Kaolin and calcined kaolin: which one a coating line actually needs

Coating lines ask for kaolin and receive two different materials under that word. Hydrous kaolin is china clay as mined and processed, still carrying its combined water. Calcined kaolin is that clay after a kiln has driven the water off. They do not interchange just because both are white.

What raw kaolin is for

Kaolin clay is the grade for ceramic bodies that need plasticity, and for paper and paint coats that can hit shade with a hydrous clay. Whiteness, residue, and a low impurity level are the checks. A forming plant also cares that the clay is plastic. A coating plant often does not.

If the hydrous clay already meets brightness and the film looks right, calcined kaolin is a more expensive answer to a problem you do not have.

What calcination adds

Calcined kaolin is the grade for paints, paper coating, cables, rubber, and plastics when raw kaolin is short on brightness, opacity, or hardness. The kiln changes the particle. Loss on ignition drops. The clay is no longer a plastic body clay, so do not drop it into a tile recipe and expect the same forming.

Opacity is the usual reason a paper or paint line makes the switch. Hardness is the usual reason a compound or a coating wants the calcined particle to stay put in the film. Brightness follows from the heat treatment, and it is compared with your own standard, not with a number printed without a method.

How to choose

Write down the job before you write down the mineral.

• The clay has to form a ceramic body, or a hydrous clay already holds your shade. Start with kaolin.
• The film or the sheet needs more opacity, more brightness, or a harder particle than raw kaolin gives. Start with calcined kaolin.
• You are rebuilding a whole ceramic body, shrinkage included. That is a blended clay conversation, not a coating-clay conversation.

Ask for a sample against the standard you already approved. Typical notes on the product pages describe the grade family. They are not a certificate of analysis, and they will not settle a shade argument. The trial will.

When you request a quote, name the line (paper, paint, cable, rubber, or ceramic), the mesh or residue you run, and whether the last clay failed on shade, opacity, or forming. That is enough to stop a hydrous sample arriving where a calcined one was required.