The photos below illustrate the how firing brings out the full color and luster of the under-glaze paints.  My little "Tropical Gecko" A very cool little dude!  Before & after firing.

There are several methods of painting ceramic tile murals. Each method having its own "feel" as well as advantages and disadvantages to the artist. There is not necessarily a "better" way, it is more the artist's preference. The following is the method I prefer.

The unfinished ceramic tiles or “bisque” starts out as “slip” (liquefied clay) which is poured into a plaster mold.  The mold absorbs the moisture from the slip, and the clay begins to adhere to the mold and take shape.  After the desired thickness is achieved, the excess slip is poured out and the piece is allowed to set.  The mold is opened to reveal the next stage: “greenware”.   

The greenware is cleaned to remove seams and small blemishes.  After sufficient drying time, the greenware is placed in a kiln and is fired at a temperature of 1830F to 1940F for 4 to 6 hours. The kiln is then allowed to naturally cool to room temperature; this may take approximately another 12 hours.  After cooling, the kiln is opened and the bisque is removed.  

The bisque in now ready for painting.  I use a commercially made bisque to ensure consistent quality and dimensional accuracy.  I prefer to use under-glaze paints, which is like painting with liquefied clay. It takes a considerable amount of practice and experimentation to get the desired results, because unlike other types of paints, under-glazes look vastly different before and after firing, as my gecko friend above illustrates.  And I may use a combination of 4 or 5 different blues and whites when painting ocean scenes. After the mural is completely dry the tiles are re-fired to bring out the full color of the under-glaze paints. The mural is then glazed with a clear glaze and refired to seal and protect the tiles.  These tile are now the same size and equally as commercial tiles you would buy from a tile store.  Prior to firing each tile is numbered and a schematic is provided so the installer will know the installation sequence.