A three-alarm fire that damaged the State Police Museum & Learning Center in Grafton this weekend broke out spontaneously when rags and sawdust tossed together in a plastic bag heated up and ignited, the state fire marshal said Monday.

Workers were refinishing wooden molding in the museum on Saturday. As they completed their work for the day, they put rags with stain on them into plastic trash bags, along with sawdust accumulated during the project, officials said.

Over the next few hours, the rags began to dry, producing heat that ignited the sawdust, State Fire Marshal Peter. J. Ostroskey said in a statement. The fire, caused by the “spontaneous combustion of oily rags,’’ was discovered around 9:30 p.m. Saturday, officials said.

“As flammable oil from rags dry out, they produce heat that can start a fire like this one which is why it is important to dispose of oily rags properly,’’ Ostroskey said in the statement. “When balled up with combustibles like sawdust, a fire can quickly start.’’

The museum, a former State Police barracks, did not have sprinklers installed, Ostroskey said.

The blaze was concentrated in the second-floor meeting room where the restoration work was underway, officials said. The museum’s exhibits and galleries, which are on the first floor, sustained only smoke and water damage, State Police said over the weekend.

State Police Colonel Richard D. McKeon, in a statement, applauded Grafton firefighters, whose work saved historic artifacts tracing the history of the only statewide police agency.

“Firefighters did a superb job containing the flames to an area on the second floor,’’ McKeon said in a statement. State Police are “grateful beyond words for their efforts in preserving the numerous artifacts and photographs from the department’s 151-year history that are on display.’’

Fire Chief Michael Gauthier said firefighters deployed “salvage covers’’ as they battled the fire to protect the museum’s collection.

Ostroskey’s office issued guidelines on how to safely dispose of oily rags:

First, hang them outside to dry in a safe area or spread them out flat, making sure they are weighted down outdoors. They should not be in a pile. Second, rags should be stored in a nonflammable container, such as a metal paint can, covered by a solution of water and a detergent to break down the oil. The can must have a tight cover.

The material should then be disposed of during a local hazardous waste collection program.

John R. Ellement can be reached at ellement@globe.com. Follow him on Twitter @JREbosglobe.