4.3 ISO Method
4.3.1 Acid Resistance (SR)
Glass samples with dimensions of 30 x 30 x 2 mm are prepared with all surfaces polished to given specifications. They are hung by platinum wire and placed in a nitric acid solution (pH 0.3) or an acetic acid buffer solution (pH 4.6) at 25°C for the length of times specified (10 minutes, 100 minutes, 16 hours, or 100 hours).
After this treatment, the loss of mass of the sample is determined using an analytical balance. Calculation of the time t0.1 in hours, necessary to etch a surface layer to a depth of 0.1 µm is performed using the following formula:
| t0,1 | : | the time (h) necessary to etch a surface layer to a depth of 0.1 µm |
| te | : | the time (h) for attack 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 |
The calculation is carried out by using the value of the loss of mass that is observed by the minimum test condition (i. e., test solution and test time) for obtaining a loss of mass greater than 1 mg/sample. If the loss of mass is less than 1 mg/sample after 100 hours exposure to pH 0.3, this value shall be accepted.
The acid resistance class SR is obtained by comparison of the pH of the test solution and the time required for the attack to a depth of 0.1 µm (h). Time scales are given in table 6.
| Acid resistance class SR | 1 | 2 | 3 | 4 | 5 | 51 | 52 | 53 | |
| pH of the attacking solution | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 4.6 | 4.6 | 4.6 | 4.6 |
| Time t0.1 needed to etch to a depth of 0.1 µm (h) | >100 | 100~10 | 10~1 | 1~0.1 | <0.1 | >10 | 10~1 | 1~0.1 | <0.1 |
| Table 6 |
In addition, changes in the surface of the sample following this test procedure are qualitatively evaluated with the naked eye. Additional classification numbers are given in table 7.
| Additional Number | Changes in the Surface |
| .0 | No visible changes |
| .1 | Clear, but irregular surface (wavy, pockmarked) |
| .2 | Interference colours (slight selective leaching) |
| .3 | Tenacious thin whitish layer (stronger selective leaching) |
| .4 | Loosely adhering thick layer (surface crust) |
| Table 7 |
For example, it is indicated that the acid resistance class (SR) is SR 3.2 for an optical glass that needs 2 hours for the attack to a depth of 0.1 µm (attacking solution of pH 0.3) and with interference colours after attack.