How to get a better Etch. Acids and etching from a chemist

Greenberg Woods

Greenberg Woods

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Dec 27, 2013
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Hey guys. Ive got a bit of a lull in classes before finals, so I thought I would write a small article about something other than wood. Etching.

Knife makers love etches, they let us show off hamons, damascus, darken steel and even clean some steel if you are tricky "Send me a message if you are interested in this"

I want to explain some things about etches and acids. This is going to assume little to no prior knowledge of chemistry, so please dont be offended.

What is an acid? Simply put an acid is a chemical, compound or solution that can donate a Hydrogen ion. .

So what is a hydrogen ion? Its H+ or a single proton. Atoms are made up of protons neutrons and electrons. Hydrogen in normally one proton and one electron. Take away that electron and you have H+, the essence of an acid. You have probably heard the term pH, which refers to the power of the hydrogen. It can be understood as the concentration of H+ ions in a solution. Any number below 7 is an acid, and each number down is 10 times more concentrated.

So, why are acids important to bladesmiths? Because metals are proton receptors. When a metal like Iron is in contact with an acid, atoms of iron become ions, generally Fe(ll) which will dissolve in water. That means you can dissolve metal and any contaminants with an acid.

The more important part is that different alloys and crystal structures react to acids differently. In a knife with a hamon, the edge is hardened ceminite, while the spine is ferrite, which are two different compounds that react differently to an acid. In damascus steel, Nickle forms a protective oxide very quickly on contact with acid called a passivization layer. This prevents the nickle baring steel from being etched further. On the other hand, manganese reacts very strongly with acid, etching quickly and leaving a black oxide pattern. This is why high manganese steels like O2 make such vivid damascus.

Now that the background is out of the way, how can you take advantage of it?

The biggest issue I see is people making their etching solutions too concentrated. Ferric Chloride is a special kind of acid called a lewis acid, ill explain that later if people want. But understand that when you mix ferric chloride with water, you make an acidic solution. But, remember how the metal is being dissolved. The metal reacts with the acid to form an ion that becomes dissolved in the water. The water dissolves. Not the acid. When someone's etch is dead, most of the time the acid is still strong enough to etch, but the water is saturated. Nothing new can dissolve in it and thus it stops etching. A rule of chemistry is that dilute acids are more efficient. You may have heard this as a long slow etch gives better results. This is because that long slow etch has lots of water free to dissolve more material.

Think of an acid like a drill bit. The H+ is the acid part that everyone cares about. That is steel. But the water is the flutes. With no flutes, you cant remove any material. A super concentrated acid is one with very small flutes. Yes it can start to cut anything, but you simply cant remove the material.

I think this is partially responsible for the popularity of the coffee etch. Its such a dilute acid it has a fantastic efficacy and can thus slowly etch surfaces evenly.

So, how can you get a better etch?

Use a dilute acid. Dilute acids are always more efficient.

Warm your solution. A warm solution not only lets the acid work faster, but a warm solution can dissolve more material and is thus more efficient.

Use distilled water when making an etching solution. The Chloramines, florides and other trace elements in water mean it has a lower capacity. Granted its not much, but it is something.

I've got to go to class, ill try to write more about my special etches and how to use acids for prepping stock for damascus soon.
 
Actually etching is an electrochemical corrosion process, involving ionic currents through the solution and electron currents through the metal. There are both anodic and cathodic portions of the metal/solution interface involved. The driving force is the difference in the relative electrochemical potential of the ion in solution vs. ion in the metal. More particularly it is the sum of all the relative potentials of all reactions involved. This is the thermodynamics which tells nothing of the kinetics or how fast the reaction will occur. The kinetics are governed by the rate limiting step which may or may not involve diffusion or migration (ion movement in an electric field). The rate limiting step is also not necessarily involved with the oxidation/anodic reaction - the metal going to metal+ in solution. It might involve, for instance, the reduction/cathodic reaction which necessarily accompanies the anodic one. The most common reduction reaction when oxygen is present is the reduction of oxygen in the solution. If you don't understand the mechanism you can't explain the reasons behind efforts to slow or hasten the process. You cannot explain etching (from a scientific perspective) without electrochemistry. It is a heterogeneous process and often has little to do with homogeneous reactions like those introduced in basic chemistry. Nothing wrong with trial and error of course.

And that's not at all the right description of the phases in the hardened and unhardened parts of a knife with a hamon.
 
Bill is exactly right. And after reading and re reading this post, I'm still not sure the point of it all. What are you trying to say here, Ben? I use vinegar and lime and dish soap (to kill surface tension) warmed to almost boiling for Hamons. I use a commercial echant (electrolysis) for my logo. And some of the most spectacular Hamons I've seen have been done with ferric chloride. So I'm left sorta scratching my head with this one.
 
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Transfer of electrons is not necessarily electrochemical. Electrochemical reactions are heterogeneous over an interface. There are two distinct regions in electrochemical processes, anodes where oxidation reactions occur and cathodes where reduction reactions occur. Each supports a half-cell reaction where electrons are transferred from species in solution to the metal (oxidation) and the reverse (reduction). This is what is happening when you etch steel. There are localized batteries that are set up based upon many inter-dependent variables, not limited to but including different phases and microstructures, geometry, concentration of species in solution, and concentration of dislocations in the metal. You can have redox reactions occurring homogeneously in a solution but this is not electrochemical. When you etch steel you are corroding it. Don't confuse corrosion, the process of metal moving to ions in solution, with the appearance of corrosion products, such as the various oxides/carbonates/hydroxides/etc. which may or may not manifest themselves. When you drive the entire surface in on particular direction, such as when you use a supporting electrode in solution and a power supply, you are doing just that - forcing one or the other to be the anode and cathode. In the etch, localized inhomogeneities create localized electrochemical potential differences, which are the electrochemical driving forces for the reactions. Acidic etching is very much electrochemical etching. You just have less control over the process. It is not at all like 'dissolving' a material, say NaCl in water.

As we all know, you don't need to understand materials science or electrochemistry to make fantastic knives. I think it's great that some people strive for a deeper understanding of the underlying processes. That doesn't make it ok for someone with a better than average scientific understanding to cobble together an ad-hoc and inaccurate description of a process and present it to the community, which is what Ben's post basically was. Nothing against Ben - he's a great guy, generous, smart, and very motivated.
 
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