| Factors | Amount of initial H2SO4 (in g/l): X1 | Amount of BaCO3 (in g/l): X2 | Temperature (in ˚C): X3 | Time (in mn): X4 |
| Level (−1) | 17.0 | 15 | 40 | 25 |
| Level (+1) | 29.4 | 40 | 74 | 60 |