Classify each of these items as direct materials, direct labor, or overhead, and provide the rationale for your classification decision.

• Glue used to attach labels to bottles containing a patented medicine.
• Compressed air used in operating paint sprayers for Student Painters, a company that paints houses and apartments.
• Insurance on a factory building and equipment.
• A production department supervisor's salary.
• Rent on factory machinery.
• Iron ore in a steel mill.
• Oil, gasoline, and grease for forklift trucks in a manufacturing company's warehouse.
• Services of painters in building construction.
• Cutting oils used in machining operations.
• Cost of paper towels in a factory employees' washroom.
• Payroll taxes and fringe benefits related to direct labor.
• The plant electricians' salaries.
• Crude oil to an oil refinery.
• Copy editor's salary in a book publishing company.