Over the millenia people have worked precious metals, they have evolved many different techniques for producing different finishes, structures and effects, however one of the oldest and still most important is casting. Although some aspects of the process have been streamlined with technology to improve speed and quality control, the basic process remains the same now as it did in the earliest metal casting processes, reckoned to date back around five or six thousand years BCE.
In this post, our master caster Andy Barnes takes us on a whirlwind tour of this ancient, and complex process…
The first step in the casting process is making a metal master, which might be produced in a variety of different ways, from hand-sculpting wax to making shapes and lettering in brass or other metals. It could even be taking a handmade, hand-chased piece of precious metal jewellery. However the master is made, it’s encased in liquid rubber, which solidifies to make a reusable mould for preparing wax masters for the casting process.
Making a rubber mould is quite a skill. It takes a lot of precise scalpel work to cut the mould to remove the original master, and make it reusable. One of the key skills is making little vents in the rubber so that when the liquid wax gets pumped in to make the wax master, the air can escape and no bubbles spoil the moulding.
The wax master model is traditionally made by setting hot wax in rubber moulds, but In the last decade, 3D printing has added complex resin forms alongside wax, which enables much more complex shapes to be produced. The shapes are usually made in multiples, attached to a tree shape (or sprue) to keep them all together when the mould is broken.
The wax master is then encased in a plaster-filled flask. In a modern workshop the plaster is mixed under a vacuum to minimise the risk of getting air bubbles inside the mould, which would spoil the surface consistency of the finished cast. The use of vacuums has greatly increased the consistency of the casting process.
“In the old days they used to cast using sand based moulds, which was hit and miss quality wise, it’s amazing the results they could get, though. The old sand-casters were very highly skilled master craftsmen.”
The finished mould is then fired twice in a kiln. The first firing is at 180 °C, at which point the wax or 3D resin melts and leaves a perfect empty space inside the plaster (ready to be filled with molten silver). The second firing slowly heats the plaster to 730°C, which gradually drives out any moisture and gasses from the fine pores in the plaster.
“It’s an organic process and never 100% reliable, but the slow kiln and high temperatures have greatly increased the reliability of mould making. The mould making takes much longer than the actual silver casting, but it’s essential for a consistently high quality finish.”
Each flask is numbered, and based on the volume of the wax master, the correct quantity of silver to fill the cast is calculated. The silver comes in the form of coffee bean sized grains, and we use a mix of 40% recycled silver and 60% newly alloyed silver to recycle materials but also maintain the correct quantity of metal alloys in the mix to deliver a durable, workable silver product.
“We recycle as much silver as possible, but recycled silver loses some of its strength and durability. We have to balance our sustainable recycling programme with freshly alloyed silver to maintaining a workable finished product.”
The silver grains are placed in a graphite crucible, and heated rapidly in an induction kiln (with an induction coil) to 1010°C, in a pressurised nitrogen chamber (which ensures the silver melts without any oxidation reactions with the air). At the same time, the plaster mould flask is placed in a vacuum chamber below. The combination of high pressure nitrogen above and vacuum below, forces the liquid silver into the mould, coating the void inside to a smooth finish.
“In the old days, we used a centrifuge to drive the liquid silver deep into the mould, and before that, silversmiths would heat the silver in a crucible, pour it into a sand mould and swing the red hot flask around their heads. Which produced unpredictable results, as well as being really dangerous.”
When the silver has filled the mould, the hot plaster is then dropped into a water bath, where the plaster explosively dissolves leaving the original moulded shape of the wax shapes and sprue, now replaced by freshly cast silver. Any traces of plaster are then cleaned off using a high powered spray.
The finished cast usually has some light oxidation on the surface, producing a dull, blue-black finish (well, it has been cooked at 1010°C and plunged into cold water!). This is removed by treating the silver cast in an acid bath, which cleans it up to a more familiar dull white finish… ready for polishing, chasing and finishing by the silversmiths in the workshop.
With over 30 years as a master caster (19 at Grant Macdonald) Andy Barnes is highly skilled at every step in the casting process. It’s a constantly evolving field, as new technologies like 3D printing mean developing new techniques to work with experimental resins, developing new plaster mixes and experimenting with new kiln temperature profiles to boost efficiency and optimise production costs.
Most workshops can’t handle the sheer volume of business to keep a full time master caster employed, but feeding our busy workshop with fresh casts – and curating our huge library of moulds and masters we’ve accumulated over the last fifty years – keeps Andy busy all year round.
With Andy’s experience and expertise, we’re always pushing the art of casting forwards and incorporating cutting edge technology into the silversmith’s traditional craft. It’s the work of people like Andy that keeps Grant Macdonald’s workshops at the leading edge of precious metal craftsmanship.
“Part of my job is the same now as it was back in the 1600s, the other part is pushing the limits of what’s possible with the latest technology. I’m like that movie Back to the Future.”