Assessment of tin surface

You might want to look up some old metallography books and look for grain size & shape assessment techniques. One I recall was superimposing a transparent grid onto a photomicrograph of a polished & etched section. With a reticulated eyepiece one could do much the same on a metallographic microscope. Anyway, the idea was to count the number of intersections that a grid line made with the underlying grain boundaries. Statistical routines were derived for reducing the gridline-grain boundary intersection data to grain size & shape. One had to count intersections in two or more directions (by rotating the grid sometimes). If you think hard on this you will be able to see how it can be made nearly bias-free. Good luck on hunting out the literature.