Venetian Red was originally made by grinding native red oxide or red hematite, but as the use of paint developed the name was transferred by English dry color manufacturers to the artificial product made by calcining green copperas with more or less limestone that gave a red pigment consisting of about 28 per cent ferric oxide and 72 per cent of sulphate of lime (terra alba or gypsum). Large quantities of this sort of English Venetian red were exported to this country, but since the American manufacturers made great progress in the production of oxide of iron paints, imports have been reduced to quite an extent. The color grinders of twenty years ago had in their oil color lists a red generally labeled English Venetian red over their own name, and in that case it was a dark red, consisting in its pigment portion of 35 to 40 per cent sesquioxide of iron, balance usually being gypsum (sulphate of lime). Venetian red in oil, consisting of less than that percentage of iron oxide, was hardly ever labeled with the manufacturer's name. One certain firm listed three shades of Venetian red in oil, light, medium and dark. The pigment used for the light or bright shade was made by mixing the highest grade of a scarlet red oxide known at one time as Forest of Dean or Crawshay red, that showed by chemical analysis 96 to 97 per cent Fe2O3 in such proportion with hydrated sulphate of lime (gypsum), that it would invariably show on analysis 20 per cent Fe2O3 and 80 per cent CaSO.4 On account of its bright color this red had a remarkable sale. The medium shade of Venetian red was a copperas red, averaging 40 per cent Fe2O3 with very little deviation, balance being sulphate of lime, dead burnt in the furnace along with green vitriol (copperas) and a trifle of silica. The dark shade of Venetian red consisted in the pigment of a mixture of Persian Gulf red and hydrated sulphate of lime (gypsum) averaging 35 per cent Fe2O3, 15 per cent silica and alumina (SiO2 and A12O3) and 50 per cent CaSo4. The mixing and grinding of the light and dark shades of the red referred to was accomplished in the dry state by first mixing the material and expelling the moisture on steam heated iron drying pans, and then grinding the material so mixed to impalpable fineness on stone mills of 36-inch diameter before mixing the pigments in oil. Either of these three red pigments could be mixed and ground to a smooth oil paste on 20 or 24-inch stone mills (esopus stones) by mixing 77 pounds of pigment to 23 pounds raw linseed oil, while by the use of fire boiled linseed oil it was possible to mix 79 pounds pigment and 21 pounds of oil. This bit of history is not related as a suggestion to follow, as present day color grinders would hardly care to do so in the face of the competition. Any color grinder who may have before him the specifications of the United States Navy Department can draw his own conclusions as to the adoption by that service branch of the government of the chemical constituents of the reds above described. These specifications are still in force at the present time with this one exception, that they originally called for a mixture of so much sesquioxide of iron, not over 15 per cent of silica, balance to be sulphate of lime fully hydrated, while some nine or ten years ago this passage was changed to the wording: - "Balance to consist of sulphate of lime that has been dead burned in the furnace, so as to render it incapable of absorbing moisture again."
The reason for this change is obvious. When gypsum is only partly hydrated it is apt to take up moisture from the air, even when mixed with linseed oil to be applied as a paint and even after application. It is also claimed by chemists that one part of gypsum can be dissolved in 500 parts water, but when it has been dead burned it cannot be dissolved no matter how much water is used. Most of the color grinders to-day place only one shade of Venetian red on their list, and that of the medium type, carrying anywhere from 30 to 40 per cent of sesquioxide of iron.
