Mac is right about the core having to be laminated or some form of
core designed to eliminate eddy currents.
Another important thing to consider is currents in the transformer. You
don't want them. This is why transformer cores are typically made from
many metal stampings which are glued together. The glue does not conduct
electricity, so electricity can't flow in the core in the direction of the
induced electric field gradient.
Kathy, This is a really neat project and there are so many things you
can do to make this and make it work with minimal headaces. This core
material will seem to be elusive with out a small study to what is
happening. I will be starting a thread soon on designing a transformer
from old parts here real soon. The eddy currents would cause the core
to get hot, and heat destroys the magnetism and that stops the current
from transfering and the primary current then goes real high and then
you have smoke everywhere. Eddy is a bad thing!
You can actually get good core material by scrapping out an old
transformer, and is a great way to recycle materials that get thrown
away needlessly. The Iron in a transformer is the most expensive part,
and salvaging one for a project is great. I would recommend reading a
little on Volt Amp characteristics to make sure you get a core of the
right size for you project. Depending on the load current, (total VA)
something like an old ballast core may work fine too. And here all you
have to do is cut the old wiring away, salvage the laminations and
clean things up, make a new bobbin and wind the transformer to your
own design. There is a LOT of information on the net to help with as
well.
Just a thought here.
Good luck
Harlan