Firefighters faced renewed challenges Saturday at a large Boyle Heights cold storage facility fire, where conditions remain highly complex as Mayor Karen Bass declared a local emergency to support response efforts.The fire was first reported on Wednesday afternoon at the 500,000-square-foot Lineage Big Bear facility at 1400 S. Los Palos Street, which is used to store frozen foods."The best way to describe this is like a giant cooler," said L.A. Fire Chief Jamie Moore, who added that the structure was built with corrugated steel walls filled with dense foam insulation and reinforced interior steel panels. The building also used ammonia in its refrigeration system to maintain extremely low temperatures for frozen food storage, which may have fueled the fire on its initial day of burning."Imagine your refrigerator having a fire. And so, you have the shell of the outside and the shell on the inside," Moore said during a news conference on Saturday afternoon. "What protects everything from the weather, or whatever the temperature is on the outside, is the rubber. ... If you can imagine, that's exactly what's happening here."He said that with the help of water-dropping helicopters, LAFD crews have controlled the fire to approximately half of the building. The unaffected half, however, is filled with food that has begun to thaw as they had to turn off the refrigeration system and remove ammonia from inside. He said that the internal temperature of the building remains at around 45 degrees because of the insulation.Moore said the nature of the materials inside has made the fire difficult to control, as the foam insulation continues to burn slowly once ignited.