When it comes to a second or even third grade line of colors, as many makers are listing, the usual practice is to extend the red still further with barytes, the one inert mineral base that absorbs the smallest percentage of oil, thus meeting the demand for very low priced Venetian reds that are often mislabeled iron oxide red. As an example, we will assume that a typical Venetian red in oil costs the maker $2.75 per 100 pounds for material, not figuring labor and fixed charges or packages; a second grade like the one mentioned, if made from the same pigment with its own weight of a first grade of barytes and pure linseed oil as the vehicle would cost $2 per 100 pounds for material, figuring on a mixing of equal parts of dry red and barytes, ground with 15 per cent by weight of oil. A red of this quality in paste form would still show by analysis about 14 per cent of sesquioxide of iron in the pigment, and on thinning for use covers up nearly as well as the normal Venetian red, but on comparison, pound for pound, would not absorb as much thinner and therefore be deficient in spreading capacity. Third grade of Venetian red or oxide of iron reds, so-called, usually do not contain in their pigment portion over 7 or 8 per cent of sesquioxide of iron, and a cheap grade of barytes is used as extender, unless the vehicle is a paint or putty oil of mineral origin, when a lighter extender, such as cheap whiting or marble dust, is introduced in place of barytes in order to keep the paste from hardening in the package. Nor is a good oxide of iron introduced as the coloring principle, though it would prove far better, but often an ordinary red ocher or clay. Being of the opinion that manufacturers who prize their reputation more highly than temporary gain, will not cater to the demand for this class of goods, we shall not dwell further on the subject and proceed to deal with Freight Car Reds in paste or semi-paste form. Nearly every color grinder of repute is familiar with the specifications at present in force for freight car red called for by the Pennsylvania Railroad Company, the first one of the railway companies in the United States to issue specifications for paint material to be used on their rolling stock and other equipment. But it may be of more than passing interest for the younger element in the business to learn how this feature developed to its present stage. The latest requirements that have been in force for some years call for a red in paste form consisting of 74 per cent by weight of dry pigment, 1 per cent of moisture and 25 per cent of oil. The pigment to consist of 25 per cent of sesquioxide of iron, not less than 2 nor more than 5 per cent carbonate of lime, balance to be inert material not less opaque than sulphate of lime fully hydrated or such silicious inert material as occurs with oxide of iron in nature, or a mixture of these. The oil to be strictly pure, well settled raw linseed oil. When the late Dr. Charles B. Dudley, who was then chief chemist for the P. R. R. Co., having charge of the laboratories at Al-toona, Pa., in 1888 first conceived the idea of issuing specifications for freight car color, he not only consulted the superintendent and master car painter at the Altoona shops, but also several prominent paint manufacturers, and his first step was to order several barrels of red oxide of iron paste in oil, paste form, that was to have in the dry pigment not less than 60 per cent by weight of pure sesquioxide of iron, while he left it to the various manufacturers what extender should be used if any were necessary. Upon investigation he found that 50 per cent of sesquioxide in the pigment was sufficient to give all the hiding power that was desired, also that there were excessive percentages of moisture present in the paint furnished, and asked for a paint that would be practically free of moisture, but upon being furnished with a trial lot that did not show over one-quarter of 1 per cent of moisture, because the dry pigment had been subjected to a heat of 350 to 375 degrees F. and mixed with the raw linseed oil, while still showing a temperature of over 200 degrees F. before running the material through the mill, he found that the paint did not at all give proper satisfaction in his tests. This was no doubt due to the partial dehydration of the gypsum constituting the inert base, brought about by the excessive degree of heat used in trying to expel all the moisture possible from the pigment. After that Dr. Dudley insisted on the extracted dry pigment from his freight car red paste, showing one full per cent by weight of moisture, and not long afterward made his specifications for this red require only 35 per cent by weight of sesquioxide of iron, which was finally further reduced to 25 per cent. The idea of requiring from 2 to 5 per cent of carbonate of lime in the dry pigment was to neutralize any traces of free sulphuric acid that might be present if the red oxide in the pigment was of the artificial sort made from copperas. The reduction of the percentage of iron oxide in the pigment was made for economical reasons, because it enabled color grinders to furnish the material at lower prices. Another point of consideration appears to be that it was unnecessary expenditure of money to place higher priced material on this equipment, as it would need repainting as frequently even with the more costly material.