The application of a d.c. potential to kaolinite and halloysite pellets during firing under reduced pressure has been found to produce significant improvements in the hardness, tensile strength, and density of the product. Studies of these properties as a function of voltage, electrolysis time and temperature, and firing atmosphere are reported. X‐Ray and i.r. studies of mullite formation at the electrode faces indicate enhanced crystallinity at the negative electrode. The electrolysis mechanism is thought to involve the removal of residual protons normally present even at high temperatures.