Final answer:
Indirect materials used in a heavily automated plant with a job-order costing system are recorded as a credit to Manufacturing overhead, reflecting the reduction in the direct materials inventory and the incurrence of overhead costs.
Step-by-step explanation:
The use of indirect materials that have been previously purchased in a job-order costing system is recorded as a credit to Manufacturing overhead. In an automated plant, indirect materials are often used in various departments and are not directly traceable to a job, hence they are part of manufacturing overhead costs. When these materials are used, the corresponding accounting entry involves a debit to manufacturing overhead to reflect the incurrence of the overhead cost and a credit to direct materials inventory, indicating that this asset has been reduced by the amount of the indirect materials used.