Framing fine art is more complex than framing a poster or a certificate. The artist usually will want to play a role in the decision. All will depend on what the artist concludes is the message they wish to communicate with the art frame choice.
Artists often choose plain wooden frames. Fine art requires that the frame does not overwhelm the artwork, that it occupies a back seat, and does not detract from the overall aesthetic experience.
Plain wooden frames, especially sympathetic hardwood frames meet all the criteria. They are mouldings that have stood the test of time. Mouldings are constantly changing in character and shape, busily being in and out of fashion and discontinued for not selling well.
Plain wooden frames are as relevant today as they have been during the last 100 years. Always a safe choice, wood meets all the character of compatibility that most artworks need. Strange but true.