Sometimes we get negatives which are too thin and weak to print even on Contrast Velox; if we developed them in the tray perhaps we were deceived in judging the density and we under-developed them, or possibly the subject itself was very flatly lighted, as often happens when the subject is an extremely distant landscape or a view across a large body of water, and from such negatives we cannot get a bright print, even on Contrast Velox.
a. Negative with dense, blocked up highlights.
b. Shows that a Flattening Reducer removes much silver from the
Highlights, less from the Halftones and little or none from the Shadows.
Sometimes, also, we may not have Contrast Velox on hand and may wish to use Special or Regular Velox. In all these cases it is convenient to have a means of increasing the contrast of the negative, and the method by which this is done is the chemical process commonly called "intensification."
Fig. 96. Showing Effect of Intensification.
In order to increase the contrast we must, of course, increase all the separate steps of density occurring in the negative, and not only must we increase them but the increase must be proportional to the steps already existing; that is to say, we must multiply them all by the same amount if we are to retain correct gradation. Fig. 97 shows a number of different steps of density before and after intensification, all the densities having been multitiplied by the same amount or increased in the same proportion.
In order to produce this increase of density we must either deposit some other material on the silver, so as to add something to the image or we must change the color of the image so as to make it more non-actinic and capable of stopping more of the light which affects the printing paper.
There are many different substances which can be deposited upon the image. If, for instance, a negative is treated with a silvering solution suitably adjusted, the silver will be deposited on the image and will increase its density, but this is very difficult to do, and it is more practical to intensify negatives by depositing, not silver, but mercury upon them.
The Eastman Intensifier is a solution containing mercury which will be deposited upon the negative immersed in it, and since the deposition is regular, it can be watched and the gain of density observed so that the intensification can be stopped at the right time.
Densities added by intensification
Fig. 97. Densities Added By Intensification.
While the mercury method is still the most popular for intensifying negatives it has never been wholly satisfactory, because mercury intensified negatives are apt to undergo changes that affect their quality after a time.
Another method of intensifying a negative is to bleach it in. the Velox re-developer and then re-develop the bleached image with the sulphide solution used for obtaining sepia-toned prints. By this method the image is changed from silver to silver sulphide, which has a brownish-yellow color and is much more opaque to actinic light than the original silver image, so that a negative treated in this way will show much more contrast than before treatment. This method has proved very satisfactory and it is believed that re-developed negatives will prove as permanent as redeveloped Velox prints.
It must be understood that intensification is only suitable for the increase of contrast, it cannot improve a negative which is seriously under-exposed; no amount of intensification can introduce detail which is not present before the intensification is commenced; but occasionally intensification will enable us to adjust the scale of contrast of a negative so that better prints can be obtained than are possible without the intensifying treatment.