We regularly see questionable information online concerning how to mosaic and how to do other art and craft projects. Mostly this information is questionable because it was written without regard to durability or how the project could have been used as an opportunity to make real art (personal, unique) instead of making clones of something already mass produced. Sometimes we even see instructions that are potentially dangerous or even likely to be dangerous. The recent fad of tiling floors with copper pennies may be an example of this, at least in certain situations.
In micro amounts, copper is a nutrient and is found in all plant and animal life. On the other hand, excessive exposure to copper can be toxic, particularly if a person has a genetic predisposition to a condition called Wilson’s Disease, in which the liver is unable to remove excess copper from the body.
Sure we all handle copper pennies every day, but this is different from lining a living space with them, especially on a floor where foot traffic will abrade the pennies and household cleaners will accelerate the forming of copper oxides and other toxic compounds.
Different websites mention coating the pennies with urethanes and other sealers prior to use. That is probably good in theory, but I’m not confident that the thin layer of sealant will isolate the copper for very long under normal use conditions. Keep in mind that copper is fairly soft and fairly reactive. What is acceptable for a wall or a mosaic sculpture is often inadequate for a floor.