Bronze Casting

November 2005

More Cuttlefish Casting

12th November 2005

Bronze tsuba, cast in cuttlefish bone

Bronze tsuba, cast in cuttlefish bone

Finally! A tsuccessful tsuba!


Mjölnir

Mjölnir

And a tiny bronze Mjölnir.

As usual these were poured in Jarkmans's workshop at the Pontypool Blobworks.


 

Cuttlefish

Cuttlefish bone is a mould material that has been used since antiquity.

Cuttlefish bone casting

Cuttlefish bone casting

Cuttlefish (like octopus and squid) are invertebrates and don't have any skeleton, either inside or outside. The "bone" or "cuttle" is a rigid support inside an internal flotation chamber they have, like a fish's swim bladder. Being a float has two consequences: they're lightweight and easily worked, and also they wash up on beaches after the cuttlefish die at sea (does this make them vegan?).

Cuttlefish bones are available from petshops (cage birds sharpen their beaks on them) or can be collected from the seashore. I've been using petshop cuttlefish, but they seem very dry - maybe fresh would be better.


Cuttlefish moulds, carved and ready for closing up

Cuttlefish moulds, carved and ready for closing up

Simple moulds like these can be carved directly into the cuttlefish bone. I use a riffler file and a rounded dentist's probe for smoothing. A hard pattern can also be pressed into the bone, making a negative impression. This seems to need softer (fresher?) cuttlefish than we've had so far. It's important to align the two halves – I use notches around the edge and pressing a loop of wire into both halves to mark it out.

One of the attractive features of cuttlefish casting is the "woodgrain" texture that it can produce. The cuttlefish bone grows as concentric layers, like a tree (although cuttlefish only live a couple of years at most). A flat surface in the mould cuts through these layers. Brushing the mould gently can expose the harder part of these layers, developing the "woodgrain" which then transfers itself to the casting.

The top of the mould has a gate and runners carved into it, where the molten metal will be poured. My first attempt (the bronze in the pictures) used a central runner. This gave a good fill, but there was erosion of the mould under the flow of metal. Now I'm using split runners into each side of the rim, to try and fill these less delicate areas first and preserve the detail on the central web.


Mould, wired and ready to pour

Mould, wired and ready to pour

The two halves of the mould are fastened together and tied with wire.


Pouring a crucible of molten bronze

Pouring

A glowing crucible full of molten bronze is lifted from the kiln with tongs, then poured into the cuttlefish.

The crucible is steel pipe with a welded end-cap. We have a little trouble with the steel burning and we think even dissolving into the melt. Thin steel crucibles don't last at all well.


Poured moulds

Poured moulds

For support, the cuttlefish are placed in a sand bed.

Packing sand around their sides also avoids problems if there's any leakage. Tiny leakage in a cuttlefish mould soon burns through to a torrent, and a spoiled casting.


Cuttlefish bone moulds, opened up after casting to show their burned surfaces

Moulds, after casting

The cuttlefish don't survive well! This is probably one reason why the casting process stinks so much. Although the cuttlefish bones look like inert chalky minerals, there's clearly some fishy goodness left in there. Supposedly the moulds are re-usable a few times when casting pewter, but bronze is much too hot.


Completed cast tsuba

Completed cast tsuba

Two tsuba, unfortunately they didn't fill the moulds fully in the thin part of the mould. The woodgrain pattern casts nicely though. Part of the central hole is deliberate, as it is the location for the sword blade tang.

The small "teapot" is a sand-cast sword pommel.


Early thick tsuba, and a Japanese original

Early thick tsuba, and a Japanese original

The intention of all these strange ovals is to cast a pair of tsuba, the hilt-guards on a Japanese sword. I'm planning to use these for a sword re-mounting project.


 

Lost-Foam Casting

So far this has been our most successful process.

Halloween Jack-o-Lantern pattern in foam

Haloween pumpkin pattern

First a master pattern is carved out of cheap builder's insulation foam. This is a Halloween Jack-o-Lantern, about 1 ½" high.


A poured sand mould, with the hot melt still glowing in it

A poured sand mould

The foam pattern is buried in sand, along with a foam pouring gate and sprue. This much is just like traditional sand casting, but the difference with lost-foam casting is that you leave the pattern underneath the sand. Then the melt is poured in, on top of the foam, which melts/burns/vapourises (anyway, it disappears).

The advantage is that the mould-making process is really easy, compared to a wooden pattern that's removed to leave a hollow mould.

The disadvantage is that the pattern is destroyed and you need a new pattern for every piece.


Kiln, closed

Kiln, closed

To melt the bronze we're using Jarkman's propane forge with the Ron Reil burner. It's a two person job to open the door and remove the melt promptly - speed is important as the melt cools quickly once outside.


Inside the kiln

Hot crucible, freshly loaded with a stack of cold coins

A crucible loaded with bronze coins takes about 15 minutes to melt.


Robot, freshly cast

Robot

This is one of Jarkman's tiny robot army, fresh from quenching. The black grot is burned sand, which cleans off easily.


Robot, with casting sprue

Robot

The large lump is the sprue and gate. These are often larger than the intended casting, but it all goes back in the melting pot later — as do the mistakes!