Shortly after the birth of his daughter, an event he seemed to welcome with a fervor greatly out of keeping with his usual coldness, Curwen resolved to sit for a portrait. This he had painted by a very gifted Scotsman named Cosmo Alexander, then a resident of Newport, and since famous as the early teacher of Gilbert Stuart. The likeness was said to have been executed on a wall-panel of the library of the house in Olney Court, but neither of the two old diaries mentioning it gave any hint of its ultimate disposition.

In 1766 came the final change in Joseph Curwen. It was very sudden, and gained wide notice amongst the curious townsfolk; for the air of suspense and expectancy dropped like an old cloak, giving instant place to an ill-concealed exaltation of perfect triumph. It was after this transition, which appears to have come early in July, that the sinister scholar began to astonish people by his possession of information which only their long-dead ancestors would seem to be able to impart.