The Riace Bronzes, Museum of Reggio Calabria - discovered in the sea off Riace in 1972
There are a number of problems still outstanding in our explanations of ancient bronze techniques. Not least among them is how the ancients actually melted their bronze. I am a sculptor who has run a simple bronze foundry where invention took the place of investment. I am not at all convinced by the illustrations on The Foundry Cup in Berlin on which archaeologists rely. The furnace depicted might conceivably melt a small quantity of bronze but for the monumental torso and legs such as those of the bronzes of Riace one would need approximately 300 kg of molten bronze and that is a big undertaking. It is necessary to raise the temperature fast in the final stages because if you keep the bronze hot for too long you burn out the white metals, lead, zinc and tin. They simply evaporate leaving copper unalloyed which will not pour. It has the consistency of porridge.
I have been reading the first chapters of Charbonneaux's still classic Greek Bronzes and I have found in them interesting references to the excavations of a foundry at Olympia. Edilberto Formigli, the man who has conducted the most exciting experiments with the techniques of Etruscan gold-smithing, has recently published I Grandi Bronzi Antichi . He has gathered together a team of like-minded archaeologists and technicians and though they have been most enlightening in the field of bronze soldering, at which the Greeks excelled, their efforts to try to reconstruct the ancient methods of bronze casting have not met with the same success. Neither the firing of the mould nor the melting of the bronze as they describe it is adequate for one of the bronzes of Riace let alone the great statue of Athene from the Acropolis which predates them.
The sheer immensity of the task of casting a torso and legs for a bronze of Riace needs to be experienced. I am fortunate enough to have assisted in casting a complete figure life-size in one piece (equivalent to the torso of a Riace bronze) . So I have some idea of what is involved. Verrocchio died of the effort of casting his Colleoni and Cellini thought he was about to die in the middle of casting his Perseus. I offer my thoughts, which need to be tested by further experiment.
In the case of the excavations of a big foundry (at Olympia), it seems that the evidence was misinterpreted at the time (1937) by Kluge, who thought it was a sand-foundry. Sand-casting is a technique that was used a millennium later for casting very large quantities of bronze, for cannon or bells. Later still it was also used for monumental sculpture. The technique leaves very characteristic traces: the core can be removed easily (the lost wax process usually has the terracotta core still inside). It is now generally agreed that Kluge was mistaken, there really are no bronzes of the period that seem to have been cast by the sand process all have very much the character of wax that has been worked with a freedom and clarity that account for the lovely results.
I believe that what misled Kluge was the remains of Cellini's mysterious method that he only half describes. It has an element in it that is close to the sand casting technique. Kluge observed that the outer case was made of a mixture of sand and clay and grog (crushed pottery). This sounds like sand casting. The casting-pit found at Olympia was sloping, that is the figure would have been laid down with feet perhaps one meter lower than the head. In Cellini's account it is clear his figure was orientated vertically. Cellini was very circumspect in the information he gave. One gets the distinct impression that he did not want to provide his rivals with the fundamentals of his method. You have to have considerable experience of the problems of casting large bronzes to be able to guess his method. It was clearly very successful, considering what a very tall, ambitious casting he undertook. There is always a lot of repair and finishing to do (the Perseus took Cellini more than a year after casting).
Where he was outstandingly successful was in the firing of his mould. The danger with firing a big mould is that it needs so much heat to get the top of the mould up to temperature (that should be 630 °C to burn out the wax, a bit more to fire the clay to terracotta, say 670 °C plus depending upon the type of clay in use). By the time you have reached the temperature you have over-fired the bottom and weakened it. Cellini got round this difficulty by introducing what he described as a fence of bricks to protect the foot of the mould (see diagram 1).
Chinese potters were able to reach stoneware temperatures (1250-1300 °C) with fairly large wood-fired kilns; this is the temperature needed to melt bronze. Chinese kilns were usually set into a hillside directed towards the prevailing wind. The sequence of kilns up the hill constituted a fairly long chimney.
The same principle could be applied to heating a mould. That is one would have a separate fire box and use the mould chamber as a chimney to draw the fire into itself (see diagram 1). In this way you could have a very gentle initial drying out followed by a brisk firing to take the kiln up to temperature to fire the terracotta to 650 or 700 °C. This is what Cellini did.
He moved his mould after firing, it therefore must have been strong i.e. well fired, perhaps to as much as 750 °C. Because to move such a tall and awkward mould could easily have resulted in damage. Though he used iron straps to cradle it, iron has little strength after firing and would no longer be precisely the right shape, as the terracotta would have shrunk in the firing. Cellini used first a slow fire to draw off the wax, he then protected his mould with a fence of bricks and the real heating took only 48 hours where a modern foundry with a well-controlled gas kiln would take upwards of a week. Nowadays we use a plaster based mould that would not be nearly so strong and is much more cumbersome. How did Cellini do it?
See diagram 2
Cellini says there were many holes in his mould. This greatly aided the exit of the wax and the burning out of the wax residue. (If the wax takes a long time to run out it gets absorbed into the drying mould material and takes a long time to fire out again, if it comes out fast it leaves the mould much cleaner.) Furthermore the holes where the bronze is eventually going to enter will at this stage allow air to circulate through the mould and aid combustion of the residual wax. The mould should reach a dull red-hot if it is properly fired. Cellini would have allowed the mould to cool very slowly, keeping some fire going for a few hours then he would have closed up the kiln and insulated it as well as possible. At this stage I leave Cellini's description and transfer to the very incomplete evidence of the casting pit found in Olympia.
