It has been awhile since I have done any copy work but I would buy several rolls of my favorite slide film and set the object to be photographed either on a table or floor depending on how large the object is. I am assuming that this a painting or some version of 2 dementional art. It that is the case, I would place hot lights at 45 degrees of the object and then I would use a gray card to "guessitmate" the exposure. After that I would bracket exposures on both sides of the gray card exposure. then move to the next piece of art work. Have the film processed and pick the best exposure