Rotring Technical Pen Ink is formulated to work with technical pens to provide maximum opacity black line work.
To achieve this opacity the pigment, namely soot, is machined to the finest possible tolerances.
Rotring Technical Pen Ink is a suspension type – particles of pigment suspended in a liquid – as compared to a solution – dyes dissolved in a liquid.
The liquid in this case contains glue and shellac which combined with the soot is a recipe for clogging a pen unless care is taken to keep it flowing.
Regular cleaning of the pen, especially the breathing helix, will help maintain flow.
Old ink should never be used as it separates with time. Shaking it to mix it will not work and will only produce grey lines until the pen clogs.
Start checking for settling of the pigment if your ink is more than 2 years old.
The Rotring brand is the fastest seller and will therefore be the freshest type you can buy.