Even if you know the cinnabar and lead, if you don't know the firing process it is of no use. It all depends on the power of practical application; if you deviate even slightly you won't crystallize the elixir.
This passage establishes the firing process as the indispensable operative knowledge in inner alchemy, superior even to knowledge of the raw materials, and frames deviation from its precise timing as fatal to the work.
, The Taoist I Ching, 1986thesis