Mystic — The Mystic Art Association is planning to buy the historic Emporium building on Water Street late this summer.

Bill Middleton, treasurer of the art association’s board of directors, and Evan John Nickles, co-owner of The Emporium gift shop, confirmed today that the two parties signed a purchase-and-sale agreement on the 1859 building this afternoon.

“They are wonderful stewards for the legacy of that building,” Nickles said in a phone interview. “I think it could be a wonderful thing.”

Neither Nickles nor Middleton would reveal a sales price for the building, which was on the market for $799,000. But Middleton said the sale included a very generous contribution by Nickles and co-owner Robert Palmer Bankel back to the art association.

“It is our ‘thank you’ back to Mystic,” Nickles said. “It put the deal in a position where it could actually happen and give the arts center some momentum.”

Middleton said the arts center, which is celebrating its centennial year in 2013, had no immediate plans for the building and would likely lease it in the near term. In the long term, discussions have included the possibility that the 4,250-square-foot Water Street building — or another structure on River Road donated recently to the nonprofit — could be used as an artist’s retreat, he said.

Other possibilities, he said, include use of the space as a commercial venture related to the art association’s mission. The association doesn’t currently lack gallery space, but it could need overflow capacity in the future, he added.

“Whenever you have the chance to buy a contiguous piece of property (and have funding to do it) ... it’s usually a pretty good idea,” Middleton said.

Middleton said that in addition to The Emporium owners’ contribution to the association, another substantial donation was promised by a member of the community to make the deal happen. He added that Chelsea Groton Bank is expected to provide a mortgage on the property.

Judi Caracausa of Market Realty in Mystic represented the sellers, while Randy Russ of Russ Real Estate in Pawcatuck worked on behalf of the art association.

Nickles announced earlier this year that the nearly half-century-old Mystic Emporium — a fun and quirky stop for three generations of shoppers — would be closing. But he said that with the art association deal not expected to be finalized until late August or early September, the gift shop will stay open longer than the initial June closing that previously had been announced.

The extra time will allow The Emporium to host one last art show, probably at the end of June or first week of July, celebrating all of the artists whose works have been exhibited over the years in the shop’s gallery. Nickles estimated at least 250 invitations to the “So Long Peter Pan” celebration — named after one of the Emporium’s signature pieces — will be sent out.

Nickles said the show will represent a passing of the Emporium building’s baton to new owners. He added that he has been in discussions with the art association for three and a half years, but negotiations became earnest in the past year.

“It’s a new beginning for that building,” he said. “This is a very exciting moment for Mystic.”