Viscosity builders, by definition, refer to the resistance of a fluid to flow. As viscosity increases, the thickness of the drilling fluid, or its resistance to flow, increases as well. While the viscosity of water is 26 sec/liter, the ideal viscosity for drilling mud typically ranges between 35-45 sec/liter. The viscosity of the mud is measured in the field using a Marsh Funnel and a viscosity cup. Excessive viscosity within the well can raise pump pressure and cause drill cuttings to pack around the drill bit. This increases the risk of formation damage. On the other hand, insufficient viscosity prevents the mud from carrying the drill cuttings, which leads to accumulation at the bottom of the well, an increase in pressure at the drill bit, and a decrease in core recovery efficiency.