Paints for Dipping.
It need hardly be said that the paint employed for this purpose must be of a special character. There are several firms of manufacturers who make a speciality of this class of paints and supply them either ready for use or in paste form, requiring only the addition of white spirit or other suitable thinner to bring them to the right consistency. The following may be considered as the necessary qualities of a paint suitable for this purpose:—
1. It must dry hard in not less than six hours.
2. The pigment must be of such a specific gravity that when mixed with a suitable vehicle or thinner to form a paint the pigment will not quickly deposit at the bottom of the tank or settle out.
3. The consistency of the paint must be so arranged that only a minimum quantity will run off, while it must not be so thick as to give rise to tears or runs.
White lead which has a specific gravity of about 6.750 is for the reason mentioned usually considered too heavy, and in the case of a white or grey paint, zinc oxide which has a specific gravity of 5.470 is used in preference. The following colours may also be successfully employed, because of their low specific gravity, viz., Oxford ochre (s. g. 2.822), Venetian red (s. g. 3.560), Indian red (4.732), golden ochre (3.107), Italian raw sienna (3.081), burnt sienna (3.477), Turkey umber (3.496), Prussian blue (1.956), bone black (2.319).
It will be observed that the earth colours, such as ochre, sienna, umber, etc., are all light pigments, and as they are also recognised as being the most durable, they are eminently well suited for use in many positions, although the colour is not always very agreeable. Precipitated barytes (s. g. 4.144) can be used up to, say, a proportion of 10 per cent.—not more; red lead (s.g. 8.681) cannot be used for this purpose, as it is far too heavy, and the same applies to English vermilion (s. g. 7.726). If, however, a bright colour is required, it may be obtained by dipping in Venetian red or Indian red and giving a second coat of crimson lake (s. g. 1.898), but a coat of varnish should be sprayed upon this for protective purposes.
In considering this subject, it is well to remember that much will depend upon the thickness of the vehicle used, as, clearly, a fairly heavy pigment will settle out in a thin vehicle much quicker than it would in a comparatively thick one. The successful paint, therefore, is one in which both considerations are taken into account.
Very rarely indeed does it pay manufacturers to attempt to grind or prepare paints themselves, and much greater satisfaction will be obtained by getting supplies from a reputable firm who have made a special study of the subject.
Sometimes a white paint is required for dipping, and in that case 30 lbs. of sublimed white lead, 10 lbs. of zinc oxide and 6 lbs. of gilders' whiting, with 4 lbs. of asbestine pulp, mixed with 9 lbs. of raw linseed oil, will be found to give good results.