PD has also been useful in developing latent footwear impressions on porous substrates. Thus, PD is useful for detecting many latent prints that cannot be found as a result of other processes. In fact, it has developed identifiable prints on test materials that were last touched 30 years before being processed. PD is a silver-based solution that reacts with lipids, which are not soluble in water. It is generally used after the ninhydrin process and instead of the conventional silver nitrate method of latent print development. Using PD requires more labor, time, and preciseness than other processes, but familiarity with the process reduces these disadvantages over time. Explanations of the formulation and use of PD, photographs, addresses from which to obtain further information, and 19 reference notes.