After an ignominious stay in a Fairfax County courthouse vault - amid heroin, cocaine and assorted stolen guns - the much-traveled wills of George and Martha Washington have recently been restored by the Library of Congress and returned to the Fairfax courthouse.

This time the county's most valuable documents will be kept in a vault free of drugs and guns.

The criminal evidence vault, where the wills have been kept before their restoration, "is just not the kind of a place where you keep those priceless things," said Fairfax Circuit Court Clerk James E. Hoofnagle, the man in charge of guarding documents that begin: "I George Washington of Mount Vernon - a citizen of the United States and lately President of the same. . . ."

In the new vault, which Hoofnagle says has no drugs or "anything with a smell to it," the wills are kept inside a clear plastic box. That box is inside a wooden box, which is inside a specially made "preservatory," a small refrigeration unit that controls temperature and humidity to slow the deterioration of the documents.

The only problem with the new storage technique, according to Hoofnagle, is that no one can look at the wills. The county plans to correct this in 1981 when, with the completion of the new county courthouse, the documents will be put on display in a climate-controlled display case.

Washington's will, written in the first president's own handwriting in 1799, addresses the Mount Vernon landholder's opposition to slavery, his interest in universal education and his confidence in the nation he helped found. Washington wrote: "Upon the decease of my wife, it is my will and desire that all the slaves which I hold in my own right, shall receive their freedom."

Washington's will and that of his wife have not always been in the safekeeping of Fairfax County, which under Virginia law is assigned to guard them.

During the Civil War, Washington's will was taken to Richmond to keep it from falling into the hands of raiding Union troops. During the trip to Richmond, the will was folded and nearly every page was damaged.

Martha Washington's will was stolen during the Civil War by a Union lieutenant. J. P. Morgan, the financier later bought it and exhibited it in his museum. Through a lawsuit that reached the Supreme Court, Fairfax County managed to force the return of Martha's will in 1915.

In early 1977, after the Virginia General Assembly granted Fairfax County special authority, George Washington's will was returned on loan to Mount Vernon, where it had been drawn up 177 years before. It remained there on display for six months.

Hoofnagle yesterday took the wills over to the meeting of the Fairfax County Board of Supervisors, where he was praised for the care he has taken since 1976 in guarding the documents.