Egg shell porcelain

[peterp]

Thin porcelain is not necessary eggshell porcelain. While both may be translucent the thickness still differs considrably. Even if you have never seen genuine eggshell porcelain before, you will recognize it as such as soon as you see it. It looks fragile as if the pressure of a thumb could easily break it.
If you are in doubt whether a porcelain piece is eggshell or not, then it likely isn't.

How eggshell porcelain is made:
Normally you have clay body and glaze on the outside and inside. With eggshell porcelain initially only one side is glazed and then the piece is fired. Afterwards, the clay body is ground down bit by bit on the unglazed side, until almost no clay body remains. Then the other side is glazed and the item is fired again.