PAINTER-WORK.
The disadvantage of white lead paints is that they darken with age, and become discoloured when exposed to the fumes of sul phuretted hydrogen which exists to a greater or less extent in the atmosphere of all large towns.
For this reason paints made on a white lead base cannot be used on gas-holders or in the neighbourhood of a gas-works, as they would rapidly turn quite black.
On account of the many disadvantages of white lead paints a large demand has arisen for "leadless" or non-poisonous white paints. The chief pigments used in the manufacture of non poisonous white paints are zinc oxide (zinc white) and titanium oxide. These pigments, besides being non-poisonous, are a purer white colour than white lead, and hence lend themselves ad mirably to all interior paint-work. Other pigments used in the manufacture of non-poisonous white paints are lithopone and zinc sulphide. Titanium white was only introduced as a paint pigment during the second decade of this century, but is rapidly coming into favour on account of its remarkable permanency and stability under all atmospheric conditions, and also because of its obliterat ing power or opacity, far greater than that of white lead.
A white paint made with a mixture of zinc oxide and titanium white pigment as the base, and a vehicle consisting of processed oil or synthetic resin varnish, gives an excellent durable paint suitable for exterior use.
This paint has excellent covering and obscuring properties, wears well over a long period of years, and—unlike white lead— will not discolour or turn black even on exposure to sulphuretted hydrogen, acid fumes or other noxious gases.
The large variety of light shades or tints in common use are obtained by the addition of a small proportion of various coloured pigments to white paint; thus for example white paint and a suit able black give greys from the lightest to the darkest hue, de pending on the proportion of white and black used. Buff is obtained by tinting white with yellow ochre, cream colour results from adding a touch of chrome yellow to white and so on.
The amount of oil required to convert a pigment into paint form varies with the specific gravity of the pigment; a heavy pig ment such as white lead (sp.gr. 6.75) will require considerably less
oil than a bulky pigment, such as zinc oxide (sp.gr. 5.47) or vegetable black (sp.gr. 1.72).
Enamel or Varnish Paint.—Enamel paints dry to a brilliant surface of varying degrees of gloss. They are made by finely grind ing the selected pigment or mixture of pigments in a varnish medium, and their nature and properties depend on the type of varnish used. A quick drying variety is made from a synthetic resin base varnish (phenolic or alkyd). It dries in two to four hours and, contrary to experiences of earlier days when such quick drying enamels were made from short oil, natural resin or rosin varnishes, these quick drying enamels have a high degree of tough ness and abrasion resistance, retain their gloss and fullness against repeated cleaning, and have proven themselves highly suit able for all interior work. For outside enamelling, slow drying processed oil base enamels are still used to some extent, but the synthetic base enamels of a somewhat longer oil, more elastic type than for interior work, have become increasingly popular and have practically displaced the processed oil and copal varnish enamels of an earlier day.
Flat type of paint is really a flat drying enamel. it is made in the same way as the gloss enamels, except that the pigment is less finely ground, and the vehicle contains less varnish and more volatile solvent than ordinary enamel. Owing to their pleasing decorative effect, they are used for interior decorations, but are not suitable for outside use.
Anti-corrosion Paints.—These paints are largely used for protecting iron and steel structures from rusting. A good anti-rust paint is red lead paint, which is made by mixing red lead with raw linseed oil. As red lead in linseed oil rapidly sets into a hard mass if kept for any length of time, it is necessary to mix it just before use. A non-setting red lead has been introduced which obviates this defect and will keep, when mixed in linseed oil, in a liquid condition for any length of time. The use of inhibitive pigments, such as lead and zinc chromates, has also become quite general, as has the use of a preliminary phosphoric chromic inhibitive wash.