Glass samples were delivered polished to the specified polishing conditions. Each glass sample was weighed using a analytical balance. One sample of each glass type were hung by platinum wire into 0.01 mol/liter sodium hydroxide at 50 deg C for the length of times specified (15 minutes, 1 hour, 4 hours or 16 hours). After this treatment, the samples were weighed again and the loss of the mass of the sample were calculated. Calculation of the time t0.1 in hours, necessary to etch a surface layer a depth of 0.1 um was done using the following formula:
t0.1 = (tedS)/[(m1-m2)*100]
where:
t0.1 = the time (min) necessary to etch a surface layer to a depth of 0.1 m
te = the time (min) for attach in the experiment
d = the specific gravity of the sample
S = the surface area (cm2) of the sample
m1 = the mass (mg) of the sample before the test
m2 = the mass (mg) of the sample after the test
Phosphate Resistance Class PR 1 2 3 4
Time t0.1 needed to etch to a depth of 0.1um/min >240 240~60 60~15 <15
In addition, changes in the surface of the sample following the treatment are qualitatively evaluated with the naked eye. Additional classification numbers are given according to the second table.
Additional Number Changes in the Surface
.0 No visible changes
.1 Clear, but irregular surface (wavy, pockmarked)
.2 Interference colors (slight selective leaching)
.3 Tenacious thin whitish layer (stronger selective)
.4 Loosely adhering thick layer (surface crust)
Six samples did not exceed the 1 mg weight loss. However, all eight samples would be classified as AR1.0 as the two samples that did loose over the 1 mg did so only after 16 hours of soaking. None of the samples had any visible abnormalities.