Retting on the field is also known as 'dew-retting’.
Retting: 'loosening' of the bast fibre from the core material in the plant stalk
After harvesting, the hemp straw remains on the land for several weeks. During this period, microorganisms weaken the glue layer between the bast fibre and the woody core in the plant under the influence of water (rain and dew); this process is called 'retting'. During retting, the hemp is turned several times to make sure the retting process is homogeneous and optimal. By weakening the glue layer (pectin layer), the fibre and core material (shives) can eventually be separated during the further processing step (§6.1 for technical application and §7.1.1 for textiles). The exact duration of retting depends on the weather; decomposition depends on rain, dew, and temperature.
For technical applications, the retting lasts about 2 weeks and the stems are turned every 2 to 3 times. For textiles, the retting duration is up to 5 - 6 weeks and the stems are turned once or twice.
When the retting process proceeds beyond needs, the stems are over-retted: the hemp fibres become weaker and have a lower value. Over-retted hemp results in relatively large share of short fibres during scutching and hackling (§7.1.1, 7.1.3). Conversely, when the straw is under-retted, separating the bast fibre from the core material will require more energy (§6.1), which also results in damages to the fibres.
Determining the retting degree
During retting, the stems turn from green to yellow and finally grey (see image below). The degree of retting can be determined with a relatively simple test (the so-called Fried test):[1]
Place a piece of hemp stem of 8 cm length in a slim bottle/flask.
Add boiling water until so that the entire stem is submerged.
Vortex the bottle/flask for 10 seconds at 2500 RPM.[2] Additionally, shake firmly vertically for further 5 times.
Determine the extent to which the bast fibres detach from the core.
[1] http://www.global-sci.org/v1/jfbi/Biological-Treatment-of-Raw-Flax-with-Fungus
[2] https://www.ika.com/en/Products-LabEq/Shakers-pg179/Vortex-2-25000258/