The question has been raised about the coloration of the iron and steel elements in damascus. The smith will select materials for the elements in the damascus and then also the finishing chemicals that will create the desired appearance in the finished product. The color of the material after etching is determined by the alloys in it. Steel with a manganese content will typically etch black. Iron will etch gray through charcoal. Nickel will produce silver and chromium a light gray. The smith can use several different types of steel for his damascus and create layers in the pattern with varied shades from silver through black in the oxides formed by the etchant. But, there’s more. If the damascus is deeply etched, it creates topography (high and low areas) on the surface of the material. Whatever element that is least affected by the etchant will be proud of the element that was more eaten away by the etchant. When lightly sanded, this higher element will become lighter in color, or silver. Typically, it is the higher carbon content material (steel) that etches quicker. This would leave the iron layers standing above the steel layers. So after sanding, the iron would finish silver. If the damascus is then blackened or browned after etching and sanding, the colors could again change, depending on which element has the greatest affinity for the coloring chemicals. So, it would be a stretch to categorically state the either the iron or steel will always be a certain color. It is all dependant on the finishing process.
In the photo posted earlier of the defects in the barrel pattern, it was obvious that the condition was caused by decarburization of material that had been exposed to the fire for an extended period of time. Decarburization would have occurred over the entire surface of the barrel tube during the forge welding and shaping process of the tube. The decarburized outer material would be ground away during the finishing process on the tube. In the area of the flaw however, the exposed edges of the material extended below what would become the outer surface of the finished tube. Once the welds were closed on the decarburized material, there was no possibility to grind them away in the finishing process.
Surface decarburization is something that is critical for a bladesmith to understand. A blade must be forged with enough material left to remove from the outer surface to eliminate all decarburized steel. This is especially critical at the blade’s edge. The thin area of the edge is very susceptible to decarburization. Carbon loss from the steel at the edge will cause a loss in hardenability of the steel and result in inferior edge holding ability. It was long thought that it was possible to increase the carbon content in steel by using a reducing fire in the forge. A reducing fire being one that had insufficient blast to sustain complete combustion of the forge fuel and therefore leave available carbon in the forge atmosphere that the steel could absorb. Metallurgical testing in recent years has disproven this concept. Decarburization is especially problematic at the heats required for welding damascus. The high blast rates necessary to bring the fire and steel to a welding heat will certainly cause decarburization.
Another fairly recent metallurgical learning that can affect the appearance of finished damascus is carbon migration. It was long believed that forge welding high carbon and low carbon steels together would result in layers with the same carbon content as the original steels. It has been metalurgically observed that carbon will migrate across the weld boundaries from the high carbon steel into the low carbon steel. The carbon is essentially trying to form an equilibrium throughout the matrix of steel. This migration happens in a relatively short amount of time and happens quicker at higher temperatures. So if two steels are used that only have different carbon contents and no other alloying difference between them, there is a possibility that carbon migration will render them substantially the same by the end of the forging process. Being nearly the same, they would not be so differentially affected by etchant and coloring chemicals. This could be a plausible explanation for damacus barrels that do not display a distinct pattern.
Carbon migration can be stopped by placing nickel between the layers of steel. This is often done by modern damascus smiths. The result is a very high contrast damascus. The nickel finishes bright silver and will not color if the damascus is blackened. Nickel is not a suitable material for use in a knife blade, as it will not harden, so this combination of materials is only useful for knife fittings or other damacus items that do not require heat treatment for hardness. A question I have concerning old damascus gun barrels is whether the silica content of the wrought iron used for barrel making had the ability to mitigate carbon migration. There is not much wrought iron used in modern damascus, so I know of no testing that has been done in this field. I have sent a piece of an old damascus barrel to a friend and fellow knifemaker, Kevin Cashen. Kevin is arguably one of the most knowledgeable metallurgists on the planet. I have asked Kevin to do some testing on the barrel section, to include carbon migration examination. Kevin’s web site includes a tremendous amount of metallurgical information and is worth a visit.
