Nobody who has examined the figure sculpture on the West Front of Wells Cathedral, both before and after conservation, can deny that there has been a dramatic change in its condition. As often happens in conservation, however, technical rationalisation of the change has lagged behind the skill of the craftsman who produces the change. When this investigation began in 1980, there was ample evidence that the lime treatment caused a major improvement in the condition of the decayed Doulting stone. Nobody could say, on the other hand, which part(s) of the treatment had caused the change, how they had worked, nor how long the change would last. The aim of this investigation was, therefore, to provide a technical rationale for the treatment. The investigation also gave the opportunity to compare the performance of the lime poultice with two other cleaning techniques, air abrasion and water washing.