As a general rule, hospitals and dirt don't get along.

In the Culpeper Civil War hospital that has become known as the Graffiti House, however, dirt has literally saved history.

At some point, probably years after the war ended, the owner of this old house decided to cover up the graffiti that soldiers from both armies had written--mostly using charcoal--on the walls of the makeshift hospital.

"When the wall was whitewashed over, this thin layer of dirt protected it," says historical conservationist Chris Mills.

Mills, who operates Christopher Mills Conservation Services in New York City, is in the process of cleaning an upstairs bedroom wall at the Graffiti House in Brandy Station.

This room, called the J.E.B. Stuart Room because of a wall signature believed to have been written there by the Confederate general, is the last to release its secrets in the house that was constructed about three years before the Civil War began.

"When we cleave off the whitewash, the layer of dirt allows us to separate the lime wash from the plaster with no damage to the writing," says Mills.

Of great help too is the fact that the soldiers used mostly charcoal from the several fireplaces in the home to leave their messages on the walls (there are also some pencil inscriptions and drawings).

"Charcoal is inert," says Mills. "The carbon doesn't deteriorate with time."

The work in this room is a bit trickier than in the rest of the house, however, because the owners added some sort of blue tint to the whitewash.

"It is an amazing circumstance that it is still all here," says Mills.

Not only is Mills, in surgeon-like manner, painstakingly removing the whitewash, the conservationist is also stabilizing the plaster, which in some places is cracked and pulling away from the wooden laths that have held it in place for more than 150 years.

In some cases, previous owners have used strips of porous tape, covered with some type of spackling, to keep the cracks from widening. Removing these foreign substances makes Mills' job even tougher and results in some minor but unavoidable damage to the graffiti underneath.

Once the tape is removed, Mills pins the cracked plaster to the laths with nail-like plastic fasteners. When the pins are removed, the holes they made are used to inject an alcohol solution into the plaster.