I don't think you can tell for sure if a nail will be a good magnet or
not just by looking at it. Nails are made out of many different kinds
of materials, and some are more magnetic than others. There are some
important clues you can get from a nail's appearance that might help
you spot a good candidate for making a magnet:
1) Is the nail made out of plastic? You can usually identify
plastic nails by looking at them -- in general, they won't be magnetic.
But -- some nails may have plastic coatings on them, or paint that may
make them look like they're made out of plastic.
2) Some nails are made out of copper, and others are made out of
aluminum. These also make lousy magnets. Aluminum nails are also weak
and hard to use for other reasons, but they are fairly common. Copper
nails may have that reddish tinge to them like pennies, or they may
have a green or black oxide to them. Aluminum nails usually have a
dull, gray finish, and are easiest to identify by their weight.
3) Stainless steel nails will also make disappointing magnets.
Unfortunately these will look like other steel nails, as long as the
other nails are kept clean and shiny. The stainless steel ones won't
rust. Maybe a few flakes of rust are an indication that the nail will
make a good magnet. But this eliminates new, shiny nails. Many nails
that aren't made out of stainless steel will have coatings of various
colors on them (dull browns, greens, or blues) that prevent rusting.
These might be good candidates, but I cannot guarantee that every such
nail will make a good magnet!
The best way to tell, of course, is to try to pick them up with a
magnet. The really good ones will be hard to pull off of your test
(published on 10/22/2007)