Unfortunately, very little factual and useful material on the details of bone charcoal color case-hardening has appeared in print as most of the craftsmen who practice this art were and are still very secretive, closely guarding their trade secrets and taking them to their grave if they do not pass them on to an apprentice. This is illustrated by the experience of the Colt Mfg. Co. in the mid-1950s when they experienced some difficulty in restarting the manufacturing operation for their single-action revolvers. After a 15-year hiatus in making these guns, they had lost several key older employees in the interim who had kept the details of their color case-hardening process to themselves and in sparsely written records. An expensive and time consuming research and development effort was then required to redevelop their color case-hardening operation. (Wilkerson, Don, The Post-War Colt Single-Action Revolver, Taylor Publishing Co., Dallas, Texas, 1978).

There is, consequently, a great deal of mystery, myth and folklore, and in some cases misinformation, surrounding the subject of color case-hardening. Most of what has appeared in print echoes many of the myths that have been around for decades. This includes tales of the use by certain practitioners of special ingredients such as charred leather, horn and other organic material, or even human bone to obtain certain distinct colors. Certain admixtures to the quench water are also said to have been used to enhance the colors. These materials may or may not have been used; however, their effectiveness to process is open to question.

The fact of the matter is that with only animal bone charcoal and hardwood charcoal as the carburizing agents, and with ordinary water from a municipal water system as the quench, any of the colors and patterns of colors found on originally color case-hardened firearms of the 19th and early 20th century can be closely duplicated. The art of color case-hardening is still alive and well today as several firearm manufacturers in Europe still use bone charcoal color case-hardening to finish some of their better guns, and Colt still color case-hardens some of their revolvers. In addition, there are a small number of craftsmen in this country who do beautiful restoration work using bone charcoal color case-hardening closely duplicating the original firearm colors or yore.

The purpose of this article is to try to take some of the mystery out of the subject of color case-hardening by reporting the details of the author's experiences learned through several years of research and experimentation. This study has led to a better understanding of the scientific principles underlying color case-hardening and the development of techniques that allow one to restore the case-hardening colors on high-quality double guns and other quality firearms. Color hues and textures, and patterns of colors that are extremely close to the original ones can be achieved using these techniques. Perhaps others who may wish to try their hand at color case-hardening can then benefit from the author's experiences and mistakes.

The process of case-hardening itself has been known for over a thousand years. It is speculated that it was known and used in China as early as the eighth century A.D.. A written description of the process of case-hardening hand files was provided by a Benedictine monk, Theophilus Presbyter, in the latter half of the ninth century indicating that the art was fairly advanced at that time. Relatively crude case-hardening was also utilized in the manufacture of some weapons and armor during the middle ages. With the industrial revolution, case-hardening became crucially important in the manufacture of tools and machinery. In the early part of this century, case-hardening of steel parts was still an extremely important and widespread process used in industry including the manufacture of bicycle and automobile parts. With the availability of modern alloy steels, case-hardening of steel is not so important today.

Case-hardening involves a process called carburizing which produces a thin layer of very high carbon steel on the surface of an iron or mild (low carbon) steel substrate. If the surface carburized steel is then heated to a high temperature (above the so-called critical temperature of approximately 720 degrees Centigrade) and then quenched rapidly by immersion in water, the high carbon steel layer (case) undergoes a phase transformation and becomes very hard and is extremely wear and corrosion resistant. The steel beneath the carburized skin or case is unhardened and remains as tough and resilient as it was before the carburizing and quenching. Both the carburizing and the quenching and hardening constitute the case-hardening process.

Carburizing can take place by a variety of methods. One method, which is also one of the oldest, is a process called pack hardening where the mild (low carbon) steel substrate is packed in a closed, but not air tight, container with the carburizing agents such as a bone and wood charcoal mixture. At elevated temperatures, the carburizing agents chemically produce an atmosphere in the container that is predominately carbon monoxide plus a small amount of carbon dioxide. At these high temperatures, carbon monoxide reacts chemically with the iron near the surface of the mild steel substrate to form the chemical compound iron carbide, which diffuses a short distance into the substrate steel. Pure carbon or wood charcoal alone is not an effective carburizing agent as it produces very little carbon monoxide in a closed container at elevated temperatures. Another substance, usually called the energizer, is added to the wood charcoal to facilitate the production of carbon monoxide in the closed carburizing container. This is the main function of bone charcoal in the carburizing mixture.

