A tool for evaluation of fired clay bricks is both valuable and necessary to assess the extent of decay. In the present study, a recently developed methodology was extended to make it more generally valid by increasing the variety of the test material. It was found that high deterioration depth in these non-homogeneously fired clay bricks, requires that both pH and silicon be measured, while for more traditional weathering of calcareous bricks pH measurements are sufficient to determine deterioration depth.