1. n. 
A class of polymers added to a drilling-grade clay mineral during grinding, or added directly into a clay-based mud system, to enhance the clay's rheological performance. In concept, clay-extender polymers achieve the type of rheology needed for fast drilling with fewer colloidal solids and lower viscosity at high shear rate (at the bit). This is the concept of a "low-solids, nondispersed mud" system. Extenders are usually long-chain anionic or nonionic polymers that link clay platelets together in large networks. Anionic polymers are highly effective but can be precipitated by hardness ions. Nonionic polymers are less effective but also much less sensitive to hardness ions. Excessively long, linear polymers may break up under mechanical shearing. Either by precipitation or breakup, extender polymers can quickly become ineffective if poorly chosen and used improperly. A drilling-grade clay that has no extender is that which meets the standard for API nontreated bentonite. API bentonite and OCMA-grade API bentonite usually contain extender polymers.