Observation on the density of negatives

Echelle de gris

From a workshop to the other one, the reference density of negatives (I indicate here between 1,10 and 1,20) can vary. This is of in the ultraviolet light source used for the sunstroke of patches. Indeed, a lively and intense source increases the contrast of the image (and can pull a correct density in 1,05), while a low and diffuse source eases it (in that case a contrast of 1,30 can be justified). It’s the same of the nature of the light source. As the bright spectre includes more or less of visible light (outside the zone U.V.), the light will penetrate differently the thickness of the coat of gelatin, what will pull a significant variation of the contrast.

The quantity of dichromate in the sensitive coat also makes vary the contrast. More it is raised, more the contrast will be raised.

It is indispensable that every collotype practitioner takes into account these indications, and determines at the beginning of its works the reference density of its negatives, and the quantity of dichromate according to its equipment. This will avoid many wastes of time.

Grey scaleTo realize this calibration, a simple solution consists in realizing a negative of a scale of grey. Measure it contrast by means of a transmission densitometer. Expose to UV a collotype plate with this negative and to make an drawing. Compare the tests obtained with the original scale of grey. With some tries of this type, you should be capable of correcting the contrast of your negatives until establish the one who has to serve you as reference. You will take advantage of it to establish a curve of correction if your negatives are printed on transparent.