This works well technically but lacks historic authenticity. Until the situation is clarified, Rose of Jericho has decided to replace the animal hair in haired plasters and renders with a blend of polypropylene fibres and sisal plant fibre. There have been failures of lime plasters (not ours, thankfully) that have been attributed to the rapid degradation of hair in the moist alkali environment. It is thought the protective keratin proteins and natural oils in the animal hair are depleted during the sterilisation treatment. This treatment, which can include boiling and/or steam treating, appears to decrease the resistance to degradation and reduce the durability of the hair in the alkali lime environment. The plaster mixes were traditionally the same as for lath and plaster - the render and floating layers 1:3 (lime putty:sharp sand) with animal hair often added - the setting layer either lime putty on its own or 3:1 (lime putty:fine sand). The vast majority of animal hair is now imported in neat bundles from countries where anthrax is a problem, and the hair is cleaned and sterilised in the country of origin to ensure any bacteria spores are destroyed. This degradation appears to occur more rapidly in currently available animal hair. Lime plaster might be shrink after drying so animal hair of about 5 Kg is used for 1 m 2 to avoid lime plaster cracking and shrinking. Not only this mixture is used for under coat but also used as finish coat.

However, recent analytical work is suggesting that some degradation and loss of tensile strength can occur over time. Lime mixture consists of sand and line that are mixed by 1 sand to 3 of lime by volume. When examining dry historic plaster it is normal to find hair reinforcement present and apparently in good condition. Hay and straw were sometimes used as alternatives, but the vast majority of traditional basecoat plasters were reinforced with animal body hair from horse, cow, pig or goat. Subsequent coats to lathwork and all coats to solid backgrounds normally have hair added at the reduced rate of 5kgs/m3 (c.2.5kgs/tonne). The first ‘pricking-up’ coat traditionally contains hair reinforcement at a rate of at least 8kgs/m3 (c.4kgs/tonne).

This reinforcement is particularly important when applying basecoat plaster to laths (especially ceiling laths) as it reinforces and strengthens the crucially important nibs from which the ceiling plaster ‘hangs’. This means that there is a potential of human exposure to virulent spores when old buildings are disturbed. Anthrax spores can lay dormant for decades or 100s of years. Animal hair has been added to strengthen and reduce shrinkage in lime plasters for centuries. Horse hair which was potentially contaminated with anthrax was commonly used in the formulation of lath and plaster.

Animal hair plaster