Charcoal Dust Bluing
11 Minute Read
Sometimes when you are looking for one thing, you discover something else. The unexpected can change your course and you find yourself embarking on a new journey. One such turn of events introduced me to a unique metal finish and restored an old process to new life.
Nearly everyone who has researched history will agree that our ancestors felt little inclination to record details. They seemed to feel that the minutia of everyday life and particularly their work were either uninteresting or, more probably, private. The latter is especially problematic when we look for technical processes.
There is a marked lack of surviving written information on metal coloring as practiced before the 19th century. With the long-established tradition of passing information from generation to generation by word-of-mouth, tradesmen apparently assumed that knowledge handed down would never be lost. Only what they deemed as necessary for the general public to know was included in their writings, details have to be inferred or usually discovered through indirect means.
The Master/Apprentice system caused the loss of many techniques. Processes were jealously withheld from outsiders, and if they went out of use or were replaced by something new, they would quietly disappear. This secrecy prevented much of the information on processes from ever reaching the printed page. It was a sorry apprentice who would divulge his master's secrets and there was a tendency with most of the artisans in the factories to hold back details in order to garner higher esteem for themselves. One example was in the European armories, where both the etching and gilding processes were secrets. When the Hardner was busy doing his work, he would avoid 0even the presence of his trades Kinsmen.
Finally, the few processes that have been recorded involved vague instructions and exotic chemicals, or mysticism to make them more secret and mysterious. "As is often the case, the few surviving print references did not tell the same story that the surviving objects told." Some Parts were included strictly to add mystery while others were disguised methods to ensure care in the process. One such rite was orienting a knife blade due north when plunging it into the quench bath. A practice such as this added an element of mysticism, but at the same time, ensured that the handler would take great care in the process, ensuring consistent results.
The consensus today is that certain methods for bluing steel had not been invented until the 19th century. It has been assumed that any coloring on steel parts was restricted to fire blues or chemical browns until the 19th century. This opinion has held in spite of physical evidence from before that time showing traces of chemical blues and blacks.
References to steel coloring must be researched by examining armorers' publications. In the 18th century and before, the armorer was the primary source for metal finishes and other trades that used such finishes borrowed the techniques from these arms makers. Before 1800 the wealthy class was the primary patron of the armorer for armor, swords, and firearms, consequently, the richer the embellishment, the more successful was the artisan. Suits of armor were often colored black, swords were a brilliant blue, and firearms displayed both colors in their decorations. The armorer was constantly striving for better finishes and new processes. For this reason, they became the inventors and perfectors of most of the methods employed.
My original intent was to experiment with charcoal bluing. This was a coloring process known to be in use at least as early as the 18th century. It involved heating steel and charcoal together in a controlled atmosphere. The steel I used for the coloring was 2035 hot rolled steel. I picked this low carbon steel because its availability and common usage allowed me virtually unlimited access to stock which would help to insure consistent results. I used plates 1½ x 3 x ⅛" polished by hand to 600 grit. I tried several different polishes on the steel plates, from 400 grit finish to a mirror polish. I found that the 400 grit finish was rather dull and did not show the true colors. At the other end of the scale, the mirror finish from a buffing wheel involved more time in getting the piece ready than any benefit I derived from it. I settled on a 600 grit hand finish that was not as difficult to obtain, yet still showed the colors as well as the high polish. I used this finish on the experimental plates, though in later work on actual pieces, I opted for the mirror finish.
Along with the plates I colored with charcoal, I also used what I refer to as control pieces and these plates were what prompted me to change the course of my experiments. These pieces were identical to the plates I was coloring with charcoal, but they were left exposed to the air to take on temper colors. Fire blue is one of the names for the coloring they exhibited. It is also called temper blue and draw blue. This is a very transitory color that in the 18th century was considered a store finish, one that would attract a customer but would not wear well. Fire blue is a superficial oxidation of the surface by heating in the presence of oxygen and is the oldest method of coloring. It can be used as a flash color to quickly decorate a piece. A slightly more durable finish can be built up by letting the piece soak in heat for an extended period of time. The color variations of temper colors are well documented, so they made a good comparison for my experiments. The objective was to compare the color difference between the plate embedded in charcoal in a controlled, oxygen-poor atmosphere and the plate exposed to free oxygen at the same temperature and exposure time. I wanted to make sure that the color I was getting in the box was caused by the charcoal and not just the effect of the heat.
During the process of experimenting with charcoal bluing, several rimes, upon opening the furnace I found that I had accidentally allowed charcoal dust to settle on the control piece. This meant that the experiment had to be repeated, but more importantly, I noticed a difference in color where the charcoal dust had been.
In my research, I had found a reference to the coloring of sword blades in a recent translation of a 1771 German book on the trades, Handewerke und Kunste in Tabellen. It was an obscure reference, but it stuck with me. The line that attracted my attention was "rubbed with sweet oil and sprinkled with ash." This appeared to have some relationship to what I was doing and it warranted a closer look.
From my experience I had concluded that the ash mentioned was actually charcoal dust, and that the sweet oil was probably a light machine oil. My assumption regarding the ash was proven correct, but not being familiar with the oils available to the 18th century artisan the type of oil eluded me. In conversation with John Bivins of Wilmington, North Carolina, one of the translators of the above mentioned book, I learned that sweet oil was a term used for edible oils and he suggested either olive or walnut oil.
