What is an evaporation concentration?
Residues of Soap and trace amounts of starch remain in a water soaked piece of linen. As the cloth dries, water wicks its way to the surface to evaporate into the air. As the liquid makes its way to the surface it carries with it dissolved saccharides and starch fractions. As the water evaporates into the atmosphere these chemicals are deposited as a super thin coating on the topmost fibers, the very outermost fibers of the thread. Chemists say this superficial residue of reactive saccharides is on the surface of the cloth.
Ray Rogers explained it this way:
Because the cellulose was not involved in image formation, the color must have formed in impurities on the surfaces of the image fibers. Independent observations have proved that all of the image color resides in a very thin layer on the outside surfaces of colored fibers.
Evaporation concentration can explain the superficial nature of the image and the identical properties of the front and back images. It can also explain the "doubly-superficial" image, i.e., the presence of a superficial image on the back surface of the cloth as reported by Ghiberti and Fanti and Maggiolo.
When a solution evaporates at the surface of a porous solid, dissolved solutes are concentrated at the evaporating surface. The principle is illustrated in the photomicrograph with blue dye. A piece of linen was saturated with a dilute solution of blue dye, and the cloth was dried while laying on a sheet of Teflon. All evaporation occurred at the top surface, and the dye concentrated on that surface. It is obvious that most of the dye deposited on the highest parts of the weave and the upward-pointing fibers of the nap. A sheet of cloth that contained sugars and starches would deposit those impurities at the very topmost part of the weave after washing and drying.