As the name implies, these are not glazes. They are decorative colors that need a clear glaze over them to provide the sealing and durability qualities only a glaze with its glass can provide. Technically, they are any coloring oxide or combination of oxides (such as stains) use alone or mixed with other media such as gum & clay, that are fired onto green or bisque ware. Practically speaking, one could call glazes the glass coating and underglazes a colored clay coating. This definition would somewhat include slips and engobes as underglazes. Additionally, colored pencils and chalks and terrasigllata would fall under this broad underglazes definition.
The reason for using underglazes is to achieve different effects than can readily be achieved with a glaze. Since underglazes do not melt, becoming fluid and moving on the ware, more precise designs can be applied and two colors can be butted next to one another without losing definition. Traditionally, they are formulated to be applied to greenware. The high clay content and lack of fluxing or melt means there is high shrinkage comparable to greenware and little binding or fusing to hold the underglaze on the ware. Thus, without any additions, they are likely to crack off in the firing, or before, if applied to bisque, especially if a thick coat is applied. Decorating on fragile greenware risks breakage of the ware through handling, however. Be sure to read the labels of any commercially produced underglaze to determine whether it can be used on bisque. If not, the addition of flux will help. A simple addition of 2 to 3 tablespoons of clear low fire glaze to a pint of underglaze will generally do.
Note that most underglazes can be used as majolica-like decorating colors painted over an unfired glaze. In addition, commercial underglazes for bisque and properly fluxed others can be used as traditional over glazes, applied to an already fired glaze and refired. This works best on flat horizontal surfaces, such as tile. To get underglazes to 'stick' to a vertical glazed surface, the following may help:: 1) warm the ware, 2) add gum to the underglaze, 3) replace some or all of the water in the underglaze with alcohol. Test first, of course.
Application to Greenware:
Underglazes should be applied on a well-sponged, dust-free surface. Use a soft, good quality sable or camel hair brush. When trying to achieve a water color effect, load brush with color and apply underglaze with one continuous stroke. For over-all solid coverage, it is important to lay down or scrub in first coat of underglaze. This will act as a foundation for the subsequent 2 or 3 coats, each coat should then be applied diagonally across the previous coat. Bisque fire Cone 05-04.
Application of Bisque:
Basically the same instructions given for greenware may be followed for use of underglaze on bisque. Care must be taken that bisque is not too porous as it will have a tendency to grab or absorb too much color and thus give you a build-up which could peel or chip off. Due to the porosity of bisque, you may find it helpful to dampen the bisque before decorating, however, best results are obtained on greenware. The addition of a small amount of clear glaze to your underglaze can help fix an underglaze which might pop off during or after firing.
Also See Raw Materials- Stains