So you want to open sealed envelopes without getting caught?

Here’s the secret, according to one of the six oldest classified documents in possession of the Central Intelligence Agency:

“Mix 5 drams copper acetol arsenate. 3 ounces acetone and add 1 pint amyl alcohol (fusil-oil). Heat in water bath — steam rising will dissolve the sealing material of its mucilage, wax or oil.”

But there’s a warning for the intrepid spy: “Do not inhale fumes.”

Nearly a century after it was written, the recipe was released Tuesday by the CIA as part of a cache of six World War I-era documents. The documents, which deal mostly with invisible ink, date from 1917 and 1918 — predating the agency itself by decades.

“When historical information is no longer sensitive, we take seriously our responsibility to share it with the American people,” CIA Director Leon Panetta said in a statement announcing the release.

One document lists chemicals and techniques to create invisible ink for what is charmingly called “secret writing.” Another document, from June 1918 and written in French, provides the formula the Germans apparently used for their invisible writing during World War I.

All of the documents can be viewed at in the site’s Freedom of Information Act Electronic Reading Room. They will also be available at the National Archives.

The six documents were first held by the Office of Naval Intelligence during World War I, and at least one was obtained from the French.

But, as the CIA said Tuesday, the intended recipients of all this secret stuff are not always clear.

One document listing seven formulas is on Department of Commerce letterhead, and a chemist at the Bureau of Standards recommends that some of the invisible ink solutions be used with a quill pen rather than a steel pen because of the risk of corrosion.

Another document in the collection was intended to teach U.S. postal inspectors how to detect secret ink. The pamphlet, listing 50 possible scenarios in which invisible ink could be employed, was prepared by a handwriting expert in San Francisco.

“There are a number of other methods used by spies and smugglers, according to the skill and education of the criminals, such as placing writings under postage stamps, wrapping messages in medicine capsules and engraving messages . . . on toe-nails,” which later would be made visible with powdered charcoal, the expert, Theodore Kytka, wrote at the time. “The rule is to suspect or examine every possible thing. The war between the spy or forger and the expert is continually bringing out new methods.”

Any material describing secret writing falls under the CIA’s purview to declassify, the agency said. The documents still bear stamps showing that they remained exempt from declassification as recently as 1978, although some had been downgraded from secret to confidential.

Some of the methods seem wonderfully archaic, such as carrying solution absorbed into ironed handkerchiefs or starched collars. “The article thus treated is later on again put in water and a solution obtained, which can be used for invisible ink,” according to one document. “The best means for developing are iodite of potassium.”

It seems a tad odd that this kind of material would be classified for nearly 100 years when every other child with a little lemon juice and a Hardy Boys or Nancy Drew novel has dabbled in invisible writing. But, the agency said, this was all once a seriously secret business. The methods behind sophisticated invisible writing didn’t change for many decades.

In 1999, the agency rejected a Freedom of Information Act request to release the six documents, asserting that doing so “could be expected to damage the national security.”

Asked Tuesday why it took so long to release the documents, a CIA spokeswoman said that “in recent years, the chemistry of making secret ink and the lighting used to detect it has greatly improved.”

In other words, it is only now that the agency can tell us some of the fun stuff one can do in a bathtub.

Staff writer Greg Miller contributed to this report.