Mars Yellow is artificially prepared yellow ochre. It may be made by precipitating ferrous sulphate, (green vitriol) solution and alum with caustic soda, potash, or lime. The amount of alum in the iron solution governs the depth of shade in the resulting precipitated pigment. It is bright in tone, and the shades vary from pale yellow to orange, and by heat various darker colors are produced including red and purple. It is almost as permanent as the siennas and works well in both oil and water colors. Gold and russet shades are readily produced by it, and herein it has much value as a pigment.