When the mould had cooled, the Greeks would have laid it down in the sloping pit. They would have packed sand under the back till they got just under halfway up the figure (the median plane between back and front) and then would have lain in the pipes as Cellini did. The pipes could be laid on the sand at the correct angle, packed with wool and clay at the joints and then have more sand packed on top. The fact of being diagonal would have made the task much easier. To do this with the mould in a vertical position must have been very awkward for Cellini's men. Could Cellini have partially reinvented a lost Greek technique? (see diagram 3 Cellini's method done in a sloping sand ramming pit. Diagram 3 is represented symbolically at the top of diagram 2 and is a shaft furnace, air would have been pulled through the fire at a furious pace by the chimney)
When it comes to the pouring, the placing of the mould on a slope removes half the element of risk. See diagram 5. The pressure at the bottom of the mould is greatly reduced, since the height is reduced the pressure of molten bronze, is also reduced. It has been suggested that the casting pit could also have served as the kiln for firing the mould and I am sure this would work. It would save the risk of moving after firing. On the other hand it would have deprived Phidias of the opportunity of adjusting the pose in its true relationship to gravity.
Furnaces based on the same principle also built into the hillside are recorded towards the end of the Bronze Age in Europe and the Near East. I believe I have found the middle section of the chimney for such a furnace cut into the cliff face of the Acropolis within the general area of the foundry. I am convinced that careful further excavation will reveal the remains of the largest ancient shaft-furnace yet found in Greece.
The original chimney must have been huge: 2 m diameter × 40 m height. Two thirds of the cylinder is visible for at least 10 m. The section is the shape of a question mark upside down. The opening in the shaft would have been filled with bricks or stone probably held together by clay (see diagram 4) which would have become fired after the chimney had been used. Similar chimneys were cut by the Etruscans. I have found no sign of actual cutting on the Acropolis but one would hardly expect to after centuries of use. The very straightness and regularity of the shaft convinces me that it was man made at least in part. A furnace powered by bellows is capable of melting bronze for small works but I think the Foundry Cup refers to preheating for soldering purposes, or more likely in view of the man waiting with a hammer, heating scrap bronze to red heat, in order to break it up with the hammer, prior to feeding it into the crucible for melting.
If we imagine ourselves back at the beginning of the era when these great bronze statues were being made there would inevitably have been many lessons to learn and usually in foundries one learns the hard way. First there would have been breakouts of bronze as the pressure built up due to the height of the mould. The idea of laying it diagonal is a thoroughly satisfactory solution to this problem. Second there would be the problem of handling the amount of molten metal. There are five reasons why Phidias would not have dared used this method; it is fraught with dangers.
- The crucibles found on sites could contain no more than 50 kg of bronze.
- You would therefore need 10 men to do the work.
- They would need good protective clothing and...
- Less primitive handling irons than those that have been found at Olympia.
- There would have to have been five perfectly synchronised furnaces because bronze deteriorates rapidly when kept at a high temperature. This is nearly impossible.
The Greeks must have used a single huge crucible with a plug in the bottom that is knocked out at the moment of pouring (see diagram 3). Phidias could not have use another method of melting bronze with the technology available at the time. The reason why such a crucible has not been found may be because the remains of such a crucible could well be confused with a furnace lining. The small crucibles would be fine for smaller jobs.
You may well ask why had archaeologists not found the channel before. The most important reason must be because they had not foreseen the necessity of such a chimney and nothing of the sort had been found previously. I knew there must have been a chimney and I was lucky enough to find it. At Olympia there is a straight line of kinks in the contour of the hill of Chronos leading down towards the Stadium, three furnaces were found at the bottom of this line. It must be the chimney.
At first sight the channel looked like many other cave type erosions that exist in the natural rock. It was cut out at the angle of the natural stone strata, rising at an angle of about 40 degrees. It was also overgrown but the discolouration due to the heat and smoke was obvious. There are several possible sites for a similar set up at Olympia but because of the friable nature of the terrain there may be nothing left to find. Any free-standing, vertical chimney would have toppled long ago.
Note for diagram 5: When I drew the figure thus, I immediately found an answer to a question that had puzzled me very much indeed. The Greeks of this period (the beginnings of the age of iron and steel) habitually cut across and through the feet from the point where one would tie ones laces down vertically to the arch and cast this part separately. Now this not only seemed damaging because one usually relies on the leverage of the foot to keep the sculpture well anchored to its base and obviously the soldering would weaken the purchase the foot would have on the base, but it is also creates much more work. It seemed to serve no purpose whatsoever. They also cast the sex organs and pubic hair separately and the head again separately. The cutting off of the head is easy to understand it gives access to the inside of the torso and obviously received special treatment in itself. Though the sex organs also received very special treatment in terms of very precise observation common sense would lead one to weld it onto the torso at the wax stage, it is much more work to do it when it is in bronze. However the Greeks did it in bronze! Why?
There would have been a miss-cast of feet, penis and head if no air vents had been provided. The drawing shows that these three high points would have been filled with air that would prevent the bronze from flowing up into them. Today we know that it is necessary to vent these parts but an ancient Greek would have had to be a bit of genius to arrive at this cause for the constant failure of these parts. He would have had to have a mental picture of the air filling the space as if it was a solid, so that the bronze could not enter (you cannot see air, bronze normally passes through air without any problems, it is a very advanced notion!). Their solution, if to us a strange one, would be to cut the problematic parts off and cast them separately. It seems likely that this is why they did what is otherwise inexplicable . They should have turned the figure over there are no awkward projections on the back.