Last year I discovered Gordon Matta-Clark's Photo-Fry at the Whitney Museum. In 1969, he cooked Polaroids of Christmas trees in vegetable oil. As soon as they boiled, he sprinkled gold leaf onto the emulsion. After they were exhibited, he mailed them as Christmas cards to friends and art world professionals.
In Object to Be Destroyed: The Work of Gordon Matta-Clark, Pamela M. Lee writes:
"In late 1969, a small beige package, no bigger than a cigar box, arrived at the Smithson/Holt loft on Greenwich Street.... The upper half of the box bore a virtually illegible piece of a Christmas tree, a Polaroid whose cracked and welted surface took on the appearance of charred skin. Its excrescent finish served to distort and nearly efface the image, so much so that its photographic properties were rendered irrelevant in considering the object."
Matta-Clark's battered Polaroids come to mind when I receive a postcard where the artist is intentionally exploring destruction or when their image is too fragile and must be contained in an envelope. I also consider it a model of influence specifically with its incorporation of smell, often overlooked in mail art.