Prior to first time use, inserts made from bamboo, hemp and cotton should washed on a regular cycle with detergent, in water of any temperature. These fabrics will continue building absorbency with use, change more frequently during this time to prevent leaks from occurring. See the Why Am I Getting Leaks? for methods on how to prevent them.
Why does prepping new nappies make such a difference to user success?
The surface area of the fibers increases over time with washing (ie friction), the fibers become looser and micro piling occurs. The greater the surface area, the more absorbency potential , .
Covers and microfiber inserts can be washed once with detergent prior to use.
See the Adding Absorbency page for absorbency measurements of various flats, prefolds and inserts.
Scanning electron microscope images of the silk fabrics: (a), (d) before washing; (b), (e) washed with ultrasonic energy; and (c), (f) washed with washing machine for 15 cycles.
L. Quaynor, M.Takahashi M. Nakajima 2014. Effects of Laundering on the Surface Properties and Dimensional Stability of Plain Knitted Fabrics 
Electron microscope image of common fabric fibers
 C. Ganser, P. Kreiml, R. Morak, C. Teichert 2015. The effects of water uptake on mechanical properties of viscose fibers