One of Bladen Forrest’s eight slaves was a Susan Mason, who was listed by the appraiser as “old & infirm,” and whose value was placed at zero.

Union Gen. Lorenzo Thomas claimed $800 for his slave laundress, Lucy Berry, and $100 each for her children, George and Lorenzo. But the general was allowed only $219 for Lucy and $43.80 for little Lorenzo. George had “no value.”

So it went in Washington in the spring of 1862. It was a cold accounting, the search for the price of a person.

As the question of slavery in America was being tried on the battlefield, its future in the District was resolved in April 1862 through strange and pioneering legislation that freed 3,100 slaves but paid the masters for their “property.” The slaves received no money unless they agreed to leave the country.

The District of Columbia Compensated Emancipation Act became law on April 16 — 150 years ago Monday. It was the cause of jubilation among those whose chains it broke, and today it is celebrated in Washington on “Emancipation Day," a city holiday.

On Wednesday, the National Archives displayed some records that detailed how many slaves were freed, how many owners applied for compensation and how much each slave was deemed to be worth.

“It was the first time the government had officially liberated any group of slaves,” said David S. Ferriero, the archivist of the United States, and it anticipated the more famous Emancipation Proclamation by six months.

The documents offer a window into the bookkeeping of slavery and a rare glimpse into the lives of local slaves and their owners.

Assigning value was hard for the commission set up to administer the law, according to its final report. For years, slavery in Washington had been a matter of “trifling importance,” the report said, and an expert was needed.

So the commissioners brought in from Baltimore “an experienced dealer in slaves,” B.M. Campbell, to provide expert and independent opinion.

Campbell and his brother, Walter, appear to have had a prewar business trading slaves between Baltimore and New Orleans, Archives experts said, and were considered impartial judges of the value of slaves.

The new law stipulated that the government would pay masters as much as $300 for each freed slave, although, in the end, the owners were often paid much more.

The owners posted a claim and had to present their slaves for examination, Kenneth J. Winkle, a history professor at the University of Nebraska-Lincoln, wrote on the university’s “Civil War Washington” Web site.

The Campbells — like Civil War insurance adjusters — issued their valuations. And the commission decreed what the government would pay.

Official ledger sheets detailed the accounting.

William Pressy, for example, claimed a value of $100 for James Thomas, one of his five slaves, but he was awarded $21.90. Thomas might well have been a child because $21.90 seems to be a valuation given for some slave youngsters.

Some slaves were deemed to be worth nothing. The records are dotted with notations that such and such a slave, often an infant or child, had “no val.”

The commission records present an array of Washington slave holders.

On June 2, 1862, the “Sisters of the Visitation, Georgetown,” listed a dozen slaves — including a couple and their seven children in their petition. They were allowed $3,774 in compensation.

Francis P. Blair, whose family was strongly allied with President Abraham Lincoln, and whose son, Montgomery, was Lincoln’s postmaster general, filed for compensation for two slaves.

Clark Mills, the sculptor who created the equestrian statue of President Andrew Jackson in Lafayette Square and who worked on the statue of Freedom atop the Capitol dome, sought compensation for 11 slaves, for whom he was allowed $1,916.25.

And Henry Hatton, one of several petitioners described in the ledgers as “colored,” sought compensation for three slaves, Martha, Henry and George Hatton, who could have been members of his family, according to Archives expert Damani Davis.

Davis said the District emancipation records are remarkable for the personal detail they provide.

Many records from other sources don’t even provide an enslaved person’s last name. Slaves are treated “as just another form of property,” he said. In contrast, the District records provide last names, physical descriptions, personal qualities and work skills.

Mills, the sculptor, for example, spoke well of his slave Philip Reid, whom he valued at $1,500. Reid was a skilled plasterer and had figured out a way to complete the problematic construction of the Freedom statue, Davis said.

Reid was “aged 42 years, mullatto color, short in stature, in good health, not prepossessing in appearance, but smart in mind, a good workman in a foundry,” Mills wrote of him.

Another owner stated that his slave had “no infirmities or defects either morally, mentally or bodily.”

The legislation was introduced by Henry Wilson, an anti-slavery senator from Massachusetts, said Clarence Davis, of the D.C. Office of Public Records.

Because of Congress’s jurisdiction over the District, northern legislators were able to pass a D.C. emancipation bill in the absence of their departed Southern brethren.

The Senate appoved the legislation on April 3 and the House on April 12. Lincoln signed the bill into law four days after the House acted.

“I am gratified that the two principles of compensation and colonization are both recognized and practically applied in the act,” Lincoln wrote Congress.

In addition to compensating owners, the bill provided for payments of as much as $100 to slaves who agreed to move to Haiti or Liberia.

This voluntary “colonization,” supported by Lincoln and others, was rejected by most blacks, and only “a handful” from Washington accepted the offer, Winkle said.

Once the legislation was enacted, most of the District’s freed slaves “immediately left their homes and sought employment from others,” the commission’s report stated. “Many of them left the District of Columbia to join the service of officers of the army, or to go north.”