For artificial rock, Portland cement may be made to look like grey sandstone by mixing 3 parts of crushed grey sandstone with 1 part of cement; for red, use the same quantity of red sandstone. If the coloured sandstones are not obtainable, the cement may be coloured grey with lampblack, or a warm grey with umber and a little lampblack. The cement may be coloured red with red oxide of iron, toned, if necessary, with a little umber.