Mottle is a photographic processing fault that manifests as irregular, randomly distributed variations in the density or tone of a developed print or film, producing a blotchy, patchy, or unevenly textured appearance across areas of the image that should ideally be smooth and uniform in tone. Rather than the even, consistent density that correct processing produces across continuous tonal areas, a mottled image exhibits an uneven, cloud like pattern of slightly lighter and darker patches that disrupts the smooth gradation of tones and gives the surface of the print or film a visually disturbing, inconsistent quality.
Mottle can arise from a number of different causes during the photographic processing sequence, most of which relate to inadequate or uneven distribution of processing chemistry over the surface of the film or paper during development. Insufficient agitation during development is one of the most common causes, as without regular and thorough agitation, localised exhaustion of the developer solution occurs adjacent to areas of high silver density, while fresh developer accumulates unevenly in other areas, resulting in development proceeding at different rates across the surface of the material and producing uneven density. Similarly, allowing the developing material to remain stationary in the developer for extended periods without agitation encourages the formation of development byproduct gradients within the solution that further contribute to uneven development and mottling.
Other processing factors that can contribute to mottle include the use of exhausted or incorrectly diluted chemistry, inadequate pre-soaking of the material before development, uneven temperature distribution within the processing solution, the presence of air bubbles trapped against the surface of the film or paper at the beginning of development, and the use of processing trays or tanks that do not allow chemistry to circulate freely and evenly over the full surface of the material. In drum and rotary processing systems, mottle can result from uneven coating of the chemistry over the material surface during the initial loading of the drum.
Prevention of mottle relies primarily on ensuring correct and consistent agitation throughout the development stage, using fresh chemistry at the correct dilution and temperature, and employing processing equipment that promotes even distribution of solutions over the full surface of the material being processed. In cases where mottle has already occurred, the fault is generally irrecoverable in the affected print or negative, making correct processing technique from the outset the only reliable safeguard against this particular processing defect.