An underglaze pencil is ceramic colour in a pencil form, and it does one thing no brush does easily: it draws a fine, controlled line on bisque with the same hand movement as drawing on paper. For lettering, sketching and detail it is the most direct decorating tool available.
How they work
The pencil core is compressed underglaze with a binder. Drawn on a slightly absorbent bisque surface it leaves a line of ceramic colour that survives the firing exactly as brushed underglaze does. Because you can rest your hand and see the line as you make it, the result is far closer to drawing than to painting, which suits anyone who is better with a pencil than a brush.
Preparing the surface
Bisque that is too smooth gives a faint line and bisque that is too coarse eats the pencil. A light rub with fine wet-and-dry, done damp rather than dry, gives a surface with enough tooth to take a solid line. Wipe the dust away wet, because dry ceramic dust is exactly the material this site keeps telling you not to breathe.
Under a clear glaze
A pencil line softens slightly under a clear glaze and darkens a little, so a drawing that looks faint on the bisque often reads correctly after firing. Dipping the clear rather than brushing it avoids dragging the line, which is the one way this technique goes wrong. Test the whole sequence on a tile: pencil, clear, fire.
Where they are worth buying
Lettering, signing work, fine linear drawing and touching up gaps in brushed decoration. They are inexpensive, they last a long time because very little material transfers, and they are one of the few decorating tools that produce a recognisably different result rather than a slightly different one.
Questions people ask about underglaze
Do underglaze pencils need a clear glaze over them?
For functional or handled ware, yes. Left bare the line is matt and can be rubbed.
Can I use them on greenware?
They work best on bisque, which has the tooth to take a solid line and is robust enough to draw on without damage.
Why does my pencil line disappear after glazing?
Usually the line was too faint on a very smooth bisque, or it was dragged by brushing the clear glaze on. Dipping the clear avoids the second.