Huntington firefighters are investigating the cause of a massive Christmas Day fire in a vacant former chemical plant that was recently declared a public nuisance. They also expect to be called back when hot spots buried under tons of rubble flare up later in the week. Fire investigators photographed the rubble of the former Flint Pigments plant Thursday looking for the cause of a massive fire that crumbled the large industrial building.The city was already concerned about poor conditions at the former chemical plant along the busy 5th Avenue, therefore, on Monday it was labeled a public nuisance by the Huntington City Council. Because of the possibility of asbestos and chemical residue from the old operation here, debris caused by the fire will have to be treated as hazardous waste.