To design a paint, be it in paste or liquid form, requires a thorough knowledge of the materials, that are to be used; that is, their chemical composition, as most specifications are worded in chemical names and synonyms. For instance, a structural iron paint of dark chocolate brown is wanted in paste form, the specifications calling for a pigment being composed of not less than 23%, nor more than 28% sesquioxide of iron (Fe2O3), not over 2% carbon, not less than 2% nor more than 5% carbonate of lime (CaCO3), balance to be inert material, not less opaque than sulphate of lime, to match in color the standard sample submitted, etc., the pigment to be ground fine enough in pure raw, well settled linseed oil to meet certain tests, that are also described. The pigment portion of the paste to be not less than 73% nor more than 76% by weight and the vehicle not less than 24% nor more than 27% by weight. Then follow the usual rules under which shipments will be rejected. Now in order to meet the specifications it is immaterial, whether the pigment, containing the requisite percentage of sesquioxide of iron is an artificial product or natural or whether it is red, brown or yellow, so long as the color to be matched, can be obtained without the addition of more carbon black or the addition of a pigment, that would increase the percentage of sesquioxide of iron over the limit.
When the paint to this specification was made originally, the author found, that to use sulphate of lime in the form of gypsum made grinding rather difficult and after selecting the proper pigment with the required portion of iron oxide, made use of a fine grade of clay, thus making manipulation in mixer and grinding through the mill more simple, because in this way the color was not materially changed in grinding. Thus the color produced consisted of a native red, averaging 89 per cent sesquioxide of iron, the carbon black specified, balance of pigment being china clay, or in detail as follows:
1 1/2 pounds carbon black, 3 pounds whiting, 22 pounds native red as above and 47 1/2 pounds china clay or a total of 74 pounds dry pigment and 26 pounds of raw linseed oil.
It will be noticed, that for instance a burnt ocher of from 28% to 30% sesquioxide of iron would have served the purpose equally as well and made the addition of china clay unnecessary. Yet the cost of a burnt ocher of that strength or a native red ocher would be higher, than the cost of china clay and the texture of the ocher would not be equal in fineness to that of the clay. All of these points must be considered to make it possible to meet competition as well as the chemists' specifications.
Another structural iron paint, where the specifications read as follows: The pigment of which this paint is to be composed must consist solely of 60% sesquioxide of iron, Fe2O3, 23% silica, SiO2, 5% red lead, Pb3O4 5% white lead, PbCO3, 5% zinc oxide, ZnO, and 2% zinc chromate, ZnCrO4. The vehicle must consist of well settled, pure raw linseed oil and not over 10% drier, and the volatile matter must not exceed 4%.
The pigment must be 55% and the vehicle 45%, with a permissible variation of not over 2% either way.
Considering these specifications not from the chemical, but from the practical standpoint, it is difficult to see, what function the white lead and zinc chromate exercise in this paint, when it is to be used over old painted surface and not on the bare metal, as was the case here. But leaving this aside, the items of 60% sesquioxide of iron and 23% silica would indicate, that a silicate of iron is wanted, yet this is rather difficult to obtain in a red, such as the desired color must be. Judging from the prices, at which proposals for this paint are awarded, the red pigment cannot possibly be a chemically pure oxide of iron and as the lower priced grades or native reds always contain more or less alumina in addition to silica, the specifications cannot be strictly adhered to by the successful bidders. To all appearances, then it may be assumed, that the nearest approach is a paint made about as follows: 38 1/2 lbs. native red of 87% Fe2O3, 7 1/8 lbs. silica, 2 3/4 lbs. dry red lead, 2 3/4 lbs. dry white lead, 2 3/4 lbs. zinc oxide, 1 1/8 lb. zinc yellow, 40 lbs. raw linseed oil and 5 lbs. strong drier. Weight about 12 1/2 lbs. per gallon.