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Sculpting the urban landscape

Materials World magazine
7 Apr 2015

Public sculpture is an integral part of many regeneration projects, and continues to play an important commemorative role. Sharon Ann Holgate speaks to sculptors and founders about the materials and processes used in the creation of their recent public artworks.

For centuries, bronze sculptures have been cast using the lost wax process. But while Michelangelo would recognise the basic steps used by sculptors and foundries today, the process is evolving thanks to modern materials and laser-based technology. Meanwhile, computer-led design and manufacturing are changing how foundries produce art, and also enabling the creation of innovative public sculptures made from stone, resin and steel. 

New takes on old methods

While Sarah Craske, artist and co-owner of Meltdowns foundry, in Kent, is starting to see artists delivering ‘computer files of virtual objects’, most bronze sculptures begin life as physical models called maquettes. Philip Jackson – a West Sussex-based sculptor whose public commissions include the Bomber Command Memorial in London’s Green Park, and Manchester United’s sculpture Sir Alex Ferguson – makes wax maquettes for his bronze sculptures. He then scales up the design, sculpting the full-size artwork from clay that is internally supported – and, for some artworks, externally too – by a stainless steel skeleton known as an armature. 

Despite being inspired by historic working methods, Jackson values the accuracy he gets from using a construction industry laser-line generator to ensure that his finished sculptures, which are cast in pieces then welded together, are correctly aligned. ‘We mark laser lines that [visually] divide the clay replica into four quadrants by putting domed head screws into the clay. When the bronze casting is done, those screw heads show up and that allows us to check that the sculpture is in exactly the right alignment,’ he explains. 

With the screws in place, Jackson creates a mould of the sculpture by covering the clay with silicone rubber that, once hardened, is covered in a fibreglass layer that holds the mould in shape. Finally, he cuts the mould into separate sections before sending them off to a foundry for casting. 

Craske says she can make rubber moulds from whatever object an artist produces, or from a polystyrene foam sculpture machine cut either according to an artist’s 3D CAD file or after laser scanning, then digitally scaling up a maquette. The next step in the lost wax process is to cast a 5mm-thick wax replica of the sculpture from the rubber mould, explains artist Stephen Melton, co-owner of Meltdowns, which casts silicon bronze and aluminium artworks and architectural features. ‘You then envelop the wax in heat-resistant materials (Meltdowns uses plasters and a powdered ceramic called grog) before melting the wax out to form a cavity by baking in a kiln. Bronze is then poured into the cavity where the wax was – hence the name, says Melton. They create channels for pouring in the molten bronze (called runners) and for allowing air to escape during pouring (risers) by attaching wax rods to the wax replica before encasing it in plasters and grog.

When the bronze is cold, the plaster is jet-washed away before separate sections are welded together, and any bronze that has collected in the runners and risers is angle grinded off. The sculpture’s surface is then shot blasted with aluminium oxide, leaving a very fine texture that enables the chemicals subsequently applied to create the final colour or patina to settle evenly. Finally, a layer of acid-free wax is applied to seal in the colour and protect the sculpture against dirt and the weather. 

Installation art

Once delivered to site, Jackson explains that his bronze sculptures – which contain stainless steel armatures for support – have rag bolts, also made from stainless steel, fixed to the bottom. These bolts descend into epoxy grout-filled holes in the stone-clad concrete base as a crane lowers the artwork into place. 

If heavy bronze pieces are not practical or cost-effective to install – for instance, for temporary public exhibits – bronze resin provides an alternative. Jonathan Hateley, a Kent-based sculptor, and model maker for the TV industry, explains that he cast All From One, which formed part of RHS Garden Wisley’s 2014 sculpture trail, from bronze resin in his studio. He first made fibreglass and polyester resin-backed silicone rubber moulds of the front and back of his clay sculpture, then painted the moulds’ inner surfaces with two layers of polyester resin mixed with powdered bronze, at a ratio of 2:1. 

After the bronze resin hardened, Hateley built up the sculpture’s thickness to around 4mm by coating 

the interior with a resin and glass fibre mixture that he later used to join the two halves of the mould. Once the join set, he removed the mould to reveal the sculpture.

‘With this process, you end up with seam lines around the sculpture where the joins are, so I had to grind away the surface to get rid of those seams,’ says Hateley, who produces sculptures in both bronze and bronze resin. He used a miniature power tool with diamond and carbon cutter attachments for the grinding and, once additional bronze resin had been applied to the join area, to sculpt back in the details that had been lost. Polyester resin was used to fix a protruding steel mount inside each leg of the sculpture. Finally, the outer surface was patinated and painted before being sealed with wax. 

Computer aided creation

Once installed, public sculptures can be subject to a range of forces, including stresses from wind loading, accidental collisions, vandalism, people climbing over them, or even their own weight, explains Simon Hitchens, a sculptor whose public commissions include Unity in King’s Cross, and Glorious Beauty in Kensington, London.  Structural engineers are sometimes consulted to calculate these stresses, and Hitchens used the results of engineers’ stress calculations along with their computed values for the centre of gravity, toppling points and thermal expansion rates when designing supports and bases for his sculptures. 

The lost wax-cast stainless steel top of Glorious Beauty, which mirrors the garnet amphibolite boulder beneath (by laser scanning the boulder then digitally flipping the shape), is stabilised with a 600mm-long stainless steel solid rod of around 90mm in diameter. This goes from the very top, inside the hollow stainless steel boulder, through to its bottom, where it touches the stone boulder and then extends a 

further 600mm down into the heart of the stone, where it is resin-bonded in place. The sculpture is also bolted and resin-bonded to three stainless steel blocks that are welded to a buried stainless steel base plate.

Although appearing to touch, the granite half and cast polyurethane resin half of Unity have a 10mm gap between them, ‘to allow the resin to expand and contract throughout the day’, explains Hitchens. He adds that its thermal expansion rate is 0.25% and that without that gap, the daily 1cm expansion would cause parts of the sculpture to gradually wear away. 

Basque Country-based sculptor Juanjo Novella uses computers to help create small-scale models of his finished sculptures out of the same material – namely machine-cut steel – as this is ‘the best way of anticipating any issues that could arise on the large sculpture’. First, Novella hand-draws both the pattern that will be cut out and the overall shape, then translates this drawing into a computer file that feeds a cutting machine. Laser cutting is used to create the initial model. ‘On the final piece, the process is just using the same computer file but scaled up to the final size, using flame cutting instead of laser cutting. Due to the intricate shape of my pieces and the dependence on cutting machines, I developed, in conjunction with a workshop, a customised cutting program to keep the raw material cold enough to avoid deforming,’ says Novella, whose public sculptures include Durango in Abu Dhabi, and Welcome to My Safe Home, to My Sheltered Haven in Nelson, New Zealand. 

Once the pattern is cut, the steel plates are bent into their final shape using equipment in heavy industry facilities, such as navy workshops. They are then transported to site and fixed in place by welding them to a steel plate embedded in a concrete footing. ‘I always use industrial workshops and other technical facilities, which were created to develop other kinds of fabrications to create my sculptures’, adds Novella. ‘One of my main aims is to push the capabilities of those facilities to the limit and explore new possibilities’. 

With new materials and methods continuing to be adopted and developed by foundries and sculptors alike, it seems we can expect to see ever more technically challenging artworks appearing in our urban landscapes soon.