Asbestine probably helps to hold the pigments in suspension better than any of the others and answers nicely for dipping paints, but, as previously mentioned, does not allow the paint to level out when used with a brush.
China clay, on account of its low specific gravity, is much favoured as a suspension agent, but materially lessens the opacity of the paint.
A small amount of whiting is a good addition to a dipping paint, as it carries down much of the dirt and heavy particles usually produced during the process of dipping. Silica gives the paint "tooth," and by some authorities is considered an actual necessity in primers which are intended to be sand-papered.
White lead and zinc oxide are the two ideal white pigments considered from a dipping standpoint, and it will be found that the majority of dipping paste paints on the market contain a notable quantity of zinc oxide, either straight or in the form of zinc lead.
Zinc oxide is generally the predominating pigment in the white and tinted paste paints, and is usually associated with white lead (basic carbonate), zinc lead, and sublimed white lead, mixed with more or less inert material according to the ideas of the manufacturer or in order to cheapen the product.
The admixture of inert material with the coloured pigments requires some knowledge of the composition of the coloured pigment; for instance, lampblack might be safely mixed with asbestine, whiting, silica, barytes, etc., without detriment, but yellow ochre containing, naturally, considerable clay and silica would hardly permit of any great addition of china clay or silica.
The strong iron oxides, chrome greens and similar tinting colours will stand a large quantity of inert material when the paint is to be used as a body colour, but for varnish colours it is customary to use the chemically pure colours and less paste per gallon of thinner.
Paste paint for varnish colours is frequently ground in oil, in japan, or in a mixture of the two, but far better results are obtained by grinding the dry pigment in varnish thinned to a grinding consistency with a little turpentine. A varnish with a viscosity of 20 (water = 1) which gives a stiff mix with, say, 5 lbs. of pigment will, when thinned to a viscosity of that of raw oil, i.e., a viscosity of 4, take about 20 lbs. of pigment.
No set rule can be given for the kind and amount of thinners to be used in dipping paints for various purposes, but the following proportions[1] will furnish a key to the general mixtures used:—
[1] The figures given relate to an American gallon, which contains 231 cubic inches, while an English imperial gallon contains 277¼ cubic inches. For practical purposes the latter may be considered one-fifth greater than the former.