After that it needs to be 'toned' in tannic acid (or the tea of your choice). It is said than tannin toned 'types will fade/revert to blue/turn into pumpkins but I had one stapled to the railing on the back deck for an entire summer with no ill effects. Cyanotypes are one of the most archival processes as the image is made of Prussian Blue pigment.
This is most likely not, or only partially, true.
The first time I read about the tea-toning cyanotypes, I assumed the toning action was solely depended on brown organic "pigments" or compounds of the tea staining the paper as a kind of "brown dye" or watercolor paint. These compounds are probably not terribly light fast, and might fade. So for this part of the toning effect, yes, I can imagine a partial "reversal" of the toning action.
However, since than, I have understood that a large part of the toning action on cyanotypes of tannin and tannin from tea, is actually a true chemical reaction involving the prussian blue pigment and the gallotannic acid, resulting in a new very light fast pigment: called
ferrogallotanate or
ferric pyrogallate. This is what tea-toned cyanotypes gives their unique colors. It is the same pigment that is formed when making iron-gall ink, a centuries old pigment used as a normal writing ink. As
one website writes:
"Iron-gall ink was the most important ink in Western history. Leonardo da Vinci wrote his notes using iron-gall ink. Bach composed with it. Rembrandt and Van Gogh drew with it. The Constitution of the United States was drafted with it (Ink Corrosion). And, when the black ink on the Dead Sea Scrolls was analyzed using a cyclotron at the Davis campus of the University of California, it was found to be iron-gall ink (Nir-el 157)."
So, in effect, if properly and fully toned, there is no prussian blue pigment left in the cyanotype, and we might better speak of a "ferrogallotannato-type"

Actually, iron gall ink is associated with
"ink-corrosion" of papers, where the ink "eats" through the paper, leading to its ultimate destruction. However, this big problem in paper conservation of historic documents is associated with unbalanced, not properly created inks.
Actually, although both iron-gall ink and tea-toned cyanotypes contain the same pigments, the mechanism and chemical reactions by which the pigment is created is different, and the "ink corrosion" problem associated with ink is most likely not an issue associated with tea/tannine-toned cyanotypes. Most importantly, tea-toning cyanotypes does not involve the use of iron (II) sulfate, which seems to be the main culprit in the "ink corrosion" problems.