Which method is the first step in the sequence for developing latent prints on porous materials?

The first step in developing latent prints on porous materials often involves the application of iodine. This technique is fundamental because iodine fuming can reveal latent prints through a process called sublimation, where iodine vapor binds to oils and moisture present in the print. The prints appear as a temporary brownish color, making them visible against the background of the porous surface.

When developing prints on porous surfaces, it's important to choose methods that are effective with the substrate and the types of residues left by potential fingerprints. Iodine is particularly suitable for this purpose due to its ability to adhere to organic components typically found in fingerprint residues.

Other methods listed are used for developing latent prints, but they are generally applied later in the process. For instance, silver nitrate can be exceptionally effective but is usually a secondary technique after iodine or ninhydrin, which reacts chemically to reveal fingerprints on porous surfaces. Ninhydrin itself is also commonly used for porous materials, particularly paper, but typically follows iodine in the sequence of development methods. Alcohol is not typically used as a developing technique for latent prints.

In providing context to the developing techniques, iodine represents a foundational first step in the process of revealing latent prints on porous materials, setting the stage for further enhancement methods.