Whatman® Cellulose Extraction Thimbles. High Performance Cellulose Extraction Thimbles. Cellulose extraction thimbles are produced from high-quality alpha cellulose cotton linter and have excellent mechanical strength and retention. Standard single thickness thimbles have a wall thickness of approximately 1 mm (10.0 μm nominal particle retention). Double thickness thimbles have a wall thickness of approximately 2 mm (6.0 μm nominal particle retention) for applications where higher retention and increased wet or dry strength, or rigidity are required. The high purity of the materials ensures reliable and reproducible analytical results.