Stressed materials show characteristic patterns of dark lines under crossed polars, and the first blanks show no signs of stress. The third problem to afflict mirror blanks is bubbles. The blanks produced to date have submillimeter bubbles in varying quantities. Several experienced mirror makers have inspected the blanks and believe they are suitable for use, and others want to grind one before deciding. Most of the bubbles are near the top of the blank, and the bottom is the side used to make a reflecting surface.
Efforts are underway to minimize bubbles in future blanks.
There are no bubbles in the source Pyrex, they arise from the air between the stacked rods. Steve is now using the largest available rods, 1.5 inches in diameter. Another possible source material is Schott Borofloat, a glass somewhat similar to Pyrex in sheet form, up to 20 mm thick. A blank melted from this would be bubble free at least 20 mm from the surface. This material will be used in some of the next blanks.