I've spent a lot of time considering Adam's photomicrograph of the blown out barrel section and his analysis of the black spots being graphite nodules. I find the same anomalies in micrographs that I have taken of the old damascus barrel in my possession. But too, find them in micrographs of damascus that I have made from modern steels.
While I am NOT an expert in the field of micrograph analization, I have found that there are a number of reasons why black spots can appear in micrographs. They can be caused by the degree of surface prep of the sample, the effects of the type of etchant used and also the size of the material's grain structure. The micrograph posted of the blown out barrel section was taken at 20X magnification. The microscope in my shop is only capable of 20X maximum. I have found it very difficult to determine the exact cause of small anomalies at this magnification level. I hope that Adam will view the blown out section at a higher magnification level, to make certain of his analysis.
Regarding the analysis that the black spots are graphite nodules from the use of ductile iron, or any type of cast iron being used in the manufacture of damascus barrels; I am not yet convinced. I concede that the black spots appear similar to graphite nodules in the 20X image. However, the specific product named "ductile iron" was not invented until 1943. Ductile iron has magnesium added, which inhibits the graphite's growth directions, turning it into nodules. A similar graphite nodule formation can be created in gray cast iron, by heating it to 900C/1650F, and holding it at that temperature for 72 hours. I seriously doubt that damascus barrels were subjected to such a heat treatment process.
Nothing in my research of the historical manufacture of damascus barrels has indicated that cast iron was used. And my experience with making damascus steel causes me to doubt that cast iron could be utilized to weld damascus. Cast irons have a very high carbon content. The high carbon content makes cast iron what blacksmiths call, "hot short". If you heat a piece of cast iron and hit it with a hammer, it will shatter. It cannot be forged. I cannot conceive of trying to incorporate cast iron into a billet of damascus.
Again, I am not an expert and I don't intend to be argumentative. But my message to Adam, is to look closer. The use of cast iron in a damascus barrel seems improbable to me.
My research into damascus barrels has led me to a theory that the barrels were made up of a medium to high carbon steel, laminated with wrought iron. There is ample historical documentation stating that the barrels were made of steel and iron laminations. I base my theory of the iron element being wrought iron, from having read innumerable pages of old documents on iron making and usages, that were written contemporary to the manufacture of damascus barrels. In all cases that I have found, when the word "iron" was used, the material being spoken of was wrought iron. This was simply the common name used at that period of time. If cast iron was being written of, they used the words cast iron, gray iron, pig iron, etc. If they used the word "iron", they meant wrought iron.
Wrought iron was produced from cast iron, by a process of heating the metal to just below the melting point. An air blast was directed at the surface of the iron, to burn off the carbon content and most of the damaging impurities. The iron was maintained in a semi-melted plastic state and constantly stirred to mix in the iron oxides that formed on the surface of the iron from the air blast, and also silica from sand and limestone used as a flux. This admixture being basically an iron oxide bearing glass. This constant stirring of the material is where the name "wrought iron" comes from. Wrought meaning, hand worked. Once the iron was sufficiently decarburized and thoroughly mixed, a ball of the material was removed from the furnace and taken to a power hammer, or press, where the ball was forged into a bar. During forging out the bar, the globs of iron/silica are stretched out into strands, lengthwise of the bar. This first bar was often cut into pieces, restacked and forge welded, then drawn out again. The restacking and welding would reduce the size of the strands of silica and also force out some amount of it. Depending on the intended end use of the material, restacking and welding could continue until the material was nearly pure iron. A common product of the foundries was called merchant bar. Merchant bar was fairly high in silica content. The intention was that the end user could restack and weld the material to suit their needs.
Below are three micrographs that I took of a piece of wrought iron from a anchor chain link. You can see the strands of silica in the side (first) image, running lengthwise of the bar. The micrograph of the end of the bar, shows the black spots created by the ends of the silica strands. Note the image of the entire end of the wrought iron bar. The lighter hued lines are the edges of the restacked bars that were forge welded to make up this single bar.
I have a section of damascus barrel tube, that I purchased from Peter Dyson. This barrel tube is in its as forged condition. It was never finished out. It still has the chemise inside it. I sawed part way through the tube and then broke it off. Below are two micrographs of the end of the broken section. The layers of steel in the damascus are very fine grained and a flat gay color. The layers that I theorize to be wrought iron, are very course grained and include black spots; which I believe to be silica strands. The chemise appears to be of nearly pure iron. No silica strands are evident in the chemise at this magnification.
Below is a micrograph of an etched section of the damascus barrel. This image shows black spots in both the steel layers as well as the wrought iron layers. The above image of the broken barrel section does not indicate the presence silica strands, or graphite nodules in the steel layers. I can only assume that the black spots were created by the effect of the etchant.
Below is a combination of two micrographs of the ends of damascus bars that I made. The steels in this damascus are 1084 and 1018. The left image, is of the damascus after thermal cycling to reduce the size of the steel's grain structure. The right image is of the same material, after heat treating to create a spheoridized grain structure. Spherodized grain, is a very large structure. Notice the differences in how they etched. Also notice in the left image, the thin lines. These are decarburized weld lines, where the damascus billet was restacked and forge welded. This area etched less, because of the lack of carbon. In the right image, the weld lines are nearly invisible. During the heat treatment to spheroidize the material, the extra time at heat allowed carbon to migrate back into the decarb areas.
