The black mirror is a piece of glass coated on the reverse with black acrylic paint and, though it looks goofy when you use it (you look up to see an upside down version of the subject reflected in the glass), it is incredible at separating values in a subject so that you can understand the form of what you're seeing. We used it in the workshop to clearly discern the value changes
along our model's arm, from low light up to high light on the top of her shoulders.
Since then, I've discovered it's amazing at showing the value changes across subtle shifts such as on a white shirt worn under average light. While it's easy to see light and shadow in bright sunlight, it's much harder under regular indoor fluorescent panels like the ones in the studio where I teach. I'm enjoying examining the world through this new tool. Thank you, Coral!
If you've got any tools to alter your view of your painting or subject, I'd love to hear about them. A painter can never have enough ways to freshen her eyes.