Conventional techniques of process control are inadequate for the production of modern hard surface photomasks because they do not accommodate all the variables and because, even utilizing the tightest controls practical, the total variability of the process exceeds the allowable variability of the final product. A system of interactive process control has been developed and put into operation. This system uses the characteristics of the outgoing product in a negative feed-back loop to keep the process stabilized and properly adjusted. A mathematical model of the process is used to convert product data into changes in process parameters. A computerized reporting system keeps the process engineer informed of changes in mask quality and yield.