If you are looking for a consistent, rich color, use a stain as an underglaze. Stains are a type of underglaze that are a fine powder made of grounded metals and ceramic oxides. Stains are best when fired at low temperatures, which preserve the brightness of the color. Many stains contain raw metal oxides, so you may need to have a piece tested for leaching if you intend to use the final product as a surface for food.
Slip trailing is when you use slip, a type of underglaze, to write or draw small details in your design. Slip is available in small bottles and used in the same way you use a marker or a pen. Slips are perfect for signing pieces, personalizing and adding small details.
Some underglazes do not require any clear glaze over the top because they have a shiny color. However, you can always apply a clear glaze over the piece if you would like a shinier finish. Sponge the clear glaze on instead of using a brush. This will keep you from smudging the underglaze design.
You can achieve greater success with underglazes if you have the right tools. There are a wide variety of underglazing tools available, such as underglazing pencils, stamp pads, pens, markers, chalk, and crayons. Use these tools if you are drawing an intricate picture or design, shading areas or marking the identity of a piece. Mix one tablespoon of gum solution with one pint of water in a spray bottle. Lightly spray it on the piece before using the pencils to reduce smudging.