I left by Red Ball boots back in KS brother, but still have my 14" L.L. Beans smile
Drew, I pray that your mind will want to wade through my information barefoot. LOL!!
The comments about hardened steel etching darker, is a common topic in discussions between knifemakers. However, we first need to examine the damascus finishing techniques of knifemakers and understand how they are different from the processes used to finish damascus gun barrels.
Modern knifemakers typically use ferric chloride to etch their knife blades. Most often, the source of the ferric chloride is Radio Shack printed circuit board etchant solution. The Radio Shack solution is usually mixed one part etchant solution to three parts water. A fairly strong mix. After the damascus knife blade is sanded to around an 800 to 1000 grit finish, it is placed in the ferric chloride solution.
The purpose of this etching process is twofold. For one, it develops the dark coloring of the steel, as Drew has mentioned. Secondly, it creates topography on the surface of the steel, with one of the steels being etched more deeply than the other steel. A common combination of steels that knifemakers use to make damascus are, 1084 high carbon steel and 15N20 tool steel. 15N20 is almost identical to 1084 steel, with the exception of having 2 percent nickel added to it. The nickel acts as a resist to the etchant solution. So, the layers of 1084 are eroded much more quickly than the layers of 15N20. The result is a surface on the knife blade that has a lot of topography; or hills and valleys. Knife blades are often etched so deeply that the topography can be felt with a fingernail.
The darkened coloration that Drew has mentioned, is corroded steel. The total amount of time that a knife blade will spend in this strong ferric chloride etchant solution ranges from around 10 to 30 minutes. A knifemaker typically has to remove the blade from the etchant solution occasionally to card away some of the corroded steel, as it becomes a passivation layer, slowing the etching process. The corroded steel, which is the dark coloration mentioned previously, is often referred to by knifemakers as “oxides”. I am not certain that this material is actually an oxide, by the proper chemistry definition. As its’ formation is the result of a chemical reaction, I think it could be a separate product of the ferric chloride and the steel. Need a chemist to balance this chemical reaction equation to find out for sure. But, what if something other than ferric chloride was used for etching? Copper sulfate, sulfuric acid, hydrochloric acid, nitric acid; what then is created? What color will it be? I can tell you from experience, that damascus steel etched with hydrochloric acid does not develop the same dark coloration as does steel etched with ferric chloride.
Whatever this darkened, corroded steel material is, knifemakers often use it to display the damascus pattern in their knife blades. This material is not very durable and is fairly easily damaged. That is why knifemakers etch their blades so deeply. After the etching process is completed, the knifemaker carefully sands the blade, polishing the tops of the topography and leaving the darkened material in the valleys. As this darkened material is not very durable, many knifemakers have taken to scrubbing it away and then chemically darkening the blade with other chemicals; bluing, parkerizing, etc. The blade is then carefully sanded, leaving the newly darkened areas in the valleys.
The discussions among knifemakers as to what color different steels will become during etching, surrounds the use of the corroded steel material, whatever it is, to display the pattern in their damascus blades. Their obvious concern, is creating a dark material that will result in significant contrast in the damascus pattern. For those who remove this material and darken the valleys in the damascus with another coloring method, the coloring of this material darkened by the etching process is irrelevant.
I have heard in many knifemaker’s conversations and also seen posted in knifemaker forums (such as the one that Drew linked in the Vintage Barrel Composition thread), talk about what alloys in steel create a darker etch. Many have eagerly proselytized the “factoid” that high manganese content in the steel creates a darker etch. I don’t know where this idea came from. I expect that some “expert” said it to be so. I’ve never heard who this expert was. But, many are willing to spread a concept that they have not personally tested for themselves. On the face of this statement, the suggestion is that the manganese somehow changes the chemical structure of the darkened, corroded steel material. I’ve not heard anyone proclaiming this so called fact, explain the chemistry of how that would happen. I will later return to this subject.
Now, on to contrasting the process for finishing damascus gun barrels to the process commonly used for knife blades. I have never finished a damascus gun barrel with the methods commonly used for this purpose. I have however, blackened steel barrels by the slow rust method. Having read many descriptions of methods used to finish damascus barrels and the experience that I have had with slow rust bluing, I think I understand the damascus barrel finishing process. If I state anything incorrectly about the process, PLEASE correct my inaccuracies. Thank you.
Finishing damascus gun barrels is typically done with much weaker etchants and acids than are used to finish damascus knife blades. Rather than significantly destroying steel to display the damascus pattern, the damascus barrel finishing process seeks to generate and cultivate an iron oxide layer on the surface of the barrel. Some do a mild pre-rusting etch on the barrel with a weak solution of copper sulfate or ferric chloride. I assume that this is done to create a small degree of topography, to facilitate carding away some of the coloring on the damascus layers that stand proud of the other, more etched away layers. Next, a mild acid solution is applied to initiate rusting and then allowing the barrel to rest in a humid environment to generate an even layer of red iron oxide (Fe2O3). If the finish is to be black and white, the barrel is boiled in water, with perhaps some chemicals added to deepen the black. The boiling process converts the red iron oxide (Fe2O3), into black iron oxide (Fe3O4). The rusting carding and boiling process is repeated until a sufficient layer of iron oxide is developed for an even finish. Occasionally, the barrel may be subjected to a quick etch in copper sulfate or ferric chloride during the rust finishing process. This etch loosens the rust finish on the layers of damascus that are not so deeply affected by the rusting process, allowing the rust finish to be manually removed from these layers in a less aggressive manner; thus protecting the rust finish on the other layers in the damascus pattern.
Reasonably accurate description of damascus barrel finishing??
To recap; damascus knife blade finishing uses strong etchant solutions to deeply etch selective layers in the damascus pattern. This etching process creates a dark, corroded steel/chemical compound, which is probably not an actual oxide. This darkened material is often used to create contrast in the damascus pattern. Damascus blade finishing is fast and harsh. It is typified by destructive corrosion of steel. Damascus barrel finishing is a delicate process, using mild etchants to develop an iron oxide layer on the surface of the barrel. Both manual and chemical methods are employed to gently remove coloration from the damascus layers that have been least affected by the coloring process.
Now to discuss what is similar between damascus knife blade finishing and damascus barrel finishing. Both finishing processes rely on electro chemical corrosion, to either etch or rust the ferrous materials of the blade or barrel. Electro chemical corrosion of ferrous materials is easily researched on the internet. However most information concerns how to prevent corrosion, rather than how to utilize it for artistic embellishment. But an understanding of the reaction involved in electro chemical corrosion, plus the metallurgical knowledge of steel grain structure, will provide answers to the questions of what ferrous materials react to electro chemical corrosion faster and perhaps also darker.
To be continued…………
Hope no one is reading this!
