The aim of this study is to determine the impact of wood biomass ash (WBA) as a substitute for part of the clay on the properties of bricks and to validate laboratory tests in collaboration with brick manufacturers to ensure consistent product quality at industrial production level. Six samples of WBA from different locations in the Republic of Croatia were taken for testing purposes. This work examines the properties of bricks in which WBA partially replaces clay, employing the hand moulding method for brick preparation. Bricks are shaped into both disc and prism forms, allowing us to assess the impact of WBA on various properties such as shrinkage during drying and firing, compressive strength, and water resistance properties, including water absorption by boiling, absorption, and saturation coefficient. The results indicate that even a minimal substitution of only 3% of the clay with WBA significantly influences the water resistance and shrinkage properties. The sample preparation process significantly influences the properties of the bricks, which underlines the need to clearly outline how the samples were fabricated.