When was the Portrait Painted

Signature and date on Delapierre portrait.

The research team determined that the Delapierre portrait almost certainly was painted sometime between 17 May 1785 and 31 December 1785. They narrowed the date range as follows:

Mirabeau's De la Caisse d'Escompte.

The Delapierre portrait is dated "1785" just to the right of the artist's signature.
The book shown in the foreground—Comte de Mirabeau's De la Caisse d'Escompte—was printed on 17 May 1785. That date is included in a postscript in the book, along with an explanation indicating why the book was printed that day.1
The research team purchased enough first edition copies of the book—including possibly the very book shown in the painting—to verify that a real copy was used as a model for the one depicted in the painting.2 In particular, the ill-fitting blue paper "wrapper" used on the book—a temporary cover provided by the printer in Bouillon, Belgium3—is very distinctive.

Therefore, it is highly unlikely that the book shown in the painting was a notional depiction by the artist of how he thought the book might look. Prior to the actual printing of the book on 17 May 1785, Delapierre would not have known what the unusual cover was going to look like.

So we can be confident that the portrait was painted sometime between 17 May 1785 and the end of the year.4

References and notes

[1] Le Comte de Mirabeau, De la Caisse d'Escompte, 1785, p. 220.

[2] Three of the eight copies of De la Caisse d'Escompte purchased by the research team retain their original temporary paper covers. One of these temporary covers exposed the pages of the book in a manner virtually identical to the way the pages are exposed on the book in the painting. The two other original paper covers likewise were haphazardly affixed by the printer, exposing parts of the front and/or back pages. The five remaining copies had been rebound with permanent covers, but several of these had retained their ill-fitting original covers long enough to expose the outer pages to edge discoloration. And a copy of another pamphlet obtained by the research team—De la Banque d'Espagne, issued under Mirabeau's name and published a few weeks after De la Caisse d'Escompte, almost certainly by the same printer (Ref: Richard Whatmore & James Livesey, "Étienne Clavière, Jacques-Pierre Brissot and the intellectual origins of Girondin politics," Annales historique de la Révolution française, #321, July-September 2000)—has an ill-fitting cover and a label on the spine that are very similar to those shown in the painting (see picture below). All this evidence suggests that the ill-fitting covers were an idiosyncrasy of the particular printer who attached these temporary bindings, and therefore that the artist used a published version of De la Caisse d'Escompte rather than a notional copy as the model for the book in the painting.

De la Caisse d'Escompte as depicted in the painting.

De la Banque d'Espagne.

This particular inscribed copy of De la Caisse d'Escompte may have been used as the model for the one shown in the painting. Note the discoloration where the front page was exposed because of an ill-fitting cover, and the old crimp in the upper-right corner of the first page that matches the one in the painting.

[3] Antonina Vallentin, Mirabeau, translated by E.W. Dickes, The Viking Press, New York, 1948, p. 214. According to Vallentin, "...when Mirabeau wanted to print [De] La Caisse d'Escompte, he was able to do it in only the petty state of Bouillon, whence it was smuggled into France."

[4] We cannot entirely exclude the possibility that the painting was started and dated in 1785, but not completed until later.