Kahn on Invsisble Ink (Was: Transparency Spray?)

[Anonymous]

S a n d y  W r o t e:

>Mike [mmotyka at lsil.com] wrote:
>
>> I would bet that there is SOMETHING that
>> is dissolved by liquid freon. Just mark
>> your letters with the stuff and look for
>> the integrity of the mark at the other end.
>
>Or... is there something that REACTS to freon in an interesting way...?
>
>

IIRC in 'The Codebreakers' Kahn states that invisible ink can 
always be read by exposing the paper to iodine vapor, since the 
'ink' distorts the fibers of the paper. Even plain water can be 
used to write messages. This can be defeated by wiping the 
surface of the paper with a damp sponge, requiring Mallory to 
find the correct chemical to react with the ink and cause a 
color change.

However, _this_ can be detected by the smear left on the fibers 
of the paper. Perhaps this also occurs with sprayed on solvents? 
This could work on 'unprepared' mail.

As an added bonus, the iodine vapor is _supposed_ to fade out 
afterwards, unless you use too much of it as we did in science 
class trying to develop fingerprints.


The Baker Street Irregulars