Troubleshooting - Lime Hemp Plaster Colour Variation

Tŷ-Mawr ‌‌ posted this on 6 Apr 2016

Why does my hemp plaster have colour variation?

Hemp plaster can naturally vary in colour due to the retting process of the hemp. Please see below.

We recommend hemp plasters are painted and the colour variation will not show through painted surfaces.


Retting Process

HemShiv & HemShiv F Seasonal Variation - Seasonal variation or natural variation is something that users of products such as hemp, straw and other organic materials have to take into consideration. Natural variation is not limited to organic products as sands have differing colours throughout the quarry.

Weather conditions are the biggest cause of seasonal variation in organic materials but variety and husbandry can also have influences on a product's characteristics.

The growing season does have the greatest effect on the colour of the final hemp lime plasters that are made using hemp. This is due to the time and weather conditions after cutting while the crop is retting.

Retting is a natural process during which soil microbes in the presence of moisture breakdown the bonds which hold the hemp fibre to the hemp shiv. If the retting process is not allowed to take place the separation of the two parts of the stalk takes much more energy and the fibre produced is very low quality. Retting requires upwards of three weeks and some moisture from rain or heavy dew.

In a season when it is dry for retting then the crop will be left in the field on the ground for more than three weeks but because of the lack of moisture retting will be slow and this will produce shiv and fibre of a bright colour.

In a wet season, retting will take three weeks but the crop may stay wet for the whole period and become stained from the soil and excess water present the crop may lose its brightness giving shiv and fibre a grey colour.

If baling is delayed by rain after a perfect retting season then the crop may well become stained and the shiv loses its bright colour again turning grey.

A perfect retting period would have a dry week after cutting followed by two wet weeks to allow retting followed by two dry weeks to allow baling of dry hemp straw in perfect conditions. This does not usually happen.

Seasonal variations also have effects on the size of the shiv pieces, the amount of dust, and the amount of fibre in the samples this gives rise to different finishes of plasters otherwise mixed and applied in the same way. We minimise this by dust extracting and screening the shiv before packing. These three factors also have effects on the density of the shiv so bales may be a greater or lesser weight to get the same volume.

In wet hemp growing seasons, the presence of moulds in drying plasters may be increased if the plaster is not ventilated sufficiently after application, but mould growth cannot be ruled out in any plaster if the ventilation requirement is ignored during drying.


Categories: Lime Plaster/Render