It is sometimes the practice of inexperienced operators to take the plate off the bath and examine the impression by solar light. This plan should be abandoned, as it is almost sure to produce a dense blue film over the shadows.
This I am led to believe is occasioned by the action of light on the yet sensitive portions of the plate, and made to appear only by subsequent exposure to mercury, being equivalent to solarization.
There has been little said by our professors upon the subject of the position of the plates while exposed to the mercurial vapour. Mr. Hunt, in referring to this subject, says: “Daguerre himself laid much stress upon the necessity of exposing the plate to the mercury at an angle of about 45 deg.. This, perhaps, is the most convenient position as it enables the operator to view the plate distinctly, and watch the development of the design; but beyond this, I am satisfied there exists no real necessity for angular position. Both horizontally and vertically, I have often produced equally effective Daguerreotypes.” I presume from the last sentence of Mr. Hunt, that he has confined his experiments to the smaller sized plates. Hence he may not have thought of the effect of the vertical exposure of a large plate.
In America this is a subject of no little importance. When an impression is to be developed upon a plate fifteen by seventeen inches, were we to use an angle of about 45 deg., it would be found to make a perceptible difference in the appearance of the image. By examining the wood tops of our baths as formerly made, it will be found that there is a great variation in the distance from the mercury to the different portions of the plate. By measuring one of these tops for the size plate above mentioned, I find the distance to the nearest point between the mercury and the plate, to be thirteen, and the middle point sixteen, and the furthest point twenty-one and a half inches: by this we see that one point of the plate is eight and a half inches further from the mercury than the nearest point; even this is not the variation there would necessarily be, were we to adopt the angle of 45 deg. as urged by Daguerre.
Among our principal professors, the bevel top will not be found in use where the large plates are used. Should any one feel desirous to test more minutely the effect produced by a bevel top bath, I would suggest to them to place a frame, so constructed as to hold three sixth size plates, and fit it to the top of the bath, and so arrange it with openings that the plates may be placed, one at the nearest point of the mercury, the second midway, and the third to the greatest distance, and by placing the plates over at one and the same time, the experimenter will be enabled to judge if there exists a difference in the developing. In speaking of the above, reference is had to baths to the ordinary heights used by operators.
We will now proceed to examine the effect produced by mercurial vapor upon the plate at different lengths of exposure. In some investigations which I have made upon the appearance of the Daguerreotype impressions when developed over mercury at 90 deg. C. (194 deg. F.), the following was the result. Plates, coated and exposed to light in our usual manner of operating, produced on exposure of
1/2 minute, whole impression, deep blue.
1 minute, ashy and flat; no shadows; linen, deep blue.
1 1/2 minute, coarse and spongy; shadows, muddy; drapery, dirty reddish brown.
2 minutes, shallow or watery; shadows, yellowish; drapery, brown.