Howdy, Monroe. I've found the answer to this question, and I must say
I feel a bit disappointed. Those pens aren't as fancy-schmancy as I'd
thought. The most commonly-used counterfeit detector pens contain an
iodine solution that reacts and darkens if starch is present in the
paper that the bill is printed on. Cheap, wood-based paper will be
marked by the pen, while expensive rag paper (as used in genuine US
currency) will not be marked.
"The counterfeit detector pen solves the biggest counterfeiting threat
today. It used to be that a counterfeiting operation used expensive
presses and special inks and papers to create exact duplicates of the
bills. Today, the threat is much more mundane -- people with color
copiers and color printers try to create passable facsimiles of a
bill. They are not trying to make an exact copy. They are trying to
create something close enough that people won't notice anything if
they give the bill a passing glance. These folks are not particularly
careful or meticulous, so they copy or print onto normal, wood-based
paper.
The counterfeit detector pen is extremely simple. It contains an
iodine solution that reacts with the starch in wood-based paper to
create a black stain. When the solution is applied to the fiber-based
paper used in real bills, no discoloration occurs. The pen does
nothing but detect bills printed on normal copier paper instead of the
fine papers used by the U.S. Treasury."
How Stuff Works: How does a counterfeit detector pen work?
http://money.howstuffworks.com/question212.htm
"A counterfeit detector pen is a felt tip pen containing an iodine
solution that can be used to help identify computer-generated
counterfeit bills... Detector pens, although not fool-proof, are an
effective way to identify computer-generated counterfeit bills because
the iodine solution in a detector pen reacts with starch, which is
commonly found in the wood-based copy paper used by most printers.
Detection pens are easy to use and require no training. A clerk at a
cash register simply uses their counterfeit detector pen to put a
small mark on the bill. If the bill is counterfeit and the paper is
wood-based, the iodine in the pen solution will react with the starch
and leave a dark brown or black mark. If the bill is authentic and the
paper is fiber-based, there won't be any starch and the pen will not
leave a mark. (Although manufacturers of counterfeit detector pens
will sometimes add a biodegradable pastel coloring to the iodine
solution so that users can easily see which bills they have already
screened -- the pastel coloring usually fades within a day or so.)"
SearchSecurity: Counterfeit Detector Pen
http://searchsecurity.techtarget.com/sDefinition/0,,sid14_gci835821,00.html
"U.S. pat 5,393,556 issued July 13, 1993) states that the iodine tests
for starch content in the paper using an aqueous alcohol solution of
iodine and phenolphthalein (phenolphthalein is a common indicator used
in many chemistry labs).
"U.S. pat 5,662,735 issued September 2, 1997 discusses the iodine test
where... the ink does not mark the genuine currency and leaves a mark
on the counterfeit money. This method uses metallic iodine in any
number of solvents such as alcohols, ketones, esters of alcohols, or
ethers of alcohols. Some chemicals it mentions are methyl acetate,
isopropyl alcohol (rubbing alcohol), ethylene glycol (used in
antifreeze for the car), and acetone. Water is then used to dilute
the solution."
MadSci Network: What is special about the make-up of US currency, and
the Chemistry in it?
http://www.madsci.org/posts/archives/feb2001/982089722.Ch.r.html
These are the Google search strings that gave me the best results:
Google Web Search: counterfeit detector pen "how does"
://www.google.com/search?hl=en&q=counterfeit+detector+pen+%22how+does%22
Google Web Search: counterfeit pen iodine
://www.google.com/search?hl=en&q=counterfeit+pen+iodine
Thanks for an interesting question! I'd often wondered about this
myself, but I never shed enough sloth to go and find out. Ah, the
motivating power of money. ;-)
Best,
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