According to the FBI's Bomb Data Center in Washington, nearly 30 percent of bombing incidents are hoaxes. Because one can never be too careful, investigators could use a tool that would enable them to inspect a suspicious piece of mail and determine if they really need to scramble to defuse a letter bomb. Enter the See Through spray from Mistral Security Inc., a treatment that makes paper temporarily transparent so the user may examine the contents of an envelope without opening it. "Certain liquids change the optical characteristics of paper, making them transparent to light," explained Robert Schlegel, vice president of Mistral, which also produces sprays for drug and explosive detection and for fingerprint visualization for the law-enforcement market. "It's like what happens with a cotton T-shirt. You can't see through it, but if you wet it with water, you can." The effect is short-lived, he said, on the order of 10 to 20 seconds, and it works on both white and colored envelopes. See Through does not degrade the paper and will not affect most inks. "It's undetectable after it fades," he said. "There is a slight odor for five or 10 minutes afterward, but that disappears." See Through is not a new idea, per se, but a more environmentally friendly replacement for Freon- and CFC-based transparency sprays that have been banned under international agreement. "To my knowledge, there are no other sprays like it on the market," Schlegel said. Little threat to privacy Schlegel stressed that See Through will not fall into the wrong hands. "We and our distributors will only sell it to law enforcement," he said. "I had a magician come in asking for it, and we turned him down." The costs of the treatment are more reassuring, however, to those who fear it may be used to violate privacy rights. Mistral sells the spray in two sizes, 100- and 300-ml cans, which carry retail prices of $45 and $90, respectively. Schlegel said the smaller can is enough for approximately 50 envelopes, making it unlikely that See Through would be used on anything other than suspect envelopes. Nevertheless, he recognizes the wariness with which civil liberties organizations may regard the product. "Anytime you have something that could invade somebody's privacy, there will be some concern," he said.