Artichoke Juice, Invisible Ink --a Witchcraft Potion?

In September 1656, a female spy based in England wrote to her brother, who was living under exile in Antwerp. She feared their letters were being intercepted, broken into and sealed up again with counterfeit seals before being sent on their way. She urged caution: only ‘let mee know your minde – By the olde way [,] the hartichockes [,] as soone as you cane’. Her directive to communicate by artichokes seems peculiar, but it is possibly hinting at a recipe of invisible ink, concocted ‘by the olde way’, a phra