CUTTLEFISH BONE CASTING

2) treatment and storage

Cuttlefish bone is the internal shell of Sepia officinalis, a mollusk highly appreciated in the Mediterranean cuisine.
Seen from above the cuttlebone has an elongated oval shape, seen in section it has a lenticular form.
It has a hard and thin crust on the outside encompassing the internal softer part, the pulp, which is much thicker, and this is the part that can hold the impression of the model, the part that can be carved.

The pulp is composed of closely packed thin layers of alternatively dense and spongy soft material.
On the heel side the layers have a moderate slant, which increases as they approach the point where the layers are almost horizontal one on top of the other.

Cuttlebones can be obtained directly from fishermen; the bones are of many sizes, mostly small.
They smell noticeably, they must be cleaned, rinsed, strung and finally hung in the sun to dry for many days.
Or you can buy them from the suppliers, they are selected for size and ready to use.
Oddly the best kept ones are be found in pet shops, they are usually large enough, often encased in nylon bags.

If the cuttlebones are old and have been left in store for a long time they could have dried too much.
In such a case tensions will grow between the soft internal part and the outer crust as well as between the center and the periphery, with the result that cracks will arise, usually starting from the edge.
So, when you press the model in the soft part of the bone the crack deepens and the bone breaks.

Traditionally goldsmiths keep their cuttlebones whole, but I noticed that to reduce chances of cracks it is better to square up the bones before storing them.
When they have to be stored for months or longer, they should be kept in a fresh and not ventilated room.

Lay the model on the flattened surface of one of the bones. If possible the impression should not be taken in the middle of the bone’s length but in its inferior part, the one opposed to the gate, so that you have a sprue as high as possible.
The reason is that the mold gets filled by gravity, the column of metal weighing over it forces the molten metal to fill the form.
The model must be free of undercuts, you could push and pull it leaving an unscathed impression of it in the bone.
You cannot use soft materials for the model, so forget plasticine and modeling wax while hard wax and sealing wax, wood and plastics are OK. These materials, even if the have very smooth or even polished surfaces, tend to adhere to the crushed dust produced by the pressure and carry it out of the mold cavity when you extract the model. That makes for a faulty surface of the cast.
Metal models work better. It was common practice to make models in lead, it melts at low temperature and is soft, easily carved, filed and sanded. It was first cast in an approximate mould, often made with cardboard.
Molten lead doesn’t char the bone, by the way, so you can make a few casts using the same cuttlebone, not many , as the impact of the metal spoils it anyway.
Lead particles can contaminate the filings in the drawer and, finally, lead gives off poisonous vapors when melted.

Carefully drive the model into one of the bones, sinking it till it is level with the surface of the bone (for almost half its thickness in the case of a ring).
The more delicate and intricate parts of the model should be in the lower part of the bone.
A ring for instance will have the gate on the shank and the head near the edge of the bone; if the shank looks too thin to let enough metal flow through it, you can carve an additional sprue from where the gate meets the shank, down to the head of the ring.
( In the case of a ring, lay the other bone over the one with the ring, and press them together till their faces meet over their whole surface.
The model ring will be sunk for the same depth, more or less, in each bone).
When you press the model, the material of the bone crushes, pulverizes and compacts because the dust has sharp corners: the bone can take a sharp impression of the model and keep it.
In pressing the model, the crust of the bone must rest on the soft fleshy cushion at the base of the hand so that pressure can be distributed on a rather wide surface and diminish the risk of bone breakings.
While a narrow shape penetrates easily, it takes a certain force to press into the bone a wide or large model; can increase the force by placing your hands between your thighs and squeezing a sandwich composed of the bone, the model and a piece of flat glass or wood.
If the model stops and refuses to sink further (make sure that the bone’s thickness is adequate), it means that the dust has reached an excessive thickness and cannot be compacted any more.
You must remove the model to dislodge and blow away the dust before pressing on, the dust is so compacted that the point of a needle works better than a brush. pay attention not to take away too much dust.
You don’t need to carve vents, the bone is porous, moreover each ridge of the bone is a way of escape for the gasses.
I proceed this way: with the model embedded in the bone, I brush the surface to get rid of the dust and clean the ridges.Then I extract the model, its impression in the bone is untouched but the surface around it is free and dust cannot clog the ways of escape for the gasses.When molten metal flows in the mold, it forces itself along the spaces between the clean ridges, forming fins all round the cast along the parting line.

when you have a corner to cast, it is good practice to press a poined tool in the bone at the vertex of the corner. This is because when the metal cools and consequently shrinks, the sharp corners tenhd to round off a bit; the excess metal can be reduced to shape by fling later on.
( In the case of a ring, flatten the sides of the bones and engrave a couple of registration nicks across the parting line. It suffices to nick three sides, the fourth has the pouring.
Open the sandwich and) extract the model. Usually it sticks to one of the bones. You dislodge it by giving a few blows on the hard back of the bone and letting the model drop down.

Carve a tapering channel from the gate, at the top of each bone, to the mold proper. Use a thin triangular blade or a thin scraper. When you assemble the bones, see that the gate is large enough, it should be at least half an inch in diameter or you'll have problems pouring.
Assemble the bones, taking care to make the nicks meet. Wrap the bones with binding wire then embed them, gate up, in sand or hold them upright someway.

The best metal for cuttlebone casting is silver. Melt your metal and pour steadily without hurrying. Pick the bone and hold it underwater till it cools.
Open the bone, brush away the carbonised remains on the cast, cut or saw off the sprue and the fins and start working with saw, files, drill, etc. to finish the object.

1) history

In the Mediterranean countries the use of cuttlebone as a mould to cast small objects has been widespread for centuries.
I grew up in Venice and made my first castings using the (small sized or broken) cuttlebones that I used to find stranded on the beach after sea storms.

In Baden- Württemberg, Germany, two belt buckles dating from the VI or VII century have been found, showing on the back the distinctive marks left by the cuttlebone.
In the shopping lists of goldsmiths' workshops in Florence and Venice, at the beginning of the Renaissance, sums were reserved to the purchase of cuttlefish bones.
Up to the end of the fifties, firms such as Gori e Zucchi (1AR), one of the biggest gold manufacturers in the world, went on employing cuttlebone casting for mass production, mostly of rings.
Up to a few years ago I could still see a string of cuttlebones hung on the wall in old workshops, goldsmiths used to have a great number of models, mostly rings, whose number constantly improved with new commissions from customers.

Waste wax casting has now completely supplanted the cuttlebone technique for reproducing purposes.
As painting found new fields to explore after photography, so cuttlebone, no more tied to the reproduction of models, can be explored as a creative tool.
Not depending on big machinery cuttlebone casting is an inexpensive and versatile technique.

3) replicas by impression, classical technique

Square up two bones, making four cuts in the softer part.
With a fine bladed saw frame you could saw the crust, but it is time consuming.
It is better to use the blade of a hacksaw whose point you let sink through the pulp till you reach the crust, then you bend outwards the bone along the cuts as if you would open them and the crust cracks neatly along the cuts (as if it were a piece of glass after a diamond cut).

The four cuts leave you with a rectangular piece of thick soft pulp with a hard back and a soft belly.
So you saw away the belly (the small piece of soft pulp can be of use later) and flatten the bone using a piece of sandpaper lying on a flat surface.
Tthe goldsmiths of old used to rub the surface of the bones one against the other, but sandpaper gives a better result.
Blow away the dust. Working with cuttlebone means raising some dust, you shouldn’t worry about breathing too much of it. The bone is made of calcium carbonate, which is readily dissolved by the organism.

cuttlefish bone casting