Glues and Sizes for Painting and Decorating

Glues are a crude form of gelatine, prepared by digesting animal matter, mostly refuse, such as bones, clippings of hides and skins, horns, hoofs, fish bones and trimmings, etc., in closed vessels at a low temperature. The dark kinds are made from bones and hides; the light-coloured from skins, fish refuse, etc. But most of the dark glues are either bleached with sulphurous acid or filtered through animal charcoal. It is usually sold in a dry state, when it should not contain more than 12 to 18 per cent, of water and 1J to 5 per cent, of mineral matter. Glue, besides being used as an adhesive, is also employed for sizing wallpapers.

Liquid glue is prepared by adding a dilute solution of nitric or acetic acid to the warm solution, thus preventing it from gelatinising on cooling, and rendering it soluble in cold water. The addition of the acid does not affect the adhesive strength of the glue.

Sizes

Size is a weak form of glue and is used by the decorator mainly to act as a binder for distempers and water stains, and to satisfy the suction of wood or wallpaper prior to the application of varnish, and of plaster, before wallpaper is hung. It should not be used under paint, except when certain types of absorbent wallboards are being painted. It is prepared in the following forms:

Cake-glue Size: Cake glue is still preferred for the purpose by some decorators, although it is darker in colour and takes longer to prepare. Good glue will readily absorb cold water to form a jelly and a good idea can be formed of its quality by weighing a small cake dry, placing it under water in a suitable receptacle for a minimum of 48 hours and then weighing it again after removing as much superficial water as possible by means of blotting paper. The better the glue, the greater will be the increase in its weight; fine glues will increase as much as ten times their original weight. Powdered Glue {Concentrated Size): This is a more popular form, being quickly and easily prepared. The best qualities are made from skin or hide glues, which are stronger than those made from bones. Cheaper grades sometimes contain impurities which affect the adhesive properties of the material. When reduced to jelly form, it should be pale in colour; the stiffness of the jelly will give some indication of its strength.

Jelly Size: This is probably the most convenient form: when heated, it melts rapidly and is then reduced to the required strength by the addition of warm water. Good-quality jelly size will keep sweet for a reasonable period, but inferior grades are liable to putrify in warm weather.

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 redecora-tion 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.

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