The National Archives holds hundreds of letters and documents relating to the Gunpowder Plot, the failed plot to blow up King James I and the Parliament in 1605. These include letters to and from the Jesuit priest, Henry Garnett, found in one of our historic records; SP 14/216/2. This record contains letters that demonstrate a type of secret communication often used by the Catholic underground – letters written in orange juice. The orange juice acts as invisible ink: it fades while drying, and only becomes visible again when the paper is warmed. Using orange juice, Garnett was able to communicate with his friends and associates while he was imprisoned in the Tower of London. As the orange juice remained visible after it had been warmed, his friends would know if the letter had been intercepted and read.
The series of secret letters describe his capture and imprisonment, tensions between those involved in the Gunpowder Plot, and the difficulties of communicating secretly with invisible ink letters. One such letter is SP 14/216/2 item 242, a letter from Henry Garnett to Ambrose Rookwood. The orange juice is the very faded brown ink in the background.
Although the letters have been warmed and the ink is now visible heritage scientists at The National Archives wanted to find out if scientific imaging could help make the ink easier to read. By photographing the documents using ultraviolet light the orange juice ink appears darker because the orange juice absorbs ultraviolet light (below image on the left). These photographs appear grey but by combining a colour photograph with an ultraviolet photograph we can create a something called a ‘false colour’ image (below image on the right). A false colour photograph overlays the ultraviolet photograph on top of the colour photograph, which enables the information on the letter to be read more easily.
You and your adult can find out more about the orange juice letters from The Gunpowder Plot in our blogs ‘The Gunpowder and Orange Juice Plot’ and ‘Smoke and Mirrors: Revealing the Gunpowder Plot through heritage science’.