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Casein
Casein is a component of milk in the form of a whitish powder which is insoluble in water but which can be dissolved by adding an alkali, such as borax or ammonia.Mixed with slaked lime, it dissolves in water, but a film of this mixture applied to a surface hardens on drying and becomes insoluble.
This property enables it to be extensively used as a binder in the preparation of washable distempers.
Casein distempers of good quality possess certain characteristics which make them particularly valuable for certain classes of decorative work.
Chief among these are high light reflection, clarity of colour, non-inflammability, and for interior work, great permanence.
The casein film is practically colourless, and consequently reflects the true colour of the pigment, instead of obscuring it with a yellow film as is the case with an oil vehicle,
unless the oil has previously been bleached. It absorbs very little light and is thus very useful for factories or large buildings in which its application will effect substantial saving in artificial lighting.
In interior decoration, once a casein distemper dries out, practically no change takes place in its chemical compositIon, and as a result it is extremely durable.
Another advantage is that, casein being unaffected by lime,
a distemper which employs it as a vehicle can be used with some assurance of success on " green" plaster, cement, or concrete. It should be observed, however, that the pigments used must be themselves lime and alkali proof.
Casein distemper possesses what may be described as a " sizing" action, which makes it excellent for application to porous surfaces.
If redecoration becomes necessary, it makes a reliable foundation on which other coats can be given, or, if a change is desired, for oil paints, enamels, or cellulose.
The vogue for mat finishes makes casein distempers of special value, since they have a degree of flatness which no other paint can approach.
It is sometimes alleged that this makes them more prone to retain dirt, though there is little evidence in support of this theory.
If a surface finished in casein distemper gets dirty it may be washed carefully with soap and water, but will not stand up against scrubbing.