Soft distemper (sometimes known as Calcimine) is a traditional water-based coating for plaster surfaces. As it allows the wall beneath to ‘breathe’ it tended to be used as an initial scheme on newly-plastered walls – providing colour while the lime plaster cured and dried out. Some two or three years later it would frequently be replaced by the first scheme of oil paint:
It dries with a soft bloom and produces a velvety finish but is quite fragile and is not washable. As a result it is not recommended for areas of heavy traffic. It is ideal, however, for the decoration of elaborate plaster ceilings (such as the one below), where a heavy build up of paint is to be avoided.
There are many different recipes for soft distemper and although the main ingredients are constant the proportions often vary considerably. If you are tempted to try making it you must be prepared for trial and error, adjusting the mix to suit your needs. Above all, it cannot be rushed. However, with a bit of experimentation pleasing results can be achieved at little cost. It is worth bearing in mind that a soft distemper has a short shelf-life when in its wet state so only make up as much as you need. From that it will be understood that a true Soft Distemper cannot be bought ready-mixed and that anything in a tin that is labelled as such is unlikely to display the same properties. Above all, soft distemper should not be confused with a bound distemper, which has very different properties and which can cause problems in later years.
It is increasingly difficult to find some traditional ingredients – particularly rabbit skin glue – and it may be worth experimenting with wallpaper paste (methyl cellulose). In view of the different strengths of the various glues one cannot lay down exact proportions for mixing. The following recipe is therefore given as a guide only:
350gm rabbit skin glue / hide glue
Pigment or universal stainer (French ultramarine if white is wanted)
To overcome the porosity of bare plaster it is customary to apply a coat of clearcole first. This is a medium-strength size to which a little whiting has been added. The size, mixed as in the directions below, should be melted in a double-boiler and an equal quantity of water added. To this is added enough whiting to colour the mixture – in the ratio of 100gm whiting (soaked in water) to 1 litre of size solution. When applied this should appear semi-translucent rather than hiding the surface.
Clearcole should be applied hot so that it penetrates the plaster as deeply as possible. Always allow the size to harden before distempering – at least three hours after the surface appears to be thoroughly dry.
The basic material of limewash, lime, is derived from limestone or chalk (both are forms of calcium carbonate) which is ‘burnt’ in a kiln to form quick lime (calcium oxide). This highly reactive material is then ‘slaked’ by adding water, a process which produces a great deal of heat, causing the water to boil violently. The resulting material (calcium hydroxide) is known rather confusingly as ‘slaked lime’, ‘hydrated lime’ and ‘non-hydraulic: lime’, or even simply as ‘lime’. In the wet form in which it is usually kept it is also know as ‘lime putty’. To make limewash this material is then further diluted with water to form a thin paint of brushable consistency.
Slaked lime is the principal active ingredient of both limewashes and mortars. It hardens as it dries out by reacting with carbon dioxide in the air, slowly reverting to the stone form of lime, calcium carbonate. Additional binders are frequently added to limewash for a number of reasons, for example to provide greater adhesion and binding of the paint layer, more flexibility, and variations in porosity.
Different quarry sources provide different qualities of lime; high calcium (pure lime) is usually preferred for its consistency of material and its white colour. Other limes may contain impurities and be inconsistent, but can also be used to produce lime paints and may be white or off-white in colour.
Tallow is a type of animal fat. It is incorporated during the slaking process as the heat which is produced melts the fat enabling it to be evenly dispersed throughout the product.
It can be used on either interior or exterior surfaces, but it is more popular for exterior use as it is slightly less porous than other forms of limewash and can prevent excessive water penetration. It also tends to brush off on clothes, making it less popular for interior use than some other types of limewash, such as casein.
Other oils and waxes can be incorporated as an alternative to tallow. Linseed oil is a popular alternative as it provides durability, some flexibility and better adhesion to some substrates.
Casein is essentially the solid component of milk. Traditionally it was precipitated from skimmed milk by adding a weak acid. The material reacts with the lime to form a calcium caseinate, improving the binding capacity of the limewash. It can be applied to the inside and outside of buildings but is particularly popular for interior use as it adheres better and is more porous than a pure calcium carbonate paint layer.
In its wet form casein limewash is particularly susceptible to mould formation so a biocide should be added if the mixed product is to be stored for long.
As slaking quick lime is a hazardous operation, current awareness of health and safety issues means that this process is less frequently specified by architects. Most now favour the use of the ready mixed product which is available through specialist suppliers.
The two types of limewash most frequently specified are tallow and casein.
Oil Bound Bond Distemper
The addition of oil with an emulsifier such as borax (sodium borate) created a more durable and often washable distemper depending on its formulation, which can be considered a predecessor of modern resin based emulsions. It became popular as an interior paint in the early 19th century when a number of manufacturers began to produce it commercially, and it remained in common use until after the Second World War when it was finally superseded by vinyl and acrylic emulsion paints. It is now popular once again for its visual qualities.
Although less porous than limewash, oil bound distemper retains excellent porosity, and it is suitable for a variety of interior surfaces including timber. This material may be known by different names depending on its ingredients: modified limewash, water paint or, if it contains casein, milk paint. The additives are generally the same as those used in limewash. Traditional recipes that utilise casein have included lime (alkali) to dissolve the casein. (Water is lost in drying and the casein hardens to its insoluble form.) Other variations include the use of alkali silicates to emulsify oil, and plasticisers such as glycerine may be added. In recent years other products such as anti?foaming agents have sometimes been incorporated. Traditional recipes are interesting but the reasons for the incorporation of some additives is not always clear; some recipes may have been regionally produced and passed from generation to generation without being questioned.
It should be remembered that limewashes and distempers, with the exception of glue bound distemper, are generally insoluble when dry.
Waxing will only be needed very occasionally, perhaps once in a few years, if at all. Only use a good quality, unstained beeswax with turpentine polish, such as Liberon Beeswax Paste (not to be confused with their Black Bison range of wax pastes). Avoid any beeswax polish which contains linseed oil, because it is added for ease of application but will not produce a hard, lasting surface. There is nothing to be gained from the excessive application of wax, most of which is either likely to be removed immediately when buffed up or remain as a tacky film to which dust will adhere.