Terra sigillata is a very smooth, lustrous coating of clay which resembles a glaze and is virtually waterproof. The name means "sealed earth" and has been used to refer to the Classical Greek Attic black-figure and red-figure painted pottery.
These days, the name terra sigillata is used to refer to an especially fine coating of clay applied to a ceramic piece.
For centuries the secret of making terra sigillata was lost and only in the middle of this century was the true nature of this material, the technique of its creation and use rediscovered.
The silkiness and shine of terra sigilatta is due to the plate like shape of the clay particles and the use of only the smallest particles. Polishing this surface with your hand or a soft cloth lines up all the clay 'plates' and gives the surface its shine.
Most terra-sigillatas are made by a process of levigation in water which allows the larger particles to settle to the bottom, leaving the very finest, sub micron sized particles in suspension. These very fine particles are siphoned off and become the terra-sigillata.