Bone black is a natural black pigment created by destructive distillation of animal bones in the absence of oxygen. Animal bones are carefully selected and prepared for calcination. Only fresh, hard bones free from extraneous material can be used. This is accomplished by boiling or rendering the bones to remove fat and oily residues. Once the bones are defatted, they are placed into hermetically sealed retorts in kilns, through which they pass slowly during 24 hours at a temperature of 800° to 900°C. This process of manufacture leaves bone char which is the raw material for bone black pigment.
A well burned char has a firm physical structure, high porosity and white ash upon ignition. Each grain of char consists of a series of cavities connected by a great number of minute tubules and channels originally containing the nerves and blood vessels. The framework of this cellular structure consists primarily of tricalcium phosphate, which is entirely coated by a deposit of carbon in a state of very fine subdivisions.
The subsequent pigment is created by crushing the bone char and screening it into various sizes, which range from 0.3 to 44 microns. These intermediary products are then custom blended to create a wide variety of shades which are suitable for many different applications. Some area applications are artist colors, paper products, paints and lacquers, leathers and vinyls, and plastics. A unique area is in case hardening for gunsmithing.
Bone black is not a competitor or a replacement for carbon black. In tint strength, carbon black is superior to bone black, however, bone black has a high loading capacity in it's favor. Along with this feature, bone black has many properties which make it unique. First and foremost, bone black is classified as non-hazardous and does not contain aromatic hydrocarbons which are considered to be carcinogenic. Because of this, bone black can be used for food grade applications which meet the Food Chemical Codex.
Secondly, bone black has a dull black, non-glossy finish with a very jet masstone. It retains consistent color from start to finish through the milling process. Milling time is cut because it doesn't require extensive ball milling to disperse. It actually disperses easily in aqueous or oil base vehicles because it does not float or flocculate and can be used in high speed dispersing equipment. In fact because bone black is not an oil by-product, it can be dispersed in water by stirring with a spoon.
Another attractive aspect to bone black is that it does not overpower colors being toned. It actually makes an excellent toner for many colors, and allows for adjustments in tinting strength. Along with this, bone black is a non-migratory black, as well as being non-conductive.
Other properties of bone black are its low vehicle absorption and heat stability. Bone black's low vehicle absorption allows for a high loading capacity. In some applications, loading capacity is 25% higher than carbon black, which tend to get gummy at high loading capacities. Also, bone black has a greater heat stability than black iron oxide and is comparable to carbon blacks.
Again, bone black is not a replacement for carbon black, rather a special niche black used in applications where carbon blacks cannot be used. There are alternatives in the pigment industry, and with a little research and a slip back in time to discover the forgotten black.