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Joseph Florent, the marquis de Vallière, helped Jars enter the École des Ponts-et-Chaussées at Paris about 1754.
Jars began his studies in chemistry at the Jesuit College of Lyon. After working for some years in his father’s copper mines at St.-Bel and Chessy, Lyonnais, he attracted the attention of Joseph Florent, the Marquis de Vallière, who arranged for him to enter the École des Ponts-et-Chaussées at Paris about 1754. There Jars designed and built a furnace to refine the Chessy ores.
While still students, Jars, and Duhamel visited the lead mines of Britanny and the mines of Pontpéan and Ste.-Marie-aux-Mines in Alsace. In 1757 the French government sent them to inspect Central European mines, particularly those of Saxony and of several provinces of Austria, including Bohemia, Hungary, Tirol, Carinthia, and Styria. After two years Jars returned to Chessy, where, with the exception of a year at the coal mines of Franche-Comté, he remained until 1764. He was then sent to study the English coal mines and the manufacture and use of coke in metallurgical work.
In addition to his thorough examination of the more advanced English and Scottish technology, Jars visited lead mines, observed the preparation of white and red lead, the making of steel by cementation, and the manufacture of oil of vitriol. He was accorded unusually generous treatment by the proprietors of the establishments he visited. After fifteen months Jars returned to France in September 1765.
The following year he visited the Low Countries, Germany, and Scandinavia. After 1761 he toured east-central France from Champagne to Franche-Comté, with government orders to examine factory operations and advise the proprietors on methods that would bring their manufacturing “to the degree of perfection of which they are capable.” His success led to a similar survey of central France from Orléans to Auvergne. Unhappily, his mission was not completed; he suffered a sunstroke and died after a short illness.
In spite of his long and arduous journeys, Jars found time to experiment at St.-Bel. By applying coke to the melting of copper he demonstrated, for the first time in France, the melting of iron with coke in January 1769. A few months later he conducted the experiment again at the plant of the Wendel family at Hayange, where, although this process was not adopted immediately, the English procedures were successfully naturalized in France.”
Jars was elected to the Royal Society of Arts as a corresponding member in 1765. A correspondent of the French Royal Academy of Sciences since 1761, he became a member on May 18, 1768, when he shared a tie vote with Lavoisier.
It is not known whether Jars was married or not.
Antoine-Gabriel Jars, a politician, was his nephew.