In the late-eighteeenth century, De Montpetit invented the fixé-sous-verre miniature. This complex technique consisted of painting in oil on taffeta or fine silk and covering it with a translucent animal glue, before placing the protective glass over the top, resulting in a greater intensity of colour.
In 1759, the Department of Foreign Affairs commissioned three portraits of King Louis XV to be used as royal gifts: two of them were destined to be mounted on bracelets for which he was paid 240 livres each in April of that year. The third one, for which he was paid 360 livres was mounted on a box encrusted with 338 diamonds. In June 1760, he supplied a further portrait of the king to the Department of Foreign Affairs to be mounted on a gold box encrusted with diamonds, emeralds and rubies by the goldsmith Ducrollay.
According to Ann Massing ('Arnaud Vincent de Montpetit and Eludoric Painting', Zeitschrift für Kunsttechnologie und Konservierung, 1993, vol. 7, no. 2, p. 360), 'Montpetit is said to have painted forty portraits of the King.' Nevertheless, only very few of those miniatures seem to have survived. A rectangular fixé-sous-verre of King Louis XV by de Montpetit was exhibited at the Reichenberg Exhibition of 1903, owned by Prince A. de Rohan, Sichrow (see L. R. Schidlof, The Miniature in Europe, Graz, 1964, II, p. 569).