Biographies & Memoirs

Casting the Horse

Here is a story to gladden every library-mole’s heart. In February 1967 a specialist in early Spanish literature, Dr Jules Piccus of the University of Massachusetts, was in the Biblioteca Nacional in Madrid, looking for manuscripts of medieval ballads orcancioneros, when he came upon two stout volumes bound in red morocco leather, measuring about 9 by 5 inches. He was astonished to find that they contained a collection of drawings and writings described on the title-page, in a Spanish hand of the eighteenth century, as ‘Tractados de fortificación, mecanica y geometra’ by ‘Leonardo da Vinci pintor famoso’. That the library had once contained these volumes was known to a handful of scholars – they are mentioned in a couple of early inventories – but they were thought to have been lost or stolen. As it now turned out they had merely been mislaid, as can happen in great and aged libraries: they had disappeared into the miasma of the stacks.51

The earliest of the various tractados, or treatises, bound together in the two volumes is a small notebook of seventeen folios which forms the last section of Madrid Codex II. It contains detailed notes and instructions for the casting of the Sforza Horse. Two dated pages give its approximate chronology. Leonardo began it on 17 May 1491, on which date he wrote, ‘Here a record shall be kept of everything relating to the bronze horse now under construction.’ On another page is the date 20 December 1493, recording his decision to cast the horse on its side rather than upside down.52

The practical construction of the Sforza Horse consisted of three distinct phases: the making of the full-scale model in clay; the creation of the mould or form, a wax impression of the model sandwiched between two refractory layers pinned together by an iron framework; and the final casting of the statue in bronze, using the ‘lost-wax’ process in which the wax impression is melted away and molten bronze is poured into the emptied cavity between the refractory layers.53 As we have seen, Leonardo’s conception of the statue had evolved from the grandiose but impractical idea of the rearing horse to the more conventional trotting or prancing horse. His enthusiastic response to the Regisole in June 1490 may mark the transition, though it seems that the earlier idea lingered in his mind, for in one of his drawings of a mould the horse is shown rearing. But most of the casting diagrams, including those in the Madrid notebook, show it trotting, and it is pretty certain this was the final form of the sculpture in Leonardo’s mind. A sketch of a trotting horse à la Regisole has notes which sound almost like a sculptor’s mantra:

Simple and composed movement.

Simple and composed force.54

image

Armatured piece-mould of the head of the Sforza Horse, c. 1492.

He was working intensively on the construction of the mould in 1492. A sheet at Windsor has a sketch of the mould in two parts, and designs for the construction of pulleys and cog-wheeled mechanisms, presumably for hoisting the mould.55 The Madrid notebook has technical recipes:

Composition of the inside of the mould

Mix coarse river sand, ashes, ground brick, egg-white and vinegar together with your earth – but test it first.

Soaking the inside of the mould

As soon as you have re-baked the mould, soak it while still warm with Greek pitch, or linseed oil, or turpentine or tallow. Try each of them out and use whichever is best.

image

Study of diminishing power in a spring, from Madrid Codex I.

A meticulous red-chalk drawing shows the outer mould for the head and neck of the horse, held in place by the interlocking framework or ‘armature’ of wood and iron.56

In his Life of the Florentine architect Giuliano da Sangallo, Vasari says that Sangallo discussed the casting of the Horse with Leonardo, ‘disputing the impossibility’. Documents show that Sangallo was indeed in Milan in October 1492.57 The ‘impossibility’ would refer to Leonardo’s decision – against traditional practice – to try to cast the horse as a single piece. Vasari thought (wrongly) that this was the reason for the non-completion of the statue: ‘He carried the work forward on such a scale that it was impossible to finish it… It was so large that it proved an insoluble problem to cast it in one piece.’

Behind these technical notes and scribbled diagrams lie scenes of titanic industrial activity – the Corte Vecchia as a Vulcan’s smithy: furnaces, kiln-pits, hoists and derricks, a scene reminiscent of that drawing of an artillery-foundry. One has to bear in mind the mountainous scale of the statue: it was ‘colossal’, in the words of Paolo Giovio, who probably saw the clay model as a boy. A measurement in the Madrid notebook shows that the Horse measured 12 braccia from hoof to head – that is, about 24 feet: the height of four tall men. The length between the hind fetlock and the raised foreleg would be about the same. Leonardo’s Horse was thus something like three times lifesize. The quantity of bronze set aside for the final casting was 100 meira – about 75 tons.58

The clay model of the Horse was exhibited in late 1493, on the occasion of the marriage of Ludovico’s niece Bianca to the Holy Roman Emperor, Maximilian of Habsburg. The wedding was celebrated in Milan, by proxy, on 30 November. A clutch of celebratory poems was issued, all of them mentioning the Horse as part of their lauding of Ludovico. Baldassare Taccone writes:

Vedi che in Corte fa far di metallo

Per memoria del padre un gran colosso:

I’ credo fermamente e senza fallo

Che Gretia e Roma mai vide el piu grosso.

