By Margaretha Held
Editor’s Note: This article is excerpted from “Cuttlefish Bone Casting: Theory of Mold Making, Design Possibilities, Practical Casting Technique and Analysis,” a paper by Margaretha Held that was published in the proceedings of the 2019 Santa Fe Symposium on Jewelry Manufacturing Technology.
Cuttlefish bone casting is one of the oldest casting techniques. Archeological finds of cuttlebone cast belt buckles in Bopfingen, Germany, date back to the beginning of the Middle Ages. Although it’s not in much use today within the jewelry industry, cuttlebone casting offers clear advantages over other casting methods, including low costs and the ability to produce a casting in half an hour. Simple and fast, the technique is worthwhile for creating individual pieces and can be used to reproduce old dies for forging, especially for lost counterparts of earrings or cuff links.
As with any casting technique, there are numerous factors that must be ad-dressed to ensure casting success. In addition, consideration must be given to avoid issues such as porosity and oxidation. Let’s take a look at the cuttlebone casting process, how it can be applied as an element of art to reproduce a piece of jewelry, and the many design options and structures that can be created with this technique.
The first question many have when they hear about this technique is what exactly is a cuttlefish. They’re mollusks that have been around since at the least the upper Cambrian Period, about 450 to 500 million years ago. Long before the first fish, these cephalopods colonized the open seas. On average, they reach a total length of 6 to 20 inches, weigh between 6 and 21 pounds, and have a life expectancy of one to three years.
Cuttlefish have a cuttlebone, a hard but porous internal shell. The cuttlebone is 80 to 85 percent calcium carbonate in the form of fine aragonite crystals and 10 to 15 percent collagen proteins. There are also small amounts of calcium phosphate, potassium chloride, and chitin. Microscopically, the cuttlebone consists of thin aragonite plates, which are supported against each other by numerous mini columns. The formation of fine line structures is made possible by the mineral components that are attached to form chitin structures. The size of the cuttlebone is about 4 to 14 inches (10 to 35 cm) long, 2 to 4.5 inches (5 to 11 cm) wide, and 0.4 to 1.6 inches (1 to 4 cm) thick.
It is that cuttlebone that can be used for casting. Cuttlefish have a very wide distribution area: They can be found in the north, from the Faroe Islands, across much of the eastern Atlantic and the North Sea to the Mediterranean Sea. In the south, the territory extends along the West African coast to South Africa. Cuttlebones are often found as washed-up flotsam along sea coasts. They can also be sourced as a waste product of the cuttlefish factory. Jewelers can source cuttlebones from jewelry supply houses, though they’re likely to find a larger selection of smaller cuttlebones available from pet supply houses.
Cuttlebone has long been used for casting. The oldest documented finds of objects made with the cuttlebone casting technique are in the burial ground of Bopfingen (Baden-Württemberg, Germany). This grave complex, comprising about 300 tombs, was built between the first quarter of the 6th century and the middle of the 7th century. Silver buckles were found in two graves of the burial field. On the backside of those buckles are imprints of the casting technique with clear cuttlebone traces.
When casting with cuttlebone, jewelers have a few options. Molds can be created by pushing carved models directly into the cuttlebone, making it possible to make a casting with undercuts. Alternatively, jewelers can carve molds directly into one or both halves of the cuttlebone.
The size and thickness of the cuttlebone naturally limits the size of the casting that can be made, especially when using a single bone for one piece of jewelry. For a larger and deeper work, two cuttlebones can be used to create one mold. The cuttlebone should not be completely dried or cracked, and the soft inside should preferably have a slightly concave surface. The cuttlebone must be big enough to have to at least 0.25 inch (7 mm) thickness of bone all around the model.
In addition, a sprue and vent lines will need to be carved directly into the cuttlebone. The sprue should be carved directly into each side of the mold and enlarged into a funnel shape. Thin radial vent lines from the heaviest parts of the model to the outside of the mold should also be scratched.
One of the greatest problems with cuttlebone casting is porosity due to the oxidation of the melt during the casting process. This problem is amplified if the melt is poured at too high of a temperature. The result is an increased porosity and oxidation in the area of the casting’s skin and a distribution that increases significantly from the middle of the ring shank to the sprue.
Figure 1 (above, left) clearly shows the reactions during the casting process. Gas bubbles go slightly up around the shape of the ring and exit at the sprue. Figure 2 (above, right) shows the reaction with the cuttlebone mold on the cast skin. Due to the hot melt, the small separated aragonite chambers burn and release air that is slightly enriched with CO2. (The CO2 content is related to the water pressure of the sea at the depth at which the animal lived but is only slightly present in the bone.) These figures also show the influence that vent lines play on porosity. Above the vent lines, the porosity increases markedly.
So, how can the casting quality be improved and which components can be modified?
