How to Make an Underglaze Pencil

To make underglaze pencils and pastels, use a porcelain-type slip with 50 percent white firing ball clay or plastic kaolin. For dry strength in the green state, 3 percent macaloid or 5 percent bentonite should be added.

The materials, including colorants, should be dry sieved through an 80-mesh screen to ensure thorough blending. For color, you can use mineral oxides, carbonates, and prepared stains. A variety of combinations will produce a wide range of colors, although it’s important to select colorants that won’t burn out at high temperatures; not many will, but cadmium/selenium and potassium dichromate are likely to do so. The amount of colorant can be up to 15 percent. More than that will cause loss of plasticity in the raw state, making it difficult to form the pencils. The more colorant used, the more intense the color.

Mix the dry materials with approximately 45 percent water, to which 1 percent of sodium silicate per 100 grams of dry material mix has been added. This will slightly deflocculate the slip, giving additional green strength while also intensifying some of the colorants.

Form the pencils by drying the colored slip to a plastic state, and then either rolling out coils or extruding lengths of the desired thickness. These then can be left as pencil lengths or cut into shorter 5 - 6 cm lengths. When dry, fire the pencils to between 800°C and 950°C, depending on the desired hardness. A lower firing will produce softer “lead”; higher firing, harder “lead”. The short lengths can be placed in a claw grip drafting pencil (the Koh-I-Noor No. 48 drafting pencil can hold leads up to 5 - 6 mm in diameter.

Pastels normally are used from the greenware state and are not prefired unless they disintegrate with use.

excerpt courtesy of Robin Hopper