Plaster Additives
Hair
When plastering hair is added to to the plaster, the hair helps binds the plaster together and give additional tensile strength.
The
most common hair used is horse 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 mixer can 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 to 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.
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 an 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 pigments can 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 colors 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.
Return To Top
The Building Lime Company | Tel: 01980 611950 | Fax: 0845 658
7788 |