As we all know, the appearance of an ambrotype depends, first of all, on the appearance of its brightest pats, on the level of whiteness of the whites (which decide the contrast). This is one of the key reasons why so many wetplaters use the potassium cyanide fixer (cyanide is at the same time a reductor which means it will enhance the whites slightly).
It is also worth remembering that the choice of fixer is not the only way in which we can influence the level of contrast of our ambrotypes. Excellent results can also be obtained by manipulating the recipe of the developer we use, as can be seen in the image illustrating this note.