Why measure bentonite moisture?
Moisture is useful for production control, drying, milling, storage, packaging, dry-solids normalization and investigation of batch-to-batch variation.
It should not be presented as a principal API Clause 9 drilling-grade bentonite acceptance criterion.
Practical internal QC method
Use a clean weighing dish, balance, controlled oven and desiccator. A routine internal method can use approximately 10 g of representative sample and an oven temperature around 105 °C, with drying continued to a defined constant-mass endpoint.
Calculation
Let W1 be the empty dish mass, W2 the dish plus as-received sample, and W3 the dish plus dried sample.
Moisture (%) = [(W2 − W3) ÷ (W2 − W1)] × 100
Dry solids (%) = 100 − Moisture (%)
As-received and dry basis
Always state the reporting basis. Do not silently moisture-correct an API-oriented suspension unless the applicable method specifically requires it.
Method limitation
Smectite can contain adsorbed and interlayer water. The result should therefore be described as moisture determined by the defined laboratory method rather than an absolute measurement of every form of water associated with the mineral.