If you're interested in photographic history, examples of superior darkroom technique and some pleasing pictures, plan to spend some time at the current exhibit of gum bichromate prints by Mac Cosgrove-Davies at the Aberdeen Gallery in Georgetown.

Gum bichromate, a photographic process popular near the turn of the century, still enjoys some popularity in Europe but in this country has all but slipped though the cracks. Cosgrove-Davies, a 29-year-old consulting engineer from Arlington, has revived it very successfully.

Here's the way it works: You start with a sheet of water color paper. You size it by floating it in a solution of Knox gelatin and water. Then you soak the paper in formaldehyde to remove excess material. Cosgrove-Davies then hangs the prepared paper up to dry.

Next a mixture of gum arabic and water color pigments is prepared. This is mixed with a stock solution of potassium bichromate and used to coat the now thoroughly dried paper. This coating requires no darkroom, but is brushed onto the paper in low light.

Now the paper is ready and a negative is placed on it. The whole package is held in contact with a piece of glass and exposed by sunlight or ultraviolet lamp. Exposures are long and tedious, but the light physically hardens the bichromate. Once exposed, the paper is "developed" in a tray of water. The highlights become white, and the shadow and darker areas retain the color of the pigmented solution.

The process takes about two hours, and once dry, the paper can be re-coated and re-contacted with a different color. Dozens of exposures can be made to achieve a fine gradation of tone or contrast. Cosgrove-Davies has worked as long as three months on a single picture.

Each finished work is unique, reflecting the amount of color and emulsions used.

For example: The first picture in the exhibit is a four-way combination of pictures of the Sacred Heart Church in Glyndon, Md. The pictures are printed from the same negative, but colored differently. The set shows how effective gum bichromate prints can be.

Several of Cosgrove-Davies' Nova Scotia pictures of coves work very well in gum prints. There is an exceptional shot of an abandoned 1920s Ford that is tinted just right. Perhaps the best picture in the show is an 8x10 entitled "Farmyard, Maples, Va." The photography is superior and the tinting is splendid.

I didn't much care for the way the show was set up. The majority of prints have mattes that are much too large and distracting. I also found that the stark metal frames didn't fit in with the delicate, almost misty quality of the prints. Still, plan to spend a lot of time studying these prints -- once you get into this exhibit, time passes quickly.

Cosgrove-Davies began making gum bichromate prints in 1979. He uses two 4x5 cameras and an 8x10 Century Graphic that he rebuilt.

The show continues until May 20 at the Aberdeen Gallery, 3247 P St. NW. Hours are 10 to 8, Monday through Saturday; 10 to 6 on Sundays.