Fillers are ingredients added to a colored pencil's core that are not pigments or binders.
Common fillers include minerals such as kaolin clay, calcium carbonate, and talc.
Fillers help manufacturers control the pencil's firmness, texture, consistency, and application.
They can also reduce manufacturing costs by replacing more expensive ingredients.
Fillers are not always a sign of poor quality. They are an important part of every core, but excessive amounts or poorly balanced ingredients can result in weak color payoff, dry or chalky color, and increased shedding.
The amount of filler in a colored pencil makes the pencil more opaque. In the case of pastel pencil sets, filler has been added specifically to reduce translucency and create a pastel looking tint.
Fillers can also add the feeling of drag or feedback, reducing glide. High filler pencils often have that “nails down a chalkboard” feeling.
And in student grade or economy pencils, high filler pencils can shed excessively, creating a lot of dust.
Because pencil formulas are proprietary, the exact types and proportions of fillers are generally unknown.
In the Lab, I’ll usually point out “this feels like it has a lot of filler” mainly because of drag or shedding but I have no way of definitely proving it.