There is quite a variety of ceramic materials and manufacturing techniques
available in the market.
Firing temperatures can range as low as 1000ºF to as high as 2500ºF in commercial ware.
Clay bodies can be simply dug from a local clay deposit and fired in a primitive
straw-fueled mud kiln (as some mexican roof tiles), or formulated with analytical
grade ingredients in clean labs and fired with computer-controlled electric furnaces.
What makes a fired ceramic product is when a ceramic object is heated
to above 1030ºF (quartz inversion) the quartz crystals change from an alpha crystal structure to a
beta crystal structure. The object sort of fuses and can't be dissolved with water.
The higher a ceramic object is heated (up until its melting point when it turns to
a glass state) the harder and more chemically impervious it becomes.
Terracotta pots are fired to the low end of the scale. They have a high iron
content (hence red color) that lets them be relatively solid at a low firing temp.
It has a porous nature that can harbor algaes and bacteria. Terracotta should
be sterilized between aquarium applications with a hot water and bleach soak
(then properly dechlorinating).
Ceramic bio-filter media is low-fired to remain porous enhancing bacterial colonization.
Porcelains are a low-iron ceramic that is fired much higher. When fired, porcelain
is very strong, fairly chemically inert, and has a low moisture absorption. They are
generally safe for reef use.
Glazes are another issue. While they are composed of similar ingredients as a
ceramic body, they are formulated to melt and adhere at the temperature which
the ceramic body is fired. But the formulas for a glaze are altered to accomodate
color and texture with the additions of metals.
One of my favorite glazes is a matte gunmetal called pewter. It is a saturated
copper glazed that is not food safe; it leaches copper when it comes in contact
with acidic foods. Although my reef is akaline, I would use a product with this
glaze.
Similar popular glazes have a saturated barium content and leaching is a
concern. Cadmium, strontium, iron, cobalt, titanium, and lead are commonly
used metals in ceramics.
I think that unglazed terracotta is usable. Most porcelains and stonewares
should be ok. Fancy metallic finishes or ultrabright colors may call for more research.
You could probably soak the questionable ceramic in a bucket of tank water for
a week and test for the usual suspects. But I feel that If in doubt, leave it out.