For this purpose the sulphate ultramarine should be selected, and when 70 pounds of the dry blue are mixed with about 40 pounds (or nearly 5 gallons of water) and run through a stone mill until fine, the result should be 100 pounds of a smooth paste that will keep well for months in well-sealed jars.
Since the petroleum trade began to assume its great proportions half a century ago, enormous quantities of barrel paints have been made, and for a time at least the greatest consumption was in blues, because the practice was to paint only the heads of the oil barrels in white or yellow or buff; while the bilge of the barrels were mostly painted in various shades of blue, the base of which was ultramarine, because whenever Prussian blue was used as tinting color, the paste, on standing about, suffered a change in a short space of time, not only becoming much lighter in shade, but also turning to a dull greenish tint. This is partly due to the alkaline properties of the carbonate of lime (whiting) in the pigment, but more so to the acidity of the rosin in the benzine and rosin liquid, which is the vehicle for barrel paints in paste form. As a rule, blue barrel paints in paste form for the petroleum and cottonseed oil trades and whatever other trades will use them to a minor degree are composed of a low-priced ultramarine blue. American zinc white or lithopone and whiting (chalk) or a mixture of whiting and terra alba (gypsum), ground in a vehicle commonly known as gloss oil, and consisting of about 425 pounds of medium colored rosin, dissolved with or without heat in 50 gallons petroleum spirit (62 deg. benzine). The heavy benzines now on the market will not answer the purpose, being too slow in evaporation, causing the paint to require too long a time to dry. Rosin is now practically 150 per cent higher than it was five or six years since and benzine is nearly three times the price of one year ago. As nearly all consumers or concerns using these paints have their own standard shades, it is not practicable to give a formula here, but we may say that because of the keen competition among grinders the margins of profit are very small, even if the goods can be sold in very large quantities.