The disclosure describes a method by which the Tzc of a silica-titania glass article, for example, a EUVL mirror substrate, can be tuned to within a specification range by means of a selected final anneal that shifts Tzc of the article or substrate to the desired Tzc value. In addition, since different mirrors in a set can be specified at different values of Tzc, this process can be on used glass samples or pieces from a single glass boule to make parts with different Tzc values, thus reducing the number of separate boules required to fill an order.