What about paintings? Paintings can use different bases, such as canvas, board, or metal. How does your hand affect these? Canvas is a lot like paper-absorbent-so the same rules apply. Bases like board and metal are more resilient to your griminess, but the paint itself is not. Paintings are typically coated in a varnish to help stabilize the paint; when you touch them, your fingerprints chemically alter the varnish in those very spots. These spots begin to turn dark and cloudy and even attract dirt from the surrounding environment. This will ruin the original color of the paint. If the dirt you leave behind doesn't hurt them, your brute force could damage the art as well. In the case of thick impasto paintings, you could accidentally remove a whole chunk of paint. I've even seen some people pick at paintings, as if they are actually trying to remove the paint! The nerve!
But sculptures must be so stable that surely you can touch them, right? If you've read this far, you already know the answer. If the sculpture is made of metal, then your finger can do irreparable damage. The real culprit here is eccrine sweat. Keep in mind that sweat is one of your body's waste products. Human sweat differs from person to person but is always loaded with all kinds of acids like acetic, uric, lactic, and amino. If you have visited the Rock Paper Scissors exhibition or have a background in printmaking, you know that acids corrode and etch metal, a desired effect in printmaking but not in other forms of art that include metal. One of the objects in MOCA's Permanent Collection has some very distinct fingerprints etched permanently into its shiny mirrored finish.
However, it's not always about what your fingers leave on a piece. When you touch anything, you will remove trace elements from the surface. This lightly polishes the area you just touched. This part of a sculpture will not age the same way as the rest of the piece, and can even wear down. Have you ever seen a sculpture with a really smooth spot that is a lighter color than the rest of the piece? That's because people are touching it. In Verona, Italy, a famous sculpture of Juliet, the tragic, star-crossed lover from William Shakespeare's play, had to be removed because the right breast was starting to deteriorate. Somehow, it became a tradition for tourists to touch this breast for good luck. People are weird.
In conclusion, folks: DON'T TOUCH THINGS. You've always known this rule, and now you know why. The risk of you damaging the art is far greater than the reward of touching something. Please keep this in mind the next time you are tempted to put your dirty hand on that pretty piece of art.