Sound like a blueprint. These were commonly used to copy architects drawings in the nineteenth century and are sometimes found in archives. They seem to survive reasonably well. The colour comes from ferric ferrocyanide (Prussian Blue) which is non-toxic despite its scary name. There is an article on blueprints in Wikipedia. 

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On 18 Sep 2013, at 13:28, Susan Scott <[log in to unmask]> wrote:

> Can anyone help me identify the materials/chemicals/method of production of a letter which I have just come across in the archive?
> 
> The letter, dated 1897, is ostensibly written in white on a deep purpley-blue paper, with a faint chemical smell.  The purpley-blue finish is on both sides of the sheet, but where the sheet is folded you can see that the paper is white underneath.   The writer of the letter has rather helpfully explained how she made the white writing:
> 
> "I thought you might like a letter written on this paper... You write with a fluid which looks like water, and at first you cannot see what you write. Then it turns red and lastly white like it is now. ... It is rather funny writing with this paper - but the smell is enough to knock you down. I think it must be some kind of chemical."
> 
> I've seen architectural drawings that are on blue paper with the drawing in white, but usually the blue is only on one side of the paper. Is this basically just similar to that?  
> 
> Mostly I'd like reassurance that the letter is fairly stable, and not too toxic.  But if anyone does know what the paper is coated with and how the chemical changes in the ink were effected, I'm quite curious. 
> 
> Many thanks!
> 
> Susan
> 
> 
> 
> Susan Scott
> Archivist
> 
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