Which process could fade unless properly fixed when using iodine vapour?

The process that could fade unless properly fixed when using iodine vapor is iodine vapor development. Iodine vapour is used in latent print development primarily because it reacts with organic compounds in the print residue, producing a visible brownish-yellow color. However, this development is not permanent; the iodine prints can fade as the iodine sublimates back into the atmosphere over time.

To preserve the developed prints for further examination and documentation, it is essential to fix them properly. Fixation usually involves treating the developed prints with a solution that stabilizes the color, preventing further sublimation and ensuring that the prints remain visible for sufficient time for analysis.

The other processes mentioned either do not specifically involve iodine vapor or are permanent methods of development. For instance, ninhydrin treatment develops prints through chemistry, resulting in a stable color change, while fluorescence examination and physical developer application have their own distinct methodologies and outcomes that do not rely on the temporary nature of iodine vapour.