Glaze is the coloured, often patterned protective coating applied to the outside and the tobacco well of clay-based bowls (earthenware, clay, ceramic).
Unglazed bowls (earthenware and clay)
They take on the flavour: Even after kiln firing, unglazed bowls soak up tobacco juice over time through their microscopic pores. That is why an unglazed bowl should always be used for the same flavour (only double apple, say, or only mint); switch flavours and the aromas mix and performance drops.
Bowl sweating: Water and juice absorbed during cleaning stay inside the bowl, then expand and seep out when it heats up. Hard bowls fired at high temperatures sweat less. It looks messy, but if you always smoke the same flavour it actually makes the flavour more intense.
Glazed bowls (ceramic and stone)
Versatile: The glaze stops the juice from being absorbed, so you can try different tobaccos and flavours in the same bowl without any trouble.
Easy cleaning: The smooth surface makes cleaning after a session quick and easy.
The design difference: Glazed Phunnel bowls absorb almost no juice at all. In glazed Killer bowls the juice can cling to the inner surface as it runs through the holes, so a slight ghost of the previous flavour may linger; even so, they sweat little or not at all.