Cashen Blades Speaking of heat treatments; I find no mention of heat treatment processes performed on the barrel tubes in any writings contemporary to the manufacture of damascus barrels. I suspect that many of these barrels contained a sufficient amount of carbon to have been capable of hardening, if heated to a high temperature and quenched. They likely would have then needed to be tempered, or annealed, to mitigate brittleness. As a brittle gun barrel is highly undesirable, I’m certain that the smiths avoided any rapid cooling of the tube from high temperatures. They were however, doing a type of heat treatment process that I am certain that they did not fully understand. I have read in several writings of the cold hammering of barrel tubes after welding. The tubes were not actually “cold”, but in blacksmith terms, at a heat that was too cold to effectively move the steel under the hammer. This would be a low red heat, down to a temperature of about 900 to 1,000 degrees F. Greener states that this hammering “greatly increases the density of the metal”. The interesting part is that the actual hammering was totally unnecessary.
In the bladesmithing world, cold hammering of a blade edge has been taught for decades; the description of the process was called “edge packing”. The concept was that the hammering of the cold steel compressed and refined the grain structure of the steel, creating a smaller and tighter structure. The tighter grain structure would take a keen edge better and retain sharpness longer. Edge packing was taught to knifemakers as an essential part of blade forging until about 20 years ago. Finally, metallurgical testing was done, revealing that the actual hammering of the cold steel had no effect. In fact, it was probably detrimental, risking stress fractures. Too, logic will prevail, in that steel is a solid and solids cannot be compressed. So, no density changes can be made to the material by hammering. Still, changes that were visible to the naked eye occurred to the steel during this process. The surface of the steel looked smoother and actually looked denser. If a blade was heat treated and then broke in two, the grain in the cold hammered blade appeared much tighter and smaller. This was also very apparent in damascus blades. The finish of a damascus blade that was not cold hammered was coarse and grainy looking. So, I am certain that the barrel smiths also realized that their barrels had a better appearance after the cold hammering process. They too would have assumed that the hammering of the steel was making it denser; as that was the look that it had after the process.
The phenomenon of grain structure changes is purely the result of the temperatures that the steel is heated to. At high temperature (such as forge welding heat) the crystalline structure of steel becomes very large. As the steel cools, the structure reforms into smaller crystals. However, one cooling cycle is not typically enough to reform all of the crystals. So, the steel is reheated again to a temperature that is low enough not to create large crystals and it is allowed to cool to a black heat again. The lower temperature is sufficient to promote the reformation into smaller crystals, without creating large crystals again. This heating to low red heat and cooling in room air to a black heat is repeated three or more times. The process is called thermal cycling, or normalizing. I am convinced that this is exactly what the barrel smiths were doing during the cold hammering of their barrel tubes.
Addressing the questions about why stars in the damascus patterns will transition between white and black, requires an understanding of what the smith is doing and also working with. This is a bit of a generalization, but the layers of iron/steel in a damascus barrel will be around .003 to .010 inches thick. Every hammer blow displaces the material, both downward from what will become the surface of the finished piece, as well as in relation to the material immediately adjacent to the impact of the hammer face. The smith will utilize overlapping hammer blows to attempt to keep the displacement of the layers as even as possible. It is important to understand that the smith has no visual reference for what is happening to the pattern, other than the appearance of the work-piece surface and his experienced knowledge of what is happening to the layers inside the material. The pattern cannot be seen at all during forging, as the surface is totally covered in forge scale. Given the hundreds of hammer blows that fall on the material during the welding of the rods into riband and the subsequent welding of the barrel tube, it can be seen that the small margin of a few thousands of an inch in layer thickness make it impossible for the smith to place all of the layers in an exact position. If when the barrel is sent to the boring room, it does not completely clean out, it is returned to the smith to be forged down in that area to reduce the bore for additional reaming. The affect of this additional forging could easily show up in the pattern. A barrel that displays a short area where the pattern makes a sudden change could have been one that was sent back to the smith.
The barrel grinder has no contribution to the pattern development at all. During grinding, the pattern cannot be seen. The surface of the barrel is totally in the white. If the bore of the barrel is not concentric with the outside of the tube, the grinder has no choice but to remove the excess material from the thicker side of the tube. Uneven grinding for whatever reason will affect the pattern.
Control of a damascus pattern is something that every damascus smith deals with. You know what you planned for it to be. You forge it as evenly as possible. If you forged well, you will grind an even amount of material away and hopefully preserve the pattern. But, you never know for certain until the etch. Etching is like opening a present for the damascus smith. You hope to get something beautiful and if you have done well you will. If not, you discover you got the ugly necktie.
