Tom Norton’s Darkening Medium® is an additive to be used to darken the original ink. Some artists will add black India ink to darken the colour of Walnut Ink. This will work but you will sacrifice the unique watercolour characteristics of the ink and lose the ability to lift colour or lighten areas.
It is very heavily pigmented and made with the same binder as the original so it can be added or mixed with Tom Norton’s Walnut Drawing 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.