A non-erasable liquid ink for a marking instrument including a colorant and a solvent and having a water content of about 0-15% by weight and a viscosity of about 15 to 4,500 cps. Also disclosed is a marking instrument filled with an ink of such a viscosity or lower in its reservoir....http://www.google.es/patents/US6518329?utm_source=gb-gplus-sharePatente US6518329 - Liquid ink
