The forties and fifties of the eighteenth century were key years for Raimondo di Sangro’s fame, which grew and spread across the limits of the Kingdom. In 1741 he invented a cannon, which, compared with other examples of the same type, weighed one-hundred-and-ninety lb less and had a much longer range. He become colonel of the Capitanata Regiment, one of the twelve provincial units of the Bourbon army. He took part in the glorious battle of Velletri against the Austrians (1744), distinguishing himself for his courage and skill. His passion for the military art led him to publish his Practice of Military Exercises for the Infantry (1747). His competence in the subject earned him the praise of Louis XV of France and Federico II of Prussia, and all the Spanish troops adopted the useful exercises he prescribed.

Already admitted in 1743 to the Accademia della Crusca, the most authoritative cultural institution of the period, under the pseudonym of Esercitato, the following year, Raimondo obtained authorisation to read prohibited books from Benedetto XIV. So he studied the works of Pierre Bayle, the writings of the French philosophes and the radical Enlightenment thinkers, the texts of the alchemists and the Masonic tradition, and scientific treatises of all kinds. With his reading came the experiments: he put on spectacular firework shows with fireworks of colours never seen before and a perfectly impermeable material which he gave to the sovereign. He prepared a number of medicines which brought about unexpected cures and developed a method of printing polychrome patterns and characters with a single turn of the press, using typographic devices designed by himself. In the meantime, he started work on the Sansevero Chapel, which would last up to his death. In 1749, Francesco Maria Russo painted the fresco on the vault of the funerary chapel, using special colours produced by the Prince.

In 1751, the Prince was at the centre of a “scandal” which seemed “the greatest in the world”. His innate curiosity, his eminently esoteric conception of knowledge and, at the same time, his mind open to the new ideas of the European Enlightenment had in fact attracted him to Freemasonry, a secret society through which many of those new ideas were being spread. Thus, in August 1750, Raimondo di Sangro became the Grand Master of the Neapolitan Lodge. With his Bull Providas of 18th May 1751, Benedict XIV had formally condemned the “worshipful company” in the name of the Church, a sentence furthermore reiterated in July with an edict of King Charles. The Prince had no other choice but to abjure.

But his problems did not end here. At his typographer’s workshop set up in his palace, in 1751 (but dated 1750), he printed his literary masterpiece: the Letter in Defence of the Academician Esercitato of the Crusca containing his Defence of the book entitled Letters of a Peruvian woman concerning the hypothesis regarding the Quipu addressed to the Duchess of S**** and published by the same. Ostensibly focusing on an ancient system of signs (the quipu) in use by the Incas of Peru, the Lettera Apologetica touched on many dangerous subjects, citing a large number of unorthodox authors and spreading the innovative ideas of Masonry, if not also – according to the Prince’s opponents – esoteric messages transmitted using a secret code. Judged to be “a sink of heresy” and harshly attacked in various pamphlets, the work was banned by the Congregation of the Index of prohibited books, and not even the publication of a Supplication (1753) which the Prince sent to the Pope was enough to have the Apologetica removed from the list of the prohibited books.

Disappointed and embittered, di Sangro threw himself into the “study of Experimental Physics”, and installed under his palace a great furnace and a chemist’s laboratory “with every type of burner”, making new and surprising discoveries, such as that of a mysterious “perpetual light”, about which he wrote a number of letters to the Florentine Giovanni Giraldi, later translated into French and collected into a volume addressed to the scientist Jean-Antoine Nollet (1753). Energy and money were above all invested in developing the iconographic design of his Chapel, bringing into being such masterpieces as Corradini’s Modesty, the Veiled Christ, and Disillusion.