Paint

Pigments.--These

are finely divided insoluble white or coloured powders obtained from naturally occurring earth colours, or prepared by chemical manufacturing processes. In the manu facture of paint, it is essential that the colours or pigments used should be in the form of a fine dry powder; this result is achieved by subjecting them before use to a grinding and sieving process, the final particle being generally less than two microns in size.

The

pigments which are commonly used in the manufacture of 11' kite Pigments:—Basic carbonate and basic sulphate white lead, leaded zinc, zinc oxide, titanium oxide, zinc sulphide, litho pone, and to a lesser extent, antimony and zirconium oxides.

Inert Pigments or Extenders:—Barytes, blanc-fixe, gypsum (terra alba), magnesium silicate, Paris white (whiting), China clay, silica.

Yellow Pigments:—Lead chrome and zinc chrome yellow, ochre, sienna, ferrite, cadmium sulphide, organic lakes and toners.

Blue Pigments:—Ultramarine, iron blue (Prussian, Chinese, Milori), cobalt oxide, copper phthalocyanine, organic lakes and toners.

Green Pigments:—Chrome green (iron blue–chrome yellow), chromium oxide and hydroxide, phthalocyanine, organic lakes and toners, Paris green.

Red Pigments:—Iron oxide, venetian red, tuscan red, red lead, organic lakes and toners (para, toluidine, lithol, alizarine), cad mium selenide.

Brown Pigments:—Umber, sienna, iron oxide, Vandyke.

Black Pigments:—Carbon black, lamp black, bone black, iron oxide, organic black graphite.

Metallic Powders:—Aluminium, copper, zinc, lead.

For a fuller description of pigments see PAINTS, CHEMISTRY OF. Media or Vehicles.—The liquid portions of paints which act as binders for pigments.

The vehicle of ordinary ready-mixed oil paint consists of raw linseed oil—or a mixture of raw and processed linseed oil—with a small proportion of turpentine, which is added to make the paint more fluid and to promote ease of working. As turpentine is rather expensive, turpentine substitutes, made from a petro leum distillate (white spirit), are often used as a thinning agent.

A small proportion of liquid driers (terebine) is usually incor porated in the medium ; these driers are solutions of salts of lead, manganese, and cobalt, and increase the rate of drying of the oil by what is known as "catalytic" action. The medium used

in the preparation of varnish paints, which dry with a high gloss surface, consists of an elastic natural or synthetic resin varnish. (See VARNISH).

Artists' oil paints are usually made in the form of a stiff paste and sold in tubes; the vehicle consists of either poppy seed oil or sun-bleached linseed oil, which is used in order to retain the purity of colour of the pigment. A less satisfactory medium is "megilp" composed of linseed oil and mastic varnish.

The vehicles for artists' water-colour paints are chiefly solutions in water of gum-arabic, albumen, isinglass, and size. Distemper and water paints are made with emulsified glue or casein vehicles.

The mediums of the modern cellulose paints or enamels consist of a solution of cellulose esters, in acetone, amylacetate, or other suitable mixed solvents.

Manufacture of Paint.

The process of manufacturing oil paint consists in mixing the dry fine pigment with sufficient medium to convert it into the form of a stiff paste. This stiff paste is then transferred to a roller mill, or other grinding machine, finely ground, and finally conveyed to special paint mixing machines, where it is thinned to the right consistency.

Usually a small proportion of paste driers, or liquid driers is added at this stage, to accelerate the drying of the paint.

As an immense variety of paint products are manufactured for industrial uses, it would be impossible to give more than a brief description of the most important varieties in common use.

Ready-mixed Paint.

The most common and generally used paint is ordinary ready-mixed oil paint. This paint consists of pigments ground and mixed in a vehicle consisting of a mixture of raw and processed linseed oil, with a small proportion of turpen tine. Pure pigments only are today used in the preparation of many of these paints, but a proportion of some inert pigment or extender is sometimes added to aid in suspension and improve durability. These paints are manufactured in a large variety of colours. The most commonly used white paints are white lead and zinc oxide and titanium oxide paints.

For the provisions of the (British) Lead Paint Act, 1926, see