The term 'fat' usually refers to a silky matte glaze that is opaque, thick and pleasant and soft to the touch (sometimes referred to as 'waxy' in texture). The micro surface is wavy rather than flat. The classic fat glaze is a high temperature magnesia matte (made using dolomite and talc). The MgO contributes to a melt where melting glass rivulets run around less viscous islands. The phenomenon is not seen often at lower temperatures since MgO does not melt there. The most likely candidates at lower temperatures are boron and lithia.
Sign up at the home page.