Quite a while back I put some notes on the website about medieval sugar. They're all extracts from cookbooks and such detailing the types of sugar available - the color (white, brown, black) and the form (loaf, cone, powder). My main reason for digging up these references was in part to counter a statement I'd often heard that they didn't have powdered sugar in the middle ages.
Of course I couldn't directly prove that they did have powdered sugar - that seems to be an impossible task. What I could show is that they had something they referred to as powdered sugar, and that given the common kitchen tools and processes of the time it is trivial to grind sugar into a powdered form (interestingly enough, I did find an early 17th century source that says to add starch to powdered sugar to keep it from clumping - which is done modernly). While this isn't positive proof, it does make the case strongly enough to be reasonably certain.
One question about sugar that I couldn't answer this way was about its color. While they did referr to some sugar as being "white", there is no way to know if they really meant white, or if they just meant "light colored". Now to my rescue come a couple of illuminated manuscripts which actually depict sugar!
One comes from a book called Tacuinum Sanitatis - a sort of medieval book on health and wellness. It was originally an 11th century Arabic manuscript, and was translated and copied all over Europe throughout the middle ages. In the picture below from one edition from Italy in the late 14th century, there is a depiction of a sugar merchant selling white chunks.
Pretty conclusive, but it's nice to have a second opinion. So here is a market scene from a different manuscript. In the lower right is a spice merchant with a large white cone of sugar.
Again, it seems pretty clear. The artists chose white paint for the sugar instead of just letting the beige background color to show through. If sugar were commonly brown or black or beige then they would have used those colors instead. Is this positive proof? Well ... no, these could be exceptions, but taken with the textual evidence it becomes very very compelling. Time to update my sugar notes.