I ran a number of experiments with various plant oils, but with their heavy viscosity I was unable to bring out the brilliant colors I felt I should be seeing. Two other types of oils remained to be tried and I knew it would have to be a very thin oil. One was whale oil, which would have been readily available at the time, and the other was any of a number of modern, petroleum based oils which would probably have been unknow at the time. Today, of course, the situation is reversed, whale oil is no longer available, but we have a wide selection of synthetic and petroleum oils, so I selected a light machine oil that proved to work very well.
In my experiments with the coloring, I found that the 400-500°F temperature span produced the optimum color range, lower temperatures produced little color and at higher ranges colors quickly lost their brilliance. I used three plates in each run: a control piece polished and left clean to receive a temper color for comparison, a second plate dusted with charcoal without oil and the third piece oiled and dusted.
At 400° F the control piece was a red/yellow, the dry dusted plate was a darker yellow with a dark red overlay, and the oiled plate was a brilliant red/purple. Both of the latter were a deeper, more brilliant color than the control piece.
At 500°F the control piece was a brilliant blue/purple, the dusted piece showed a yellow tint, but was otherwise nearly devoid of color. The oiled plate was the most striking, it was a more brilliant blue than the control piece and with additional deep red overlay.
In later experiments I ran the plates through smaller temperature increments between the 400 and 500°F temperature ranges and achieved many color combinations in yellows, blues, and reds. It was simply a matter of stopping at the color I liked. When the piece was finished, it needed to be preserved and protected from oxidation. I found that oiling or waxing it dramatically darkened the color, so this had to be taken into consideration when coloring. I have purposely avoided relating exact temperatures for the colors I received because the length of time at a particular temperature was a factor. I also found that various steels reacted in different ways through the temperature ranges.
The process was relatively simple. A thin coating of charcoal dust was held against the surface by a film of oil and heated to develop an envelope of gas. The piece was worked to a high polish and placed on a sheet of metal so it could be handled without disturbing the coat of charcoal that was applied. I then gave it a fine spray coat of oil. I used WD40, but any thin oil seemed to work, it deposited carbon on the surface as it evaporated, which enhanced the chemical reaction the charcoal created. Next, I covered the surface with charcoal dust. I tried different granulations and found that the finest dust gave the best results by far. The grainier it was, the more uneven and mottled the color seemed to be. I ground the charcoal with a mortar and pestle and filled the foot of a panty hose leg. Wearing rubber gloves, I kneaded the ball of charcoal dust in my hand while passing it back and forth over the piece, completely coating the surface.
For smaller pieces, I used a heat-treating furnace set at the desired temperature. Before I placed the piece in the chamber, I touched it with my finger in a spot that would show when I opened the furnace. This spot did not seem to make a difference in the final finish if I kept it as small as possible. I turned on the furnace and let it reach the set temperature, then I put the piece in and let it stabilize for about fifteen minutes before I checked it. When I opened the door to check the progress, I could see the color in the bare spot. If it was not the color I desired I turned the temperature up 25° degrees at a time and checked frequently until I got a color I liked. This could be repeated past 500° at which point the pieces lost color. If I went past the color I had in mind, there was no option but to repolish the piece and start again.
For larger pieces, i used the kitchen oven. Though accuracy in temperature control was lacking, if patience was used, the temperature raised slowly and the piece checked often, very nice results could be obtained.
The colors are obtained through a chemical reaction involving the carbon content of the charcoal and the oil in the presence of oxygen with heat as the activator. Carbon is introduced into the steel by a gas-metal reaction. The heat generates carbon monoxide (CO) and carbon dioxide (CO2) mixed with free oxygen fanning an envelope of gas around the steel. The carbon compounds break down, diffusing into the porous surface adding carbon to the steels. The color and depth that result are in direct relation to the carbon and oxygen concentration, the temperature, the exposure time, and the chemical composition of the steel.
The charcoal dusting finish is a durable finish, more so than temper colors, but perhaps not quite as durable as a rust blue or caustic blue. I had expected it to be a very delicate finish until I started repolishing pieces I wanted to re-color. I found that it was difficult to remove when using a polishing compound such as 500 grit or white diamond. It is a finish that, with care, should last for many years.
With these experiments, which were repeated many times to reach some consistency in the results, I tried to bring some light on the coloring process which, I believe, has been lost since the 18th century. What I have found is by no means the last word on this process; each time I color a piece, I learn something new about it. This is a technique that I think must have been so commonly used in the past that our predecessors felt no need to record it. There are many such processes that have disappeared. However, if we remain diligent and willing to research the past, I am confident that such lost processes can be brought back to light.
Larry Niday is a metalsmith who lives and works in Knoxville, Iowa.
Maria Nicolai and John Bivins, Translators, Handwerke und Kunste in Tabellen, P. N. Sprengel, 1761, p. 21.
Ibid, p. 19.
James Anderson, "Charcoal Bluing of Rifle Barrels," Journal of Historical Armsmaking Technology, Vol. V. Friendship, IN: National Muzzleloading Rifle Association, 1993, p. 56.
Nicolai, p. 54.
George Krauss, Steels: Heat Treatment and Processing Principles. Metal Park, Ohio: American Society for Metals, 1989, p. 286.
Joseph R. Davis, ed., ASM Handbook. Metals Park, Ohio: American Society for Metals, 1991, p. 331.
Lyman Taylor, p. 116.
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