Working on a creative project up close, hour after hour, can make it difficult to get a fresh sense of how it looks - particularly the values and shapes. To solve this problem, it is possible to use various techniques to gain a new look at the art.  Perhaps the most common technique it to simply squint the eyes, or maybe focus the eyes beyond the art work.  Of course this may be wearing on the eyes after a while, so other approaches have been tried.  For example, a magnifying glass can be framed with cardboard, and used as a viewer. But, of course the image is seriously distorted.  Then there is the Claude Lorrain black mirror, used in the 18th and 19th centuries, and now sold in some metaphysical stores. The image in this device is very dark.

Another solution is to use a regular household hand-held mirrior. The mirror can be painted lightly with a transparent oil paint such as phthalo green, or any of the quinacridones or synthetic earth colors.  The paint color can be wiped using a cotton ball, and it will leave transparent blur lines on the glass. When you look at the art work through the mirror, you will be able to see the values and shapes in a way that makes them unfamiliar enough to discern their quality, without being so occluded. If needed, the mirror can be wiped off, and used again for another purpose!