Ziller Inks. Pigment particles are sub-micron size giving you a smooth feel in a variety of tools. Stirring before use is recommended to ensure the suspension of the pigment. Create other colors by mixing. Clean your pens with diluted ammonia & water or glass cleaner as you would any acrylic waterproof ink.  A thickener is also offered should you find that a thicker ink would serve your tools and paper better. They can also be thinned with a few drops of distilled water. Price differences reflect the costs of pigments used to make the inks.