Burnt sienna, whether it be the Italian, the domestic or from any other source, is the product obtained by calcining or roasting the natural or raw sienna earth, at moderate red heat. This process drives off the combined water in the raw earth to a greater or lesser extent, but hardly ever fully, as to do so would give a very dark brown product, while the brighter and redder toned siennas are much preferred and command the higher price. The brightness of tone does not depend entirely upon the temperature and length of time used in calcining, but to a greater extent upon the quality of the natural earth. The so-called red fire burnt siennas, so highly valued for their brilliancy of tone and richness of tint, that were imported years ago from Italian sources, are hardly met with now, excepting in rare instances, due, it is said, to the mines being exhausted, which may be partly true, but it is also a matter of fact that the average manufacturer will not pay the price asked, when the consumers themselves look more for tinting strength, than for transparency and brilliancy of tone. What has been stated as to the history of raw sienna applies to the burnt material equally as well, and some of the burnt siennas of exceptionally strong staining power show as high as 76 or more per cent sesquioxide of iron. As a matter of interest we give the result of the analyses of a good average burnt sienna of Italian and American origin: -
Italian. per cent
American. per cent
Combined water, H2O.....................................
Ferric oxide, Fe2O3..........................................
Alumina, Al2O3 ...............................................
Calcium carbonate, CaCO3................................
Manganese MnO2 ..........................................
As in the case of the raw siennas, the hygroscopic moisture had been removed from the samples before the analyses were made. The practical grinding trial of these two lots, after drying the material on steam heated pans for several days at temperatures averaging 135° F. required 46 pounds of raw linseed oil to 54 pounds pigment in the case of the Italian and 34 pounds of the same oil to 66 pounds pigment for the American sienna, resulting in a buttery paste of lighter bulk in the former, than was the case with the latter, which resembled more the compactness of burnt ocher. Burnt sienna, that is largely used, ground fine in oil for grainers, as well as for tinting purposes, and even in composite colors, such as olive greens and some bronze greens, as well as in pigment stains for imitating cherry and mahogany, is at its best when of a somewhat deep red, but not brown overtone, with good transparency, which is most desirable for stains, as well as for graining work. The usual run of American sienna will not answer for that purpose, no matter how closely it resembles the Italian importations. When grinding burnt sienna in oil or in japan, excessive heating of the mills tends to deepen the color and in many instances make it appear richer, but it also makes the staining power more or less deficient. The best mills for grinding burnt sienna are those equipped with buhr stones of 20 to 24 inches diameter, and these should not make over 60 revolutions per minute. The temperature of the color in oil should at no time exceed 150° F., otherwise the product will be grainy on cooling, especially if the pigment contained some hygroscopic moisture on mixing with the oil. For grinding burnt sienna in japan, 20-inch water cooled mills are best, and if the pigment consists of finely powdered sienna, the grinding stones may be of the esopus variety. In grinding sienna in japan, the less heat is developed in the milling the less the loss of volatile matter in the japan and the less tendency of the finished product to be gummy or tending to liver.
The average requirements for mixing and grinding good qualities of Italian burnt sienna in oil may be figured at equal parts by weight of pigment and raw linseed oil, while for the japan color a fairly safe average would be a mixing of 44 pounds of pigments and 58 pounds color grinding japan, yielding 100 pounds finished product, balance being lost through evaporation of some of the turpentine in the japan and the hardening of some of the color on scrapers and flange of running stone, as well as in the hopper.
On lots smaller than 100 pounds in one batch, 10 per cent loss will not be too much to figure on. As mentioned in the case of raw sienna for artists' tubes, only the old time standard of material should be selected in burnt sienna as well and the more brilliant the overtone of the pigment, the more favorably the color will be received by the user. In grinding burnt sienna for the artist, extreme care for obtaining the highest degree of fineness and absence of all grit must be exercised, and burnt sienna of that character will require not less than 55 to 60 pounds of poppyseed oil, and 40 to 45 at most of pigment. For burnt sienna in distemper, a grade of rich, deep color, not too high in oxide of iron, should be selected, so that it may work well for the grainer as well as for the fresco artist. Must be free from grit and ground fine in water, and figures may be based on 50 pounds pigment and 65 pounds water, which batch should yield 100 pounds paste as it comes from the mill.
Before closing our remarks on sienna, we may state that the specifications of the various service departments of the United States Government bar out any sienna, raw or burnt, that is not equal to the best Italian grade on the market or that contains more than 5 per cent of lime in any form.