Copyright Laws and the concept of Fair Use.
Many artists and makers who use found materials in their work are concerned about copyright infringement. Legitimate use of found materials is sanctioned under the concept of Fair Use within the large body of Copyright Laws.
There are several key provisions to protect artistic work under the umbrella of Fair Use.
Perhaps the most useful and easily understood principal of Fair Use is whether the artist's use will be confused with the original copyrighted brand name or product.
For example, I used tin cans from SLIM FAST products in a candy box titled, The Dilemma of Desire. There is little chance that this work would confuse the consumer or hurt the brand name SLIM FAST.
Fundamentally, Fair Use allows and encourages "creativity for the enrichment of the general public," but does not permit use that would supersede the original purpose of the copyrighted materials.
In legalese, the use must be interpreted as transformative, not merely derivative. In plain English, the artist or maker must change or modify the original enough to create something clearly new and different.
So what is enough difference? Obviously, copying a book, e-book, tutorial instructions or artwork is not O.K.. Sometimes the fine line may be somewhat gray. Jeff Koons lost his claim of Fair Use when he made a three dimensional sculpture based on another artist's photograph. In this case, he sent the original photograph to the manufacturer to construct the sculpture. In essence, he made a near copy of the original.
So as long as your work transforms the original content or intent to create something new -- whether you add, subtract, modify, or distort -- you're probably OK. Just don't copy someone else's work and call it yours.