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06. Developing the Daguerreotype.


This section is from the American Hand Book of the Daguerreotype, by Samuel D. Humphrey. Published S. D. Humