Limewash by Lizzie Induni
What is Limewash?
Limewash is a paint, and like all other paints contains 3 building blocks –
- a binder
- a filler
- an extender
In limewash the binder is the lime itself. Fresh lime reacts with Carbon Dioxide in air and in the right circumstances its chemistry produces a thin layer of limestone. This is what binds the limewash to itself and to the wall.
The filler in all paints provides the bulk and the physical texture. In limewash the lime does this as well as being the binder.
The particles of lime in the limewash are a very powerful extender, and this gives lime it's translucent quality.
Limewash is not the same as whitewash. Whitewash is a traditional treatment used to temporarily disinfect and decorate cow sheds.
It is made from ground chalk mixed with water. These ground chalk particles have never been heated in a kiln and have no power to react and form a cohesive layer of limestone. Whitewash has very little binding power and rapidly falls off as dust.
Distempers became fashionable in the 18th century because they were capable of adhering to low permeability substrates, such as ultra smooth plaster. They can do this because they are based on animal glue binder. Mixes varied widely. Small proportions of glue gave a ‘soft’ distemper which could be relatively easily removed with vigorous brushing.
All distempers are vulnerable to
- mould growth : animal based glues provide a food source for bacteria and moulds. This can be controlled by the addition of biocides. However, it is open to question whether any of these are wholly safe.
- attack by ultra violet light: sunlight destroys the proteins on which animal glues are based. This can happen quite quickly ( 5 years is not uncommon), and causes the distemper to flake and peel.
When to use limewash
The main reasons for using limewash are it’s beauty and individuality. Unlike modern paints, limewash does not produce flat, standardised mass produced colours. Your paint will be individual and not shared with half a million other ‘magnolia’ users. At it’s best limewash has depth, translucence and warmth of colour.
Limewash is at it’s best :
On damp walls.
Traditional buildings are rarely free from damp. Limewash is more permeable than any other paint and will tolerate damp conditions better. This does not mean it cures damp. Damp patches it will show on the limewash by making the paint look darker. This is often temporary and will disappear when the wall dries out.
On dry walls. Even when there do not appear to be acute dampness problems traditional walls need to breathe. Normal household activities such as cooking and washing generate moisture which will condense on surfaces that are sealed with modern paint. This condensation can lead tp serious mould growth Limewash will minimise this problem by allowing the building to ‘breathe'.
Other advantages. Limewash without additives does not promote mould growth, is non-toxic, hypo-allergenic, does not burn or off-gas and is unaffected by ultraviolet light.
When not to use Limewash
Modern paint is designed for ease of application. Limewash is more challenging. It is messy, runny and greater personal effort is needed to source it.
On impermeable substrates. Limewash cannot be applied to emulsion or oil paint, overworked or polished renders. A particular problem on historic buildings is that limewash will not stick to a surface bearing any trace of lichen.
On unstable substrates, such as timber lintels or actively decaying stone. Internally old distempers are likely to prove too unstable to allow overpainting with limewash, even if they are permeable enough to accept the limewash in the first place.
What is limewash made of?
Traditionally limewash was often made by slaking quick lime on site. Today this is practically impossible. The best option is to use lime putty. Limewash can also be made from dry hydrated lime (powdered lime). Whilst this is easily available, it is probably a less satisfactory binder than lime produced as putty.
What is lime?
Lime is made by heating limestone or chalk in a kiln. This starts a chemical process, known as The Lime Cycle.
Calcium Hydroxide (or Slaked lime) is available in two forms –
- dry hydrate : this is sometime known as bagged lime, and is widely available from buildre’s merchants ( but see caution on freshness above)
- lime putty : this is produced directly from quick lime by adding extra water at the time of slaking. Lime putty is expensive to transport, but can be stored indefinately. There is never a problem with the freshness of lime putty.
Hydraulic and non-hydraulic limes
- The lime cycle illustrated above applies to non-hyraulic limes. These have no other ‘set’ than the lime cycle carbonation process. They will not set under water.
- Hydraulic limes derive their name from their ability to set under water. They are made from impure limestone that contains clay minerals. These impurities react in the kiln to form compounds that set as soon as water is added.
- Hyraulic limes are manufactured to the Natural Hydraulic Lime (NHL) standard. This divides them according to their ‘set’ strength. NHL2 is the ‘weakest’ of the standardised limes. Most limes supplied by non specialist builder’s merchants are now manufactured to the NHL standard.
Hydraulic limes and Limewash
Hydraulic lime, especially NHL2, can be used as a base for limewash, but it's hydraulic set is not really an advantage. It is simpler to store, mix and use limewash based on non-hydraulic limes.
Limewash users should be aware that commercial pressures within the lime industry may not always favour the use of non-hydraulic limes.
Where to get limewash
An internet search will provide up to date details of specialist limewash manufacturers. Buying limewash ready made is the easiest way for small quantities of paint. It is probably not the cheapest way. The cheapest way is to make your own. Making your own means you have to buy lime.........
Where to get lime?
Specialist suppliers : Lime putty is a specialist material and is not usually available at builder’s merchants. As with ready made limewash, suppliers of lime putty are best located by an internet search.
Builder’s merchants : Many builder’s merchants do stock bagged dry hydrate lime. Make sure it is fresh.
How to use Limewash
Limewash is a suspension of lime particles in water. If you use lime putty all you need to do is thin this with additional water to the consistency of single cream (through a sieve). When applied, limewash should flow freely under the brush and leave few brush marks. If dry hydrate is used, it should be steeped for as long as possible ( at least 24 hours). Again if dry hydrate is used, it is essential that it is fresh (2 weeks) from the slaking process.
Limewash should be applied with a broad brush with long soft bristles. The substrate must be permeable ( water absorbent), clean and free from all foreign matter, such as lichen, dust, oil paint, emulsion paint, distemper, oil and tar stains, wall paper glue and soluble mineral salts. Salt and tar stains will produce permanent colour change.
Surfaces must be dampened before limewash is applied. Too much suction will dry the limewash before it can carbonate, and produce a dusty surface.
Wetted surfaces should be visibly damp but must not run with water. The limewash should flow off the brush creating no noticeable drag. Allow each coat to dry and carbonate overnight before applying the next. A minimum of 3 coats will be needed, but covering dark colours or creating optimum translucence may need more.
It may take weeks for the paint to dry to achieve it's final colour. Patches of varied substrate may remain visible for much longer than this. Externally this will often reappear in wet weather.
Painting with limewash is quick but messy. Delicate surfaces must be protected. New oak and many other hardwoods will stain irreparably on contact with limewash. Some fabrics can also be damaged, and contact with historic glass should be avoided.
Almost any colour is possible, but darker colours mean more pigment. More pigment reduces the binding power of the limewash.
All pigments must be lime fast. This is a particular problem with blue pigment. Dry pigments need to be pre mixed with water and poured through a sieve before adding to the limewash.
There is no perfect limewash. Traditional users have always tried to adapt limewash for particular tasks. Tallow, salt, sugar, milk, egg, blood and cactus juice have all been used, and probably many others. Historic limes were not pure and probably contained many extra materials that we could now call 'additives'. All these additives are compromises which add specific qualities to the limewash. For example 'salt' prevents over rapid drying but leads to crystallisation damage in substrates. Tallow adds resilience and waterproofing, but restricts permeability and encourages mould growth. All additives will compromise the permeability of limewash.. Know what you want the limewash to do before you use additives.
It has become fashionable to add casein ( the protein in milk curds) to limewash. This produces a chemical reaction which forms Calcium Casein ate. Limewash with casein is no longer a limewash. It behaves differently. It will flake and peel in the long term more than a simple limewash, because tension will build up within the paint film.
Health and Safety
Liquid limewash is strongly alkaline and will cause severe burns to the eyes. Wear eye protection and look after your skin!
Tools for preparing Limewash