Not an ink but an additive to be used to darken the original ink when a drawing needs that extra punch. Some artists will add black india ink to darken the color of Walnut Ink. This will work but you will sacrifice the unique watercolor characteristics of the ink and lose the ability to lift color or lighten areas. Made with the same binder as the original so it can be added or mixed with Tom Norton's Original Ink« without changing its working properties. It can also be used straight from the bottle. However, it is so heavily pigmented it will behave more like a Sumi ink and stain the paper and not lift as easily as the original.