I have a friend with her own kiln. Would glazed or unglazed (uncoated)
pieces do well in the tank? Is there a toxin problem? Has anyone ever
heard of this?

Donna

Donna,

Fired ceramics are ok for the aquarium. I would be careful and make
sure that none of the glazes had lead in them as it can leach out into
the water. I am not sure about glazes containing cadmium, cobalt or
various oxides. My guess is that any glazes that can be used for food
containers would be safe.

Donna,

It _might_ depend on the mix in the slip (liquid clay) that was used to
make the ornaments. There might actually be fewer problems if the pieces
were to be glazed.
Some slip is rather manufactured. That is, it is formulated by a
techician to perform well in the molds and is not like the clay that you
and I might dig up in the ground. Sounds funny to have a manufactured
"natural" product like a clay, but that's the way it is. The reason for
the concern...most slips designed for hobby work have an extremely high
percentage of talc in the mix. Depending on the purpose of the finished
product there may be other items added also. HOWEVER...if the clay is a
true porcelain or a stoneware product and is fired to maturity (with an
absorption of say 1 or 2 %, you don't have to worry about anything.
Place your tongue on a fired, unglazed ornament and if it seems to want
to stick a little bit...the piece is not vitirfied (it's absorbing the
moisture off of your tongue)...it your tongue feels like it's just
licked a rock...it fired to maturity.
The glazed pieces would not break down in the water as fast because the
tempeture to make most glazes vitrify are hot enough to bring the clays
to maturity also (or at least hot enough to make the clay hard enough
not to preak down in a few months or a year). Also glazes would protect
the piece from erosion from chemicals in the water. More...glazes can be
prettier if that matters in your tanks.
Speaking about chemicals. Remember that if you treat the water,
non-matured clay piece will more that likely act as a sponge and some of
the chemical will leach into the clay. You will want to clean the clay
piece of any chemicals you do not want to reintroduce to the water.
Several years ago, I worked on a bunch of modular clay products for
breeding caves and decorative retaining walls for aquariums. Made
pottery and related ceramic work for the past 25 years...sorry if the
answer didn't answer and the low tech got high...it's a judgement call
on whether to use the ornaments in your particular situation.

Jim