|You might have looked at the "Defect Etching in Silicon" module. If
not: now would be a good time to do this. Why? Because etching Si is so much
easier than etching steel!
Let's start here by considering what we would like to see after defect-etching (or just etching for short) a polished piece of steel, looking at its surface with a light microscope or, on occasion (budget permitting), with a scanning electron microscope.
|On second thought, let's start by
considering what we will not see:
| If we look at bloomery steel (plain
carbon steel with small amounts of dissolved impurities and some slag /
"dirt" inclusions), what we can see within the limits given above
|The picture below shows how some of this "works":|
|We have used an "etchant"
that dissolves ferrite, if ever so slowly, but not cementite. As a first
consequence, the cementite will now stick out. Moreover, the etchant dissolves
regions with a "high" phosphorous content considerably slower than
those with little phosphorous, leading to raised plateaus wherever you have
sufficient phosphorous known as "ghost structure". It also
dissolves some (not all) grains faster than others, leading to steps along some
(but not all) grain boundaries. Finally, it dissolves regions where a defect
ends considerably faster, leading to etch pits for dislocations and
precipitates and grooves for grain boundaries.
We produce structure by "differential dissolution", by employing a dissolution rate that is different for different structures.
You can see all of that in your light microscope. You only have to be a bit careful in interpreting what you see because grooves or steps look about the same at high magnification.
Note that in the picture above I omitted martensite, slag inclusions and "dark etching acicular aggregates" nowadays called bainite, and whatever you might find in more complex steels, so it would not get too complicated.
|Now I need to answer your simple question: How does one "design" a proper etchant? The answer is quite simple, too: Nobody really knows. To be sure, I and everybody else who knows something about chemistry in general and the chemistry of iron and steel in particular, would have some idea of how to start but etchant development is still mostly trial and error with a strong touch of "black art"; see also the module Sword Polishing and Revealing the Pattern / Structure.|
|Nobody is perfect and that also applies to etchants. That's why we have a lot of steel etchants, each one optimized for some specific purpose.|
|First I need to narrow the subject: I'm only looking at rather standardized structural etchants here, reagents that reveal the structure as outlined above. I am not considering "cosmetic" etchants that give your steel or blade a particular look. Here is why:|
|Smith lists 26 chemical, several with rather
quaint names ("Bergbutter"?), that appear on 64 or so pages of his
book. They all are more or less intended to enhance or to bring out the
structure of pattern welded or wootz blades, to embellish harnesses, etc.
I certainly will not go into that and then run the risk that you blame me because you messed up your wootz blade instead of enhancing its beauty by using one of those concotions.
So let's get straight to the No. 1 of structural etchants:
|Nital; short for "Nitric acid (HNO3) and (ethyl) Alcohol C2H5OH. Methanol (CH3OH) is also used.|
|Mix it in a ratio of acid : alcohol
about 3 : 100 and you have Nital. I won't give you precise information because
I don't want you to do it if you don't know exactly what you are doing. Nitric
acid is dangerous and some mixtures of nitric acid with alcohol are explosive.
If mixed right, etching is quick, less than a minute produces results. It works more or less as shown in the picture above and there are many examples of nital etched steel in the hyperscript; e.g. here.
|There are many kinds of steel and there are correspondingly many variations of nital and etching conditions, The temperature, for example, does have some effect, too.|
|Nital has many brethren (concotions
with alcohols and other acids like picric acid (explosive!)) and they all work
essentially as shown in the schematic picture above.
The following "etchants" work on a different principle, though.
|Color Etches: Oberhoffers etch, Steads etch, Klemm's etch and so on.|
|The principle of structure delineation with those etches - sometimes called "color etches" - is not so much the differential dissolution of the steel, but the deposition of copper (Cu) on parts with a low concentration of phosphorous (or arsenic, or....?). The Cu protect the covered areas to some extent from dissolution; the uncovered areas will the get attacked. Since copper has a definite color, the etched surface appears colored.|
|You might ask why copper should
deposit itself on an iron / steel surface? That is not a good question. The
good question is: Why does copper deposit itself only on certain parts of iron / steel samples? Any
metal A in solution will want to deposit itself on a solid metal
B surface if A is "nobler" than B. The degree of
nobleness is well defined in chemistry. The more you are asocial (no contact /
bonding to all those people / atoms around you) and lazy (not doing much that
is useful), the nobler you are. Gold is nobler than silver, which is nobler
than copper, which is nobler than lead,.... Way down on the scale is iron,
aluminum, silicon and carbon, the guys who do the work.
So copper should deposit itself everywhere on an iron / steel surface. Yes, indeed, but copper deposition is not the only reaction that can occur if you throw you iron / steel sample into in a more complex "etchant" that does not only contain dissolved copper salts but also eye of newt and toe of frog, wool of bat and tongue of dog or something else to this effect. Then several reactions might be possible besides copper deposition, for example iron oxidation or dissolution. You then have a competition of the various processes, and which one wins will depend on the local peculiarities of your sample, in particular if you add some hard-to-get scale of dragon, tooth of wolf, witches' mummy, maw and gulf.
Here are two compositions of color etches:
|You get the idea. Etching is serious
stuff that needs some experience not only with the chemistry but also in the
preparation of the specimens before your etch (perfect polishing is needed) and
the interpretation of what you see after the etch.
Here is a direct comparison of the two most prominent etches, Nital and Oberhoffer, applied to the same specimen (probably front and backside of a thin cross-sections through a knife)):
| The color etchants (Oberhoffer,
Stead, ...) selectively deposit copper on low phosphorus/arsenic areas, leaving the high
phosphorus/arsenic areas appearing white.
The reason might be - my guess - that the oxide on the phosphorous-rich areas
is just a little bit more stable than in the other regions and that prevents Cu
The Oberhoffer etch does make the P-poor or rich areas better visible than the Nital etch. Even in black-and-white. A picture with color can be found here. Looking at higher magnfication would reveal more details (in particular for the Nital) but it is clear that revealing the phosphorous distribution is best done with one of the "color etches.
|This is black art, indeed. So in our
modern times with fancy scanning electron microscopes at our disposal (provided
you can cough up at least half a million) we do not need to resort to those
archaic techniques any more. Not so. Proper "color etching" has two
huge advantages to about anything else:
|There is also a big catch, however:
|Structure etching is here to stay. As long as we use it we run the risk of missing something or interpreting something not quite correctly. One should be aware of this but there is simply no viable alternative|
Books and Other Major Sources
Defect Etching in Silicon
7.1.1 Finding Your Way in the Iron Carbon Phase Diagram
Phosphorous Steel; 9.4.1 General Remarks
Sword Polishing and Revealing the Pattern / Structure
The Frankish Empire And Its Swords
11.6.4 Metallurgy of the Japanese Sword
Scythian Special Large Pictures
Needle Scanning Microscopes
Microscopes for Science
Scanning Electron Microscope
Segregation at Room Temperature
The Luristan Project - Results from Cut Swords
The Luristan Project - Results from Cut Swords Part 2
The Luristan Project - Large Pictures of Cut Sword
The Luristan Project - Results from Cut Swords
The Luristan Project - Results
Ghost Structures in Phosphorous Steel
Additional Pictures - Chapter 7.1
© H. Föll (Iron, Steel and Swords script)