The
codified levels of the variables (temperature–X1 and time–X2)
and their real values are in Table 1. The experimental design
was composed of 13 assays; four factorial (combination of
levels
−1 and +1), four axial (a variable at the level
±˛ and
another one at zero) and five repetitions in the central point
(two variables at level zero). Experiments in the center of the
design were performed to estimate possible pure error.