Firefighters quickly doused a blaze at a Landmark Square building Wednesday morning and prevented it from spreading. Deputy Chief Tom Gloersen said some welders were working in Building 2 on Atlantic Street behind Blackstones Steakhouse around 7:30 a.m. when a hot spark apparently dropped, causing some insulation to catch on fire. The welders used fire extinguishers and nearly had the blaze put out by the time firefighters arrived. Gloersen said fire marshals are investigating the cause of the blaze.