The corrosions are not of a uniform character throughout the whole of the stack; in some places they are porcellaneous and flaky, are firm to handle, do not break up, and give the best quality of white lead; in other parts of the stack the corrosions are soft, easily crumble to a fine powder or dust when handled, and do not give a good quality of white lead. In some places the lead may be discoloured owing to a variety of causes, such as the presence of tarry matter in the acid (especially when crude pyroligneous acid is used), by droppings of coloured water from the layer of tan on to the lead, &c