BTW, there are various toning methods for cyanotype - it's mostly about the looks, not longevity. In fact, we really don't know the long term effect of toning cyanotypes, but we do know that they can last 170 years untoned. See a couple of toners (and their effects) below:
- Lead acetate: You'll get a (presumably) more robust image (less light fading if it happens...) and a considerably colder blue. No staining of paper base...
- Tannic acid and gallic acid: Brown to warm-neutral color (fully toned: brown, split toned: warm-neutral in the shadows, brown in the hihlights...), no data for longevity. (But remember, tannic / gallic acid toning will convert the image to something similar to iron-gall ink, chemistry-wise, and we know that iron-gall ink isn't particularly friendly to paper...) You may get very little or significant staining of paper base depending on the strength of the toner and the sizing of the paper. Hard-sized papers are affected less, and mild toners (along with short toning times) stain less...
Hope this helps,
P.S. If you aren't happy with the color of the straight cyanotype I'd suggest some other process instead of fiddling with it by toning. (Split toned prints are nice though. See one of my split-toned prints here.)