After five years and almost $100,000, solar renovation of the 172-year-old stone and brick Lewis Mill in Jefferson, Md., has been completed.

Without damaging the Frederick County building, the workers (several potters and their friends) encased the mill's south wall in an envelope of glass and wood designed to soak up the sun's heat. The heat was stored in the brick wall and, when needed, funneled into the mill. The ambitious renovators then sealed off the rest of the three-story structure with thick insulation and double-paned windows. The 5,500-square-foot historic mill, abandoned 54 years ago, now houses a pottery studio and shop and living space.

The project was done with the help of a $67,000 grant from the Department of Energy's program to encourage solar energy innovations.

Today, warm air from the encased wall flows through ducts into the second floor of the mill. As it leaves the envelope, it theoretically "pulls" cold air up from the basement and thus creates what its designers call a "convection loop" that should circulate warm air from the second floor to the basement.

Although it has produced temperatures as high as 80 degrees in the second-floor residential space, the anticipated merry-go-round of warm air is not bringing heat as planned to the the first floor, where the temperature hovers near a chilly 50 degrees, according to solar project partner John Hanson. Hanson said he's experimenting with fans that would push warm air to the lower floors.

Nonetheless, building partner Arthur Kanegis estimates the solar retrofit will save almost 5,000 gallons or approximately $5,500 of heating oil a year. Currently, a wood stove provides supplemental heat.

Probably the most unusual detail of the joint solar retrofit and historical preservation project is the glassed-in south wall made up of 45 "quadruple glazed" glass sheets the size of patio doors. The glass, Hanson said, sandwiches two layers of acrylic film between two layers of tempered glass, allowing the sun's heat to flow in, but inhibiting it from flowing out--sort of a thermal one-way mirror. Hanson said he thinks the Lewis Mill project is the first solar building to use this newly developed glass.

Solar architect John Darnell included another unique feature in the solar wall. He insulated the outer side of the brick wall so that its absorbed heat wouldn't escape at night. Thus, solar warmth is forced to travel around the envelope while it is sealed and heat the wall from behind. Then, the top and bottom ducts are opened and the warm air begins its circular flow.

Darnell is reserving judgment about the efficiency of the convection loop system until after he's tried enhancing the flow with electric fans. "I have mixed feelings about it," he said. "I used to be gung-ho about solar energy--and I still am--but it's a complicated science, and I really think things have to be figured out."

Once he gets the bills paid for this renovation, Hanson said he would like to try other new forms of solar heating such as photo-voltaic shingles that catch the sun's rays and convert them to electricity.

Another feature of the Lewis Mill project is a waterless composting toilet that, Hanson said, converts bathroom and kitchen wastes to useable garden fertilizers.

Open houses are being held at the Lewis Mill from 10 a.m. to 5 p.m. today and tomorrow. Pottery-making demonstrations and exhibits of indian pottery found near the site will be featured and the solar installations described.

Lewis Mill can be reached from Washington by I-270. Fork right onto I-70 West one mile and exit on Rte. 340 west. Take Rte. 340 five miles toward Harpers Ferry and exit on Lander Road. Follow the signs to the Lewis Mill Open House. For information, call 301-371-9172.