Guarde pur come è bello quel cavallo:

Leonardo Vinci a farlo sol s’è mosso

[See in the Corte how he [Ludovico] is having a great colossus made out of metal in memory of his father. I am certain that neither Greece nor Rome ever saw anything bigger. See how beautiful this horse is: Leonardo da Vinci alone has created it.]

As the bronze horse was never actually cast, Taccone must be referring to the model. Another rimester, Lancino Curzio, finds the horse so lifelike that he imagines it addressing some lines to an astonished observer.59 Vasari: ‘Those who saw the great clay model that Leonardo made considered that they had never seen a finer or more magnificent piece of work.’

By then Leonardo was already thinking about the casting process, for on 20 December 1493 he writes, ‘I have decided that the horse should be cast without its tail, on its side, because if I were to cast it upside down the water would be only one braccio away, and… as the mould must stay underground for several hours, the head one braccio from the water would be affected by damp, and the cast would not take.’60 These considerations refer to the pit in which the casting was to be done. To cast the horse upside down would require a pit 12 braccia deep, which would bring the head too close to the shallow water-table of the Lombard plain.

The creation of the great clay horse in 1492–3 is only a part of Leonardo’s work as an engineer and mechanician, as is shown by the magnificent drawings in Madrid Codex I, begun on 1 January 1493 and worked at over a period of seven or eight years. On the cover-sheet Leonardo gives the notebook the title Libro di quantita e potentia (Book of Quantity and Force). It may also be the ‘book on mechanical elements’ and the ‘technical book on physics’ which he refers to elsewhere and which are otherwise unknown.61It is a marvellous manual of ingegni, a cornucopia of custom-made industrial devices – there are textile-machines and grain-mills and protypical windmills, and a spinning-wheel incorporating an automatic yarn-twisting mechanism, and various lifting-devices, including crane-hooks designed to disengage when the load touches the ground. But it is not primarily a book of inventions. Rather than showing complete working machines, it concentrates on the basic mechanical principles and movements involved. The remit is systematic and practical, geared towards the actuality of the assembly-line. Leonardo is quite literally getting down to nuts and bolts, not to mention chain-drives and belt-drives, universal joints and knuckle-joints, roller-bearings and disc-bearings, bi-directional screw-threads and epicycloidal gear-wheels. It is a boffin’s paradise, as is conveyed infectiously by the first editor of the codices, Ladislaus Reti.

Leonardo’s assistants are of interest here. The German Giulio (or Julius), who entered Leonardo’s service in 1493, is mentioned in a note about disc-bearings supporting a horizontal axle: ‘Giulio says he has seen two such wheels in Germany and they became worn around the spindle.’62 And the metallurgist Tommaso Masin’, or Zoroastro, was also an important assistant in these researches, as he no doubt was in the casting of the Horse. A note in one of the Forster notebooks tells us that ‘Maestro Tommaso came back’ in September 1492; he may have travelled up from Florence with Giuliano da Sangallo, with whom Leonardo was conversing the following month, ‘disputing the impossibility’ of the Horse. To Tommaso we might attribute the particular alloy of metals specified by Leonardo for the moving parts of a two-piece bearing-block. The material is essentially an ‘antifriction alloy’, predating by centuries the substance patented by the American inventor Isaac Babbitt in 1839.63

Among the diagrams of Madrid I are drawings showing the moving parts of one of Leonardo’s most fascinating creations – an automaton or robot in the form of a knight in armour. Mechanisms featuring gears with alterable teeth, and ingeniously compact motors regulated by a spindle, have been interpreted as part of the robot’s ‘programmable carriage’. This automatic knight was capable of bending its legs, moving its arms and hands, and turning its head. Its mouth opened, and an automatic drum-roll within the mechanism enabled it to ‘talk’. There are sketches of the head and neck of this cavaliere meccanica in the Forster notebooks. It was exhibited in Milan in about 1495.64 These automata become fairly common in popular and courtly feste in the sixteenth century, but Leonardo’s seems to be one of the earliest. In fact his interest in automated movement goes back to his Florentine years. There is a sheet of technical drawings dating from the late 1470s which shows a wheeled platform powered by springs and controlled by a pinion-wheel. This was probably for use in Florentine pageants: it could carry some statue or carnivalesque effigy short distances. A reconstruction of it – dubbed in the press ‘Leonardo’s car’ – was unveiled at Florence’s History of Science Museum in April 2004. This earlier interest can be connected to one of Verrocchio’s most popular creations – the putto which struck the hour on the clock at the Mercato Vecchio.65 The principles of horology are an important background for Leonardo’s automata, though according to Mark Rosheim, a NASA scientist who has reconstructed a working model of the robot-knight, Leonardo was moving far beyond the limitations of clockwork: his programmed carriage for automata is nothing less than ‘the first known example in the story of civilization of the programmable analogue computer’.66

The knight is a wonderful blend of Leonardian enthusiasms – mechanics, anatomy, sculpture, theatre. He would create other such marvels, among them the mechanical lion which, in 1515, astonished King François I when it opened up to reveal a bunch of French lilies. Lomazzo says of this creature, ‘it moved along by the power of its wheels’, which for a moment gives an impression that he is explaining how it worked.67

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