Aluminium Armature Wire

This wonderfully expressive squiggle of thick (8mm), round-profiled metal is armature wire, used by sculptors, scenographers and stop-motion animators as a frame over which other materials are draped or stuck. Wires like this (but thinner) have been used to make the finger bones of beloved claymation characters like Wallace and Gromit, as well as the twisted interior of artist Eva Hesse’s fibreglass, polyethylene and resin-coated Seven Poles sculpture.

Aluminium is particularly useful as an armature wire because of its light weight, and because of its ductility, malleability and formability. This trio of properties all describe slightly different ways in which aluminium can be deformed without cracking or breaking: it can be drawn into long thin wires, can be compressed by flattening, hammering or rolling, and can be shaped in multiple directions at once without breaking. 

In the grand scheme of materials, all metals are relatively malleable and ductile, but the extent to which they can be deformed depends on their crystal structure. At the microscopic level, metals are made up of lots of tiny crystals, called grains, which are tessellated together in a pattern that looks a bit like crazy paving. When you casually bend a paperclip or push a chunk of aluminium through an industrial hydraulic press, you are forcing these tiny grains to stretch, bend, and slide past one another. 

These tiny grains are composed of trillions of billions of atoms all packed together in different patterns, and interrupting these regular patterns are dark squiggly lines called dislocations. These dislocations are imperfections in the atomic structure of the metal crystal that they are very useful to us because they can move within the crystal, allowing little bits of material to slide from one side of the grain to the other so it can change shape, deform, and slide past its neighbours. Different metals are more or less strong, brittle or ductile because of how easy or hard it is for these dislocations to move. This is partly affected by how tightly and regularly packed and how strongly bonded the atoms inside their crystals are, as this affects how easily dislocations can slip around inside the crystal. Aluminium is better at deforming than, say, tungsten because its atoms are more closely packed together but less strongly attracted to each other. Counterintuitively, this closely packed structure allows dislocations to glide and slip around inside aluminium’s crystals with relative ease, without fracturing the bonds between atoms. Tungsten’s atoms are less closely packed but more strongly attracted, making tungsten strong but brittle. Neither metal is as malleable as lead though, whose dislocations move so easily that you can bend thick pieces by hand, dent it with your fingernail, and roofers can cut it with a knife. 

The malleability of a metal is also affected by impurities: purer metals tend to be softer and more prone to stretching or bending than alloys. When alloying elements like silicon and copper are added to pure aluminium they lodge themselves in between aluminium atoms, creating obstacles that stop aluminium’s dislocations from moving as easily as they did before. Given how easily this armature wire bends without springing back at all, it is probably a commercially pure (99%+) aluminium. 

The malleability and formability of a metal is also affected by how it has been manufactured. When metals are melted and cool, they cool into a patchwork of crystals that are oriented in different directions. The seams where they meet are called grain boundaries. Grain boundaries act as obstacles that stop metals from deforming and flowing smoothly (dislocations can’t move past them), and they are also points of weakness where fracture can occur. So the more grain boundaries there are in a metal, the stronger, tougher and more brittle it is. How quickly a metal is cooled has a big impact on how many grain boundaries it tends to form, with rapid cooling resulting in more tiny grains and a stronger and less malleable metal. 

Most metals can be ‘work hardened’: repeatedly rolling, bending or hammering them causes dislocations to build up and tangle at the grain boundaries, stopping them from flowing. As metals are manipulated they become harder, stronger, and less ductile: this is why blacksmiths hammer steel blades. If you were to keep working a piece of metal continuously it would eventually reach its limit and would break, just like a paper clip does if you keep bending it. Heating the metal up again to a specific temperature (a process called annealing) can ‘reset’ the metal though, allowing tangled dislocations to move and rearrange themselves, relieving stress inside the metal’s crystals and allowing it to be worked more without breaking. 

This wire’s ability to be easily bent by hand suggests that it is a ‘dead soft’ or fully annealed piece of metal. Although this wire is very malleable, just like any metal if you bent it back and forth too much it would eventually break, which is why it is only used for smaller features like fingers and background characters in Wallace and Gromit that don’t have to move as much as the main characters.