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Additives for Lime Plaster

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Hair

Animal hair, when added to lime plaster helps binds it together and gives it additional tensile strength. The most common used is HORSE HAIR or GOAT HAIR which is usually about (2-3 inches) long and tough. Thorough mixing into the plaster is essential to ensure consistency and prevent tangled hair. It is best mixed in just before the plaster is to be used because in the presence of wet lime it will quickly degrade in strength. The easiest way to mix it is by sprinkling it evenly over mortar which has been spread out flat. Then turn the mix over several times. Although sprinkling the hair evenly into the mortar whilst it is in a MIXERcan work effectively. To test for consistency, place some mortar on a HAWK with a trowel and, with a sweeping action, cut and remove a sample off the hawk. The cut edge on the trowel should show an even distribution of hair with a "beard of bristles" clearly showing.

*Human hair is unsuitable as it is regarded as too oily.

Linseed Oil / Tallow / Casein

Linseed oil can be added in small quantities to improve the weathering of limewash. Tallow (animal fat) was traditionally added to limewash for the same purpose. Casein (protein from milk) can be mixed with limewash to improve binding and can be mixed with lime putty to make a strong adhesive for both stone and wood.

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Sand/Stone Dust

Sands and stonedusts are mixed with lime putty to form mortars, plasters and renders. Sands are usually mixed and graded at the quarry to satisfy British Standards. It is important that the sand is well graded and angular and preferably washed. Stonedusts are usually crushed and sieved limestone. Stone repair mortars and sheltercoats usually have a higher proportion of stonedusts whereas construction, pointing mortar and plasters and renders have a high proportion of sand.

Pozzolanic Additives

POZZOLANS are materials that enable lime mortars to set more rapidly and add strength. Pozzolans are naturally or artificially fired clays which are reactive when mixed with lime putty. Some examples of pozzolans are stone/ brick dust or ash. To ensure maximum effect the pozzolan must be very fine and thoroughly mixed into the mortar. You would usually need approximately 1 part of pozzolan for every 9 parts of mortar (by volume). This must be done only just prior to use as the mortar will start to set once the pozzolan is added.

Pigments

To produce a coloured limewash various PIGMENTScan be added. Earth pigments are most commonly used to produce a wide range of beautiful colours ranging from pale creams and pinks to strong yellows, dark purples and rich browns although lighter colours are more usual and more durable. The pigments are added when the limewash is made. Depending on the depth and shade of colour required, experimentation with the amount of pigment will vary.