The former shows more silica and less sulphur and sodium in an analysis, and when tested for tinting effect with white gives a clearer blue, less grayish tint, than the latter. The better grades of ultramarine blue show average analysis as follows: - Sulphate ultramarine blue + Silica, Si02 - 48.5 per cent.; alumina, A1203 - 23.2 per cent.; sulphur, S - 9.6 per cent.; sulphur trioxide, S03 - 2.6 per cent.; sodium, Na20, 12.8 per cent.; water, H02 - 3.3 per cent. Soda ultramarine blue - Silica, Si02 - 40.8 per cent.; alumina, A1203 - 24.3 per cent.; sulphur, S - 13.6 per cent.; sulphur trioxide, S03 - 4.4 per cent.; sodium, Na20 - 14.7 per cent.; water, H2o- 2.2 per cent. The former is much paler in shade, the latter very much deeper. The better grades of soda ultramarine blue are preferred for bluing white pigments, because the whites so blued are not so apt to green off, especially when varnish is used in reducing the white for application with the brush or for dipping.
Color grinders know what a large variety of grades ultramarine blues are offered in and how the selling prices of the dry blue differ, as well as the advance in the market prices of to-day, as compared with those of years ago. There was a time not so very long since when there was spirited competition and the makers of ultramarine sold the color at ruinous prices, but since the consolidation of interests both in this country and abroad the prices have gone up at least 75 per cent from those referred to. It is not our purpose here to extol any special brand or manufacture, but we must state that the brands manufactured in the United States are always preferred on comparison of quality and prices to those imported from abroad, excepting in rare instances, where extra brilliant, soft material is required and higher cost not considered. There being a dozen or so of grades listed, of which at least one-third will interest the color grinder for his various purposes, it is up to him to test samples offered him by the manufacturer for brilliancy of tone, shade, fineness, softness of texture, tinting power and clearness of tint produced with white. One part of the dry blue mixed with 25 parts French zinc white, rubbed out to the utmost is the best guide for this. As the percentage of silica and alumina, etc., in ultramarine blue does not figure much in the oil absorption for grinding the oil color, it is not necessary to go to the trouble of grinding specimen batches, as in the case of blacks, rubouts being sufficient for testing ultramarine blue.
For grinding ultramarine blue in oil for the trade the one that has the strongest staining power should be selected, and the average mixing will require two-thirds by weight of pigment to one-third by weight of linseed oil. Refined oil is preferable to raw or boiled oil, as it will give the blue a clearer tone and appearance. For coach color the blue with the cleanest tone is preferable to that of the greatest staining power.
For coach work and automobile painting this blue should be ground in pale gold size, as the brownish color of the ordinary coach japan is apt to impart dullness to ultramarine blue. Care must be taken that the pigment is bone dry before mixing, and for extra fine work the blue is best ground in pale rubbing varnish with a portion of turpentine, 40 per cent by weight of gold size japan or 32 per cent of rubbing varnish and 6 per cent turpentine to 60 or 62 per cent by weight of pigment will be required to produce a smooth paste.
For ultramarine blue enamel a small portion of best French zinc added to the pigment, not enough, however, to affect the shade to any great extent, will be beneficial, as it will tend to hold the color in better suspension when reduced to brushing consistency. The pigment in this case, when intended for air-drying enamel, is best ground in pale gold size, but when it is intended to serve as the base or coloring for baking enamel, that is to be stoved, at fairly high temperature, the grinding vehicle should be baking enamel varnish of the proper selection and the pigment and varnish should be about equal portions by weight.
Artificial ultramarine blue, while not as permanent as the natural, is on the lists of artists' tube colors, as it is, when properly selected, deeper and more brilliant and very much lower in price. The finest and softest imported ultramarine blue in the form of drops should be ground in poppyseed oil to rather stout paste, and if good aged oil is used in the grinding the oil will not separate from the pigment when the color is squeezed from the tube on to the artists' palette. The separation of oil and pigment can be entirely avoided if the blue is ground in a mixture of 75 parts of bleached poppyseed or nut oil and 25 parts of palest lithographer's varnish, which latter will in no way interfere with the durability and permanency of the color, 65 parts by weight of the ultramarine blue in drops to 35 parts by weight of the oil mixture will be about the right proportion.