Because the cuttlebone is a product of nature, there are few adjustable parameters associated with the material itself, such as moisture content or the presence of cracking in the as-received cuttlebone. Factors that can be controlled are the preparation in the cuttlebone, such as the position of the model, the size and orientation of vent lines, and the shape and size of the sprue.
• Position of the model. The position of the model depends on its size. There must be enough space around the model in the cuttlebone. If the cuttlebone is too small, the mold heats up more quickly and the effect of combustion and oxygen absorption is intensified. Therefore, it is better if the cuttlebone is too large rather than too small.
• The sprue. A sprue that is too long or even too thin causes the melt to cool down too much on the way to the casting object and reduces mold filling. A simple aid to practice is to make the length of the sprue about 1 inch (2.5 cm or two fingers wide). The width depends on the model; the smallest part should have the width of the model and enlarge in a funnel shape.
• Thickness of the vent lines. There appears to be a direct correlation between the thickness of the vent lines and the amount of porosity that occurs. Experiments showed that increasing the thickness of the vent lines also increases the surface roughness of the cast pieces. The faster the air can escape from the mold (through larger vents), the faster the melt flows in. This acceleration leads to a greater rate of combustion associated with greater porosity.
• Position of the vent lines. In addition to the thickness of the vent lines, the position of the lines also appears to play a role in how much porosity occurs. During experiments, three angles (30, 45, and 60 degrees) were defined, one at the bottom of the shank and one at the sprue side of the shank. The results showed that the position of the lines influences the quality of the cast rings. The angle of the vent lines with the highest density is 30 degrees at the bottom and 60 degrees at the top. To minimize porosity and increase form filling, casters should make two vent lines 30 degrees below and two 60 degrees above. (However, when positioning vent lines in practice, it should also be considered that they are easy to clean and do not influence the design.)
Figure 3 above shows the results of attempting to cast a filigree part with just talcum powder: (from left to right) the used mold, the initial model, the first casting with missing parts on the wings, and a semi-complete casting.
Figure 4 shows the same mold when cast with a potato, resulting in the complete filling of the mold.
The casting of filigree parts with the cuttlebone technique has a problematic na-ture because, unlike in other types of casting, there is nothing but gravity to push the metal into the mold. This makes casting filigree parts in this way particularly tricky.
For filigree parts or a detailed surface design, it is recommended to use talcum powder as a release agent. The talcum powder compacts the surface of the mold, thereby improving the as-cast surface, and the talcum powder hardly reacts with the melt.
But even with talcum powder, casting thin filigree parts can still be problematic. That’s where “potato” casting may help.
In this process, the molten metal is poured into the cuttlebone mold and a raw potato is pressed directly onto the sprue metal as soon as it is cast. As the water in the potato heats up, it creates steam at the top of the sprue, which wants to escape because of pressure. The mold has the more porous structure, so the steam drives the melt into the mold cavity. The process could be called an extremely primitive overpressure casting technique.
It is important to immediately press the potato onto the sprue metal while it is still liquid in order to enhance form filling. It is also important that the potato has contact with the sprue metal for quick steam generation. Also key is to tie the mold very well with binding wire, as the mold is at risk of breaking.
No other casting technique can be used as an element of art like cuttlebone casting. Since every cuttlebone has its own structure, every casting is unique. The following points show some possibilities.
Structured Metal Sheets
The structure of the cuttlebone can also be cast directly on a metal sheet. A normal mold is prepared. A thin, flat cavity is carved out and the waste is removed. In order to highlight the lamellar structure of the cuttlebone on the cast sheet, the cavity can be brushed out with an old toothbrush. It is important that the cavity is not too thick; 1.5 to 2 mm is enough. Individual shapes can then be easily sawed out from the cast sheet.
Free-Hand Model Carving
One of the biggest benefits of cuttlebone casting is the possibility of carving the shape directly in the mold. The soft inside can be worked on with wax molding tools, knives, files, etc. The vent lines can be scratched in following the lamellar structure so that they are not seen on the cast object. After carving, the waste should be brushed out of the cavity so that the structure of the bone is reproduced in the casting. In the example shown here, the model was carved into the bone to form a heart. The cast results show the direct copy of the lamellar structure, and the vent lines are invisible on the back of the heart.
Cuttlebone casting is a good technique for customizing designs. For example, for a customer wanting an old stamped part with a concave back to be convex on both sides, just press the model in one side of the mold and close it to create an imprint on the other side of the mold. Open again and use the imprint on the other side to align the stamping and press it into the second side. The resulting casting will have no front or back. This technique can be also used to reproduce old dies for forging.
Cuttlebone casting is a technique with many possibilities, beginning with the simple reproduction of a piece of jewelry. Further advantages are the low cost and the speed of the process; a casting can be done in half an hour from start to finish. Cuttlebone casting is very well suited for training because of its simple application, which does not require much previous knowledge. It achieves a quick result and provides a basic understanding of the casting process.