Lost-wax casting is a technique based on the principle of pouring molten metal in an impression (negative) made of refractory material; thus the sculptures that are to be turned into bronze, will be made in wax first, hollow on the inside. For this reason, it is necessary for the wax to have the same thickness desired on the final bronze sculpture (or whichever other metal used). It is an ancient technology, developed throughout the centuries, still very much used nowadays not only for artistic purposes but also on an industrial level. It is the best technique to use when reproducing a very detailed and precise object or when traditional execution would be too time consuming. For example, today the most important elements of planes’ reaction engines are manufactured by lost wax casting. The internal blades of the turbines must withstand peripheral speed of about 450m/s, reaching temperatures over 12000 degrees with 750m/s flow velocity. The cooling system works via microscopic holes and channels of such a thinness that they can be manufactured only by lost wax casting.

Origins

The origins of lost wax casting are deeply related with the manufacture of artworks since the VI century b.C. The most ancient and elementary moulds were made of stone, terracotta or sand. Throughout the centuries techniques and materials have been refined, but the basic principles remain unchanged. In the beginning, the casts would be solid, like in the case of the Sardinian Nuragic Civilisation's bronzes or VI century b.C. Greek sculptures. In these examples, the largest sculptures obtainable were between 45 and 50 cm. Since the 3rd millennium b.C. in the East, the civilisation developed in the area of Mohenjo-Daro (Pakistan) manages to perfect the casting technique. The introduction of a core allowing a hollow cast was a truly revolutionary discovery. In VI century b.C. Greece, the technique to obtain a uniform thickness is finally mastered. In the 3rd century b.C., the system of plaster piece-moulds is invented, allowing the reproduction of more than one copy for each model not just in plaster but also in wax. It is not possible to follow a linear evolution of lost wax casting because there are times when extremely elaborated techniques were completely forgotten. Cellini, for example, worked modelling the core which was then covered with wax, risking to lose the original every time. Vasari describes as a novelty the system of plaster piece-moulds allowing the reproduction of more than one wax copies. From Giambologna onwards, there is a progressive technical evolution and a change in the very conception of the casting process. With the improvement of assembly methods and the introduction of electric welding, the dimensions of bronze casts grow considerably. Larger artworks nowadays are split up into “pieces” up to 150x100 cm wide, which are cast independently and then chased and welded back together. As aforementioned, electric welding simplifies the process and a final manual chasing removes all traces. The times when artisans would take pride in a single casting of the whole artwork, are over; too time consuming and expensive. An example of large statues made in a single cast is the equestrian statue of Carlo IV of Spain in the square of Tolosa, Mexico City. The statue was made in august 1802 in a single casting of approximately 50 tons... but the sculptor was busy then with the finishing touches for over 14 months!

The Shapes

There are no limits to the kind of shapes that can be cast as any “object” can be reproduced. The wax used in the process, in fact, is extremely flexible, elastic and allows for very high definition. As there is only a thin layer of wax covering the surfaces of the object (around 5mm), everything can be cast!

Direct Lost-wax casting method

It is sometimes possible to perform direct lost-wax casting on certain pieces, but only at three fundamental conditions: 1. That the material of which the model is made can be burned and consequentially dissolve at a temperature not over 650 degrees without leaving too many debris; 2. that the final bronze piece is solid; that there is no further need of the original model as this will be destroyed in the process. Thus, it is possible to work with very different materials including those of vegetable or animal origins.

Indirect lost-wax casting tecnique

In the indirect process the wax replica is created form the outside in. In other word the liquefied wax is brushed, poured or pressed in to the piece-mould untill it forms a shell of the desired thickness. This hollow wax is then filled with refractory core material and, if necessary, equipped with an armature and assembled. From this point, the casting steps are identical for direct and indirect cast

Accurate Details Reproduction

Another fundamental feature of lost-wax casting is the accuracy with which it can reproduce detailed patterns. The surface of the mould that will be in contact with the moulted metal during the casting, would have been already “carved” by the wax. The wax gets “lost” - i.e. it melts away - when the moulds are placed in ovens to dry with temperatures up to 650 degrees. Wax is thus the perfect material to use when reproducing detailed surfaces. It is common knowledge that wax is flexible, elastic, resistant, soft, plastic and allows for great precision. It is not difficult to imagine how easily one can leave fingerprints on warm wax. Well, the same fingerprints will be transferred onto the core of the mould and thus accurately reproduced in bronze. No other technique allows for such precision. Not even the best terracottas, ceramics or porcelains. Not even glass. Take a look for example at the accuracy of execution of Benvenuto Cellini’s saltcellar or Giambologna’s rooster, turkey, owls and eagle.

Process Schedule

Lost-wax casting takes a very long time. This is due to the following reasons: The silicone mould must be made entirely by hand, pressing the rubber with the thumbs in order to be able to imprint even the smallest detail of the surface. Modern technologies allow the use liquid silicone rubbers poured into small frameworks containing the model. The process is much quicker but not as accurate. Furthermore, melting the wax, drying it off and sintering the mould containing the wax model with all the spurs, also take a long time. After placing the moulds in special ovens, temperatures are increased gradually. This in oder to avoid the cracking of the mould while driving out any moisture and emptying of melted wax. After 36 hours the oven goes up to 100 degrees and then, after three days, to 350 degrees. Two days later it reaches 650 degrees. In order to obtain the most perfect sintering, the moulds are kept at constant temperature for 48 hours. The whole process takes roughly nine days, on top of which are also the first day (when the moulds are left to dry after completion) and other two more days to cool off once the moulds have been extracted from the ovens. Thus a total of twelve days. The processes of cleaning, chasing and patination are much quicker - depending on the difficulty of the chasing.