This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
1147. A landscape negative may in itself possess practically perfect qualities and give an excellent print. It may or it may not be artistic when considering balance, light and shade and general composition. No matter what strong points such a negative may possess, there is bound to be something lacking. A poor sky, in the majority of cases, is responsible for this defect. An absolutely white sky lacking in gradation of light and a horizon entirely lost, detracts entirely too much from the landscape proper, ruining to a great extent, a correctly composed subject. If the natural clouds appear in the sky, this error would be eliminated. Many times proper cloud effects will greatly improve a poorly arranged, or composed, landscape foreground. The amount of contrast between the light blue of the sky and the dark green of the foreground, is so great that when properly exposing the latter, the sky will be so tremendously over-exposed that it blackens up immediately. When the plate is inserted in the developer and becomes perfectly opaque, a print from such a negative will always give a blank, white, chalky sky. To remedy this error, it is advisable to employ some method of inserting clouds to match the foreground and landscape and, as such a method of procedure requires two negatives - a landscape negative and a cloud negative - the process is called double printing.
1148. The landscape negative may already have some cloud forms indicated in the sky, or the sky may be perfectly blank. By applying the methods herein described, which are extremely simple, requiring only a little practice and patience, you will be able to improve many negatives, which otherwise might appear quite ordinary.
1149. A combination print to be successful must be entirely natural. There should be no indication of double printing. In order that the results may be true to nature certain conditions must be considered. First, there must be no false lighting; second, the picture should be harmonious as to definition. Suppose you have employed a selective focus to accentuate the foreground, so that the perspective distance will be out of focus, it would be entirely wrong to print in, next to the horizon, a perfectly sharp cloud negative. Third: There must be no sharp line betraying the junction or line where the print from the cloud negative meets the outline from the original negative. The two portions must be insensibly merged into one another to give a natural result.
Points That Must Be Observed. The junction line may include the horizon, the object projecting into the sky, such as a church steeple, roof of a house, or trees beyond a horizon. The landscape may have been taken late in the day. In this case probably the horizon will be light, and if so the clouds above it must be light. Therefore, if they are vignetted into the landscape the union will not show, for clouds should be printed lightly. If, however, the horizon is dark, you can vignette both light and dark clouds into it without the union showing. In this instance the clouds will generally be rather dark. In both cases you can vignette, but it would be very unlikely that you would have a light horizon with dark clouds in nature. Therefore, such a combination is uncalled for and need not be considered. If the objects are dark, - for example, the trees, the clouds are printed straight over them, for being only slightly printed they will not show in the slightest. But if the objects are light, for example, a church steeple, monuments, etc., and it is desired to print in dark clouds, it will be necessary to mask them, as we will describe later. These cases, however, are few and far between.