Many coins look nicer on black, but white is easier to use, trust me.
I like white because when you place it in a document it means the coin stands independent on the white page. Though you will often discover that some backgrounds you thought were white were not completely white and editing is required.
Black backgrounds can be very difficult to convert to white because there are often black or near black shadows on the edges of the coin, in cracks etc. that are
still part of the coin. When you convert black to white these parts go from being a dark
part of the coin to background - in other words you lose small bites out of the edges of the coins. As very few coins (maybe some photos of AR coins) have white parts along the edges the phenomenon does not exist when you convert a white background to black or any other color.
I spent a great deal of time doing this for my book. A not insignificant number of the images that various firms kindly let me use
had backgrounds the required editing.
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