Firefighters plan to patrol the perimeter of the fire through the night and focus on hot spots, according to a statement by the Idaho Department of Lands.

Aircraft equipped with infrared tools will locate the fire's hottest areas and give that information to fire managers, "enabling them to focus attack efforts (Thursday) where intense heat is detected near the firelines," according to the statement.

As of 8:30 p.m., the size of the fire was estimated at 430 acres and was about 20 percent contained, though fire experts cautioned the containment estimate was not complete.

Extremely dry fuels in the Robie Creek area will continue to be a challenge for firefighters battling the Karney Fire Wednesday, but cooler weather and the work of fire crews overnight kept the blaze from growing.

Firefighters were able to set some burnouts around homes and other entry points in the Wilderness Ranch subdivision closest to the blaze, which kept the flames at bay. The biggest issues for firefighters overnight were some spot fires along the Wilderness Ranch Road, but crews have been able to knock those down before they could grow, officials said Wednesday morning.

Fire crews reported no significant growth in the blaze as of 3 p.m. Wednesday.

The fire, which Boise County sheriff’s officials say was deliberately set by an 18-year-old volunteer firefighter near his home Monday, was especially active along its eastern flank Tuesday afternoon. There it is threatening the western edge of the Wilderness Ranch housing development, coming within a quarter-mile of the subdivision’s homes.

In anticipation of the fire’s approach, crews burned out fuels Tuesday between the flames and three homes — an operation that “was instrumental in saving them,” according to a statement by the Idaho Department of Lands.

Sheriff’s officials gave a voluntary evacuation notice to about 10 homeowners in the Kelly Gulch area and about 50 more along Robie Creek Road on Tuesday. Owners of 20 other homes near Robie Creek Road received notices Monday.

About 75 to 100 homes in the Wilderness Ranch development are considered threatened, Boise National Forest spokesman Dave Olson said. So far only one home has burned.

Boise County sheriff’s officials say Nathaniel Fay Bartholomew admitted starting the blaze. He is being held at the Ada County Jail with no bond on a felony arson charge.

Sheriff’s officials say that it appears as if Bartholomew set the blaze in some pine cones and other fuel alongside Robie Creek Road, near Majesty Heights Lane, where it spread to a nearby home and then to other parts of the forest.

A motive in the arson is unclear, but Boise County Chief Deputy Dale Rogers said Bartholomew might have set the fire to get the attention of his father, who is also a firefighter. Bartholomew told investigators that working with his dad on fires was a bonding experience, Rogers said.

Rogers said Wednesday Bartholomew was identified as a suspect Monday afternoon after someone saw him walking away from where the fire started. It wasn't long before sheriff's deputies found Bartholomew working the fire with the Clear Creek volunteer fire department and took him into custody.

More than 200 firefighters are working to suppress the fire. Air attacks, including retardant drops and helicopter water drops, helped slow the its advance.

The Idaho City Hot Shot crew was credited with containing spot fires that crossed Wilderness Ridge Road in several locations.

The Red Cross has set up a shelter in the Idaho City High School gym for Wilderness Ranch residents who can’t get to their homes.

Boise County commissioners declared the fire a disaster emergency — a move that allows the county to apply for state and federal financial assistance, the land department statement said.

The firefighting operation is being complicated by late summer conditions in the Boise National Forest.

“This fire is really showing just how dry the fuel up here is,” Olson said.

The first report of the fire was called into 911 just before 4:30 p.m. Monday. By about 9:30 that night, it had burned about 80 acres in the Robie Creek area.

The fire became more active Tuesday morning, officials said, especially on its southern flank.

On Tuesday night, five engines and a hand crew patrolled for flareups near homes. Additional resources expected to arrive by Wednesday morning include 10 more fire engines, three hand crews and one bulldozer.

Despite Tuesday’s setbacks, crews are optimistic about their wildfire efforts going forward, Department of Lands spokeswoman Emily Anderson said. “Through the night they’re probably going to make some really good progress,” she said.

Patrick Orr: 377-6219, Twitter: @IDS_Orr