The higher the temperature and the longer the substrate is left in the container at this high temperature, the greater the depth of the carburized layer. Strangely enough, the equilibrium chemical kinetics of the carburizing process produces higher surface carbon concentration at lower carburizing temperatures albeit with a correspondingly thinner layer of carburized steel. This phenomenon has great significance in the case-hardening of firearms parts (with colors) as only very thin carburized skins are usually necessary for firearms in most cases, particularly if a good quality low-carbon mild steel forging is used for the parts. Other organic materials can also be used for carburizing and mixtures of organic materials with certain metallic salts as energizers are used in commercial pack carburizing materials today. In most commercial pack-hardening operations, the carburizing and quenching are two separate steps in order to conserve the reusable carburizing material. The carburizing and quenching can also take place in the same step by merely dumping the hot substrate and carburizing material together into the quench bath after carburizing has taken place. This is the usual procedure for small parts and is, probably, how color case-hardening was discovered. When small parts that have been carburized with bone and wood charcoal are quenched in water in this manner, they often, but not always, show interesting decorative colors and color patterns. These colors range from straw color to brown, dark blue, light blue, white and shades of red, and are closely related to, but not identical to, temper colors that one obtains by heating polished steel in air to temperatures of 400 to 600 degrees F. The composition of the thin-colored layers will be discussed below.

Early gunmakers were undoubtedly aware of and utilized pack case-hardening in the fabrication of firearms. Indeed, the flintlock and snaphance systems rely upon the frizzens being properly case-hardened. High carbon steel was also available in small quantities and was used to make springs and other small critical parts. The high cost of this material precluded using it for large parts in early firearms. Hard and tough alloy steels that we know today were not generally available until the second decade of this century. The moderate cost of mild steel and the resilience and the wear and corrosion-resistant properties of case-hardened steel made it very sensible to use in almost all parts of early firearms except for barrels.

It is not known exactly when color case-hardening was discovered and when it was first used on firearms. John Bivens ("Metal Finishing for the Custom Gunsmith," Rifle Magazine, No. 50, March-April 1973) describes a pair of French flintlock dueling pistols ca. 1730 with striking original case-colored lockplates. He also estimates that, based upon surviving specimens, the color case-hardening of firearms did not become popular until about 1800.

If one carburizes a piece of polished steel by heating it in a closed container in a mixture of bone and wood charcoal, and the dumps the hot contents of the container into a water quench, colors may or may not be seen on the part when it is removed from the water depending upon a number of factors. If air comes into contact with the hot steel before it enters the water, it will develop an ugly scale oxide covering the entire surface. This happens if the contents of the container (both the charcoals and the part) are emptied too far above the surface of the water and the protective envelope of carbon monoxide and other gasses escape and no longer keep air from reaching the hot steel surface thus producing strong uncontrolled oxidation. If the contents are dumped close to the surface of the water, coloration of the steel occurs randomly and inconsistently during quenching. A number of experiments including tests with irregularly shaped pieces show that colors are more consistently obtained on surface areas that are not immediately washed clear of the charcoal mixture as the steel parts drops into the quench water. These experiments have shown rater convincingly that colors occur during the quenching process only on areas of the steel where the bone and wood charcoal mixture remains in relatively intimate contact with the steel surface. During the quench, which takes place on a time scale of a small fraction of a second, the quench water contacts the hot steel surface producing hot water and steam that evidently interacts with the calcium phosphate that is the major constituent of bone charcoal. Chemical reactions evidently take place involving the calcium phosphate as well as the dissolved oxygen in the water producing the colored layers on the steel. From a practical point of view, any steps that can be taken to allow the charcoal mix used in the carburzing to remain in relatively intimate contact with certain areas of the steel during the quenching process will assure that colors will be produced in those areas. The precise way this is done determines the hues, texture, and the patterns of colors produced.

In order to study the colored layers that are produced by this process, several small samples of mild cold-rolled steel measuring approximately 1/8 by 1-1/2 by 2-1/4 inches were first annealed and polished and then color case-hardened under a variety of conditions using techniques that will be subsequently described. These samples were than subjected to scientific analysis by X-ray energy dispersion spectroscopy to determine the atomic elements present in the colored layers. The topography and morphology of the layers were also examined with a scanning electron microscope. These samples were color case-hardened with packing of different mixtures of bone and wood charcoal ranging from 100 percent to 10 percent bone charcoal with the balance wood charcoal. For comparison, a sample was prepared by "temper-heat" coloring with an oxygen-rich-torch. The analysis of the atom content of the colored layers of each of these samples was then performed. The results of these analytical tests show that the largest percentage of atomic elements in the thin films that constitute the color case-hardened layers are iron and oxygen indicating that these layers consist mainly of iron oxide. These tests also show only a small amount (approximately one atomic percent) of these layers consist of calcium and a smaller percentage of phosphorus. This is not too surprising as bone charcoal contains approximately 80 percent tri-calcium phosphate which dissolves readily in hot water which is produced in profusion near the surface of the hot steel during the quench. This can lead to the production of trace amounts of chemical compounds containing these elements. Iron oxide is, of course, the same material present in "temper" color layers on the surface of steel heated in air to temperatures between 400 to 600 degrees F.

The most interesting and surprising property of the color case-hardened surfaces is shown by the scanning-electron microscope examination of the surface topography of the samples with magnifications of X 3000 to X 5000. The sample that was "temper" heat colored has a relatively uniform thin oxide layer that follows the microscopic contours in the steel substrate produced by polishing with occasional open spots where the oxide layer has flaked off. The color case-hardened samples, on the other hand, have surface layers with an extremely high degree of structure. These samples show layers consisting of stacked irregular shaped globules that are closely packed and appearing to be quite thick. These globules consist predominantly of iron oxide and the globule size ranges from 0.5 to 20 microns in size (one micron is one millionth of a meter and .001 inch is equivalent to about 25 microns). The higher the concentration of the bone charcoal in the pack, the larger the dimensions of these globules. There is also evidence that a thin continuous film layer of iron oxide lies below the globules next to the surface of the steel.

These tests provide some clues that help to explain the characteristics of the color case-hardening color layers as well as the chemical processes that take place in their formation. The colors exhibited by temper heat coloring have been long known to be due to a thin-film optical reflection and interference effect much like the spectrum of colors seen with thin uniform films of oil on the surface of still water. The thin film in this case is a solid continuous thin layer of iron oxide on the surface of the polished steel. With this effect, the colors observed at a given spot on the surface depends to some extent upon the angle at which the spot is viewed relative to the surface - the colors changing slightly as the angle changes. The range of colors observed depends upon the thickness of the oxide film.

With color case-hardened surfaces, the colors observed do not in general change with the viewing angle relative to the surface except for colors produced with very small percentages of bone charcoal. These colors are also produced predominantly by optical reflection and interference affects in the thin continuous iron-oxide film immediately overlaying the steel surface. The layers of iron-oxide globules stacked on top of the continuous thin film of iron-oxide also play a role and effect the properties of the observed colors. Reflections of light from the interior and exterior surfaces of these globules and from the steel surface can also produce interference effects and alter the color of the reflected light. The predominant effect of the globules is probably their acting as miniature lenses and prisms which diffuse the reflected light in all directions, and the color is determined primarily by the thin continuous oxide film on the surface of the steel. The resulting reflected light waves from this interference/reflection effect are in a narrow range of optical wavelengths and are emitted from the surface in all directions producing the appearance to the eye of colored surface which is independent of the viewing angle. There is a possibility that small quantities of calcium and phosphorous compounds could also be occluded in the iron-oxide globules producing some coloration of the globules themselves. The large globules present when high percentages of bone charcoal are used also explain the dull matte finish that is characteristic of case-hardening colors produced in this manner.

Chemical reactions that take place near the surface of the steel during quenching produce the irregular globules of iron oxide that are the fundamental reason for the properties of case-hardening colors. These reactions most certainly involve calcium phosphate which is the majority constituent of bone charcoal. This of course is the reason that true case-hardening colors are produced only when the bone charcoal is forced to remain in relatively intimate contact with the steel surface during the quench. Experiments in which case-hardening colors are sought using only wood charcoal as a carburizing agent and with fresh unused quench water fail to produce true case-hardening colors. Very weak faded colors obtained in this manner are probably produced by oxidation of the steel surface in the form of thin continuous films by dissolved oxygen  in the quench water. Adding only a small amount of bone charcoal to the mix under the same conditions does produce true case-hardening colors. There is also considerable evidence that using only wood charcoal as a carburizer and quenching in previously used quench water in which bone charcoal has been allowed to stand for some time and calcium phosphate has dissolved, also produces true case-hardening colors albeit fairly weak and thin. The colors produced with small percentages of bone charcoal are very thin and show no surface matting, and are dependent upon viewing angle to some extent because the globules produced in this case are of a size that is equal to or less than the wavelength of visible light. This results in reflection and interference effects that are almost identical to that obtained with continuous uniform thin films of iron oxide as is the case with temper colors.

The exact chemical processes that take place during the formation of case-hardening color layers is not precisely known. It is certain that hot water and steam produced as water contacts the hot steel surface can interact with and dissolve some of the calcium phosphate in the bone charcoal which remains in close proximity to the steel surface. Oxygen dissolved in the water and also carried along in air trapped near the steel during the quench can also play a role in these reactions. One very likely scenario is the production of iron phosphide when the phosphate ion, present in dissolved and decomposed calcium phosphate, contacts the hot steel surface. Iron phosphide then reacts immediately with water to form iron oxides which are insoluble in water and which can precipitate and aggregate as a thin film on the surface of the steel and as the stacked layers of iron oxide globules. This reaction also produces phosphine gas in small quantities which very rapidly is converted to other chemical compounds in the water. This conversion is very fortunate as this gas is highly toxic and, as a precaution, good ventilation should always be used when quenching with bone-charcoal mixtures. If this is indeed the chemical process that produces the globules which are responsible for the properties of case-hardening colors, most of the oxygen that forms the color layers derives from the oxygen present in the water molecules and in the calcium phosphate by means of this chemical reaction, and dissolved oxygen in the water plays only a minor role. This is consistent with observed results as colors can be enhanced by increasing quench water oxygen content, but this enhancement is not essential to achieve case-hardening colors.

Experiments were also performed on small steel samples where pure tri-calcium phosphate in the form of fine powder was substituted for the bone charcoal in the carburizing mixture, as well as with pure wood charcoal as the caraburizer along with this chemical dissolved in the quench water. Both experiments yielded true case-hardening colors. The colors were restricted, however, to the areas of the samples that were located in close proximity to a steel fixture used to hold the samples. These results add further evidence that the above model of the chemical processes involved in color case-hardening is probably an accurate one. This also seems to indicate that the rate of cooling during the quench, and the effect of the granular structure of the bone and wood charcoal on the cooling rate, play a significant role in this process.

At this point, it is interesting to examine the mechanical properties of steel color case-hardened in this fashion. Examination of several original American-made double shotguns that were originally case-hardened and had never been refinished indicated that all had hard steel surfaces that passed the so-called "file test" whereby a hand file passed over the surface with moderate pressure applied, would merely slide and not cut the metal. Standard Rockwell hardness testing of case-hardened parts of these guns indicated only a very superficial hardening had been originally performed. All of the parts tested indicated a Rockwell hardness very close to that of modern cold-rolled mild steel. This is because in the hardness testing, a small extremely hard steel ball is pressed into the surface of the steel being tested under various loads, and the depth of penetration of the ball measures the hardness of the test piece. In the above tests, the hard ball merely stretches the extremely thin layer (case) of the part and deforms and measures the hardness of the steel substrate itself.

Similar results were obtained with hardness testing of the samples described above. Only samples prepared with percentages of bone charcoal greater than 50 percent produced measured hardness greater that the cold-rolled steel substrate. A sample produced by carburizing in 100 percent bone charcoal for two hours at 715 degrees C. and quenched at this temperature was cross-sectioned and examined under a metallurgical microscope. The surface-hardened layer was found to be only .002 inch thick and had undergone a true transformation to the Martensite phase of steel indicating that true hardening had occurred. Since the percentage of bone charcoal also affects the production of carbon monoxide in the pack and thus the amount of carbon introduced into steel, even thinner layers would be expected with steel casehardened with smaller percentages of bone charcoal under similar heating and quenching conditions. This is borne out by the hardness measurements of these samples.

These results indicate that there is strong evidence that most of the early color case-hardened guns were probably very superficially case-hardened with relatively small percentages of bone charcoal in the wood/bone charcoal mix, and they were also probably carburized at temperatures near the critical point and quenched at this temperature or slightly below this temperature. Experiments have demonstrated that colors matching those on most early original firearms can best be obtained by carburizing at or near the critical temperature and then allowing the container to cool as much as 100 degrees C. before quenching. Although this does not produce optimum case-hardening, a durable high-carbon steel layer is produced that is file hard, and very attractive colors can be obtained in this fashion. Quality of the decorative colors rather than obtaining optimum hardening may have been the primary concern of many of the early gun makers especially after good quality low-carbon steel forgings became available.

The above tests and other experiments have demonstrated that there are several variables that control the color hues, the texture of the colors, and the color patterns that one obtains in the bone charcoal color case-hardening process, which, if recognized and controlled properly, allow one to consistently and repeatedly obtain excellent results with the colors and coverage of colors on case-hardened parts. These variables are the percentage of bone charcoal in the carburizing mixture, the quenching temperature, the distance of the carburizing container above the surface of the water during dumping of the workpiece and charcoal mixture into the quench water, the quench water temperature and oxygen content, and finally and perhaps most importantly, the method used to slow the washing of the charcoal mix from the surface of the workpiece by the quench water in the areas where colors are desired. The temperature and duration of carburizing and the temperature at quenching also affect the mechanical properties of the surface.

Part II will be presented in our Spring 1997 issue.

Images from this article - click on the thumbnail to see the big picture

Scanning electron micrograph of the surface of temper-colored sample.

Scanning electron micrograph of the surface of one of the color case-hardened samples.

Some of the test samples used for surface analysis of the color case-hardened layers.

Parker Bros. VH ½ gauge 1-1/2 frame gun, color case-hardened by the author.

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