sort of coating remnants peeling off  (abraded).

Whatever is left is still solidly attached.

So the question is:  Does it helped at all?

Debatable but here is what i often see.

Yesterday while re-conciliating my physical count with my data records, I came across an early Canadian plane, Dawson, from Montreal.  So even if I said I was done and had already cleaned up and put stuff away, you know, I just could not sleep if I did not stop the rust carnage's 

Dawson Montreal

1851-1874

And the poor blade stuck inside.  Its a small smoother.

Look pretty sad, but nothing that i cannot fix.

So put my tally sheet down, looked around and came up with a few more blades

In progress. Still shows brown spots after the wire wheel, 

back soaking after a quick sanding with 80 grit

Recognized the blade?

Evaporust, brushed, wire wheel, spot sanded, rust eraser then wiped and buffed with Autosol

The blade may be practically worn out, but there is still cutting steel at the end and plenty of good metal to raise a good edge.

Talking of worn out blade, that one is definitively pass its best before due date.

So why bother?

Because it is part of the tool history.

All I am after is to stop further rust damages.

Pic in progress

Done.  Also addressed some damages to the edges

I came across lots of damaged top edge, from using a metal hammer harder than the softer iron blade.
That would be just about any decent hammer BTW.  A Brass hammer head  would deform before it deformed the soft iron.  These must be addressed to stop the damages and to render the blade safer to be hit later on.

I often see also lateral damages to the blade, from hitting the sides to adjust lateral.  Which is why Stanley invented the lateral adjuster later on :-)

This one has damages to both top and sides, but it is the cap iron, not the blade!!! 

Here is an interesting details I came across in this short marathon.
How the cap iron is fastened to the blade.

Notice how the screw shrunk.

Interestingly, the bolt diameter and thread sizes seems to have remains

 pretty well the same all along and not just Stanley.

Which just add to the confusion, which belongs to which?  

Hint the bolt should go all the way thru cap iron and be flushed on the nut side.

These last two have the same threads but not cut the same.

Take a look at the base

The screw length changes along the way it was attached.  I have no idea which methods came first in what order, but we all know it finished with the shorter screw to attach a thin steel cap iron to a thin steel cutter.

In no particular historical order

This one, rarely come across this one,  use a separate nut and screw

The nut is filed to slide between the slots of the blade.

Attached

This one is yet different

No more loose nut to loose.  

The nut is solidly embedded into the cap iron

The one most often seen, mostly British tools

The brass dome nut is also embedded solidly 

This is a new one on me.

That is from my Dawson blade

The nut is not attached, but is machined from a thicker stock

Pretty solid, probably the most secured

And of course the Stanley design we are all familiar with.

The top horizontal slot is to engage the depth of cut adjuster.

This design forced Stanley to move the hole location on the blade slot from the top to the bottom.

This design also introduced a small irritation factor with today's unwary woodworkers.
Because we rely on that slot located at the top of the chip breaker to adjust our cutter in and out... Guess what?

It had to be adjustable to compensate for the inevitable wear and tear on the blade which will be sharpened umpteen time in its life.  That's the reason for the slot in the blade.  Problem is, they are a lot of Stanley clones out there but no one ever seems to came up with a standard location ...

Notice anything different??

If you do, that is the reason why it gives you headaches when you attempt to swap blades :-)

And if you thought that the two outer ones were the same...

NO, not even close. Both now on right.

From L-R Stanley's regular bench planes iron (first 3), 

Stanley No 113 compass plane blade, and a transitional plane blade.

In these pics I lined them up by the cap iron edge, not the cutting edge.  The three regular Stanley are various vintages and the hole location changed a bit, but not enough to cause much trouble.
The Compass plane No 113 however will have a hard time to give you any cutter exposed to cut if someone swapped the cap iron or the whole blade assembly with a regular one.  That is because of the flex sole, the frog surface is longer than on regular planes.  Buyer beware.  Oh and also the lever cap is slightly different, longer. Also sometimes swapped on unsuspecting would be buyer.  Finding original spare parts?   You will have better luck lining up to get your bucket of prop wash filled :-)

Similarly, in a transitional plane, the yoke adjuster sit higher on the frog, so the slot location is yet a bit higher.  because the small frog sit on top of a 2-1/2 in thick chunk of wood.

And to add to the fun, others would be Stanley clones used all kind of slight variations in slot location, more pronounced than the slight variation shown.  Oh and they varies location thru the years.  Ever noticed why most modern blades maker have a disclaimer on their Stanley replacement blades? May not fit all models produced thru the years etc

Moral of this story, to keep your sanity and let the plane works as intended, if you want to swap blade, you may want to re-use the original cap iron, just saying.

And since I had the Evaporust bath set up again, I may as well ...

This is what I ended up with and it is all Dawson's fault :-)

And then I drank a Coke to dissolve all the rust I have no doubt ingested.

Like my Dad, I am a Coke man, we both flunked the Pepsi challenge :-)

Dad was so pissed when they tried to changed his Coke.  It taste like $@^# Pepsi!!

Bob, all rusted out from de-rusting umpteen tools saving them from a slow but inexorable death.
You know, they used to say, You are dust you will return to dust when you die.  Well with today's micro plastics being everywhere, soon it will be: You are plastic you will be recycled when you die :-)