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.
Many times very weak negatives are condemned on account of their poor printing quality, they being thin, and printing fiat, caused by insufficient developing, under-exposure, or other causes. Many dark room men judge their negatives by brilliancy, regardless of their printing quality. The most beautiful appearing negatives do not always yield the best prints. On the contrary, some apparently poor negatives have the qualities which are essential in producing the finest prints.
130. A thin negative, if stained brown or yellow, will have better printing quality than one which is blue; therefore, a tinge of brown color is the best. Always judge your negative for printing qualities alone. There are times when, through misjudgment, a plate is not developed far enough, being removed from the developer too soon, resulting in a thin negative with little contrast between the highlights and the shadows.
131. Generally, in landscape negatives of this kind, the sky portion being thin will print gray, and the shadows will be weak. In portraiture the highlights would be flat. Such negatives may have the proper color (brownish tint), which would be to their advantage, yet they lack the snap necessary to produce strong brilliant prints where all the proper shades from the highest lights to the deepest shadows will appear as the eye sees them, with good detail. The remedy for such a negative is to intensify it, thereby strengthening the highlights and shadows.
132. In many cases this treatment of the negative improves its printing quality so much that one not knowing how the negative had been treated would declare that the prints made before and after intensifying were not from the same plate. Intensifying is simply increasing the opacity of a negative. There are many methods employed for this purpose, but two are chiefly used, both being distinct from each other in the action of the chemicals employed. We will describe both, but recommend the use of the first.
Methods Of Intensifying. The first is simply increasing the density by thickening the deposit of the metallic silver. The second is by substituting another metal for the silver, having a more opaque color, causing the negative to become less transparent.
We recommend the first method, as it is the most simple to apply, and there is less danger of staining and other failures.
Kind Of Negatives Which Can Be Successfully Intensified. All weak negatives cannot be successfully intensified, but all can be improved, in some cases, however, but slightly. The following are the kind of negatives which can be materially improved by intensifying: Negatives which are fully timed and under - developed; negatives which are slightly under-timed and under-developed, having some detail in the shadows but lack strength; negatives which were overtimed and under-developed, having plenty of detail, but no strength to the highlights.