Tintypes and ambrotypes are one-off, hand-crafted photographs on metal (originally tin, now more often aluminum) or glass made using the wet-plate collodion process. They are beautiful objects that will last for many generations, true heirlooms worthy of being cherished. They have an almost three-dimensional feel to them that is difficult to reproduce digitally. When made on glass they can be even more luminous, as the image can be created on what will become the reverse and then be viewed through the glass, adding even more dimensionality.
Invented in 1851, the wet-collodion process was popular for several decades. The heart of the procedure involves pouring chemicals onto a plate in order to make it sensitive to light--in essence, turning it into a piece of film (which had yet to be invented). The plate then has to be exposed and developed before it dries (hence the "wet" portion of the name; collodion is simply one of the chemicals used to coat the plate). The entire procedure is dependant on many factors including temperature, the behavior of one's chemicals that day, and how a given plate is poured. The final product often has small flaws. Many consider that this adds character. Certainly, no two are ever exactly alike.
Wet-plate photography requires a lot of light, so is either done outside in open shade during the day, or in a studio with strobes and/or continuous lighting. I work both ways, although these days I use the strobe much more often, which allows an instantaneous exposure, reducing the risk of blur. Each shot takes approximately ten minutes to prepare and execute. The result is viewable in a minute or two, right after the plate is developed in the darkroom.