Why We Use Man Made Fibre

The reasons behind moving away from animal hair to man made fibre are firstly mechanical and secondly ethical.

Mechanical:

Reinforcement performs a vital function in any situation where there is a potential for background movement, be it seasonal movement or wind loading on a timber frame or deflection stresses on a ceiling. This is especially the case when the means of carrying the plaster is timber lath. When plastering onto lath it is always discussed how vital it is to push the plaster through to form a good key behind. While this is indeed the case, the critical part of the system is the small ’neck’ of plaster that connects the key at the back to the 20mm+ of finished plaster in front. When looking at failed lath and plaster, it is virtually always the case that the finished plaster has sheared at the point where it passes between the lath, leaving the remaining plaster flush with the lath face, as illustrated in the photo below. Although there can be many contributing factors to a failure such as this, by far the most common is lack of, or degraded animal hair reinforcement.

Why does the hair degrade?

Historically hair would have been supplied by a tanner, freshly removed from the hide of a cow, or horse, it would have been beaten, then added to the plaster at the point of use. 

The modern tanning industry chemically removes the hair, so it is no longer available locally and in it’s untreated condition.

Animal hair is generally imported from China, where it has been treated and washed, mainly for use in the brush industry, the process of washing the hair strips the naturally occurring oils from the fibres, leaving them much more vulnerable to environmental degradation.

In our opinion, modern hair if left exposed to the hostile, highly alkaline environment of wet lime plaster for more than a week, will degrade to the point that it has lost most of it’s tensile strength and therefore is no longer fit for purpose. Even if the hair is added at the last moment before use, depending on the temperature, time of year and humidity around the new plaster, it may remain damp for much longer than a week.

There is also the question of the volume of reinforcement that can be added, scientific measurement of the hair content in C17th chalk plasters show a hair content up to and probably beyond 18kg per m3, as illustrated in the following photo, often more hair than plaster. 

It isn’t reasonable to expect this volume of reinforcement to be added on site and even if it could be, the resulting mix would be very difficult to use especially if the requirement is ’straight and flat’.

With the very fine fibre that we use in Warmcote and Limecote, 150g is the equivalent of 280 linear miles of reinforcement per 25kg bag. The strength matrix that results enables the use of both plasters to extreme depths of 100mm+ in one pass, without the risk of shrinkage, slumping or cracking, it also gives the plaster enormous flexibility and the ability to cope with exceptional background movement.

Ethical considerations:

As mentioned, the majority of animal hair has to be sourced from China. The hair that is ‘easiest’ to add into a plaster mix at a greater volume is drawn goat hair, though coarser pig bristle is also commercially available.

Given the well grounded concerns about the Chinese approach to human rights, let alone animal welfare, we are not at all comfortable with the potential ‘back story’ behind the supply of Chinese animal hair.

There is also the consideration of supplying a ’traditional’ plaster reinforcement that involves a vast amount of road haulage and international shipping, derived from an animal that would never have been historically used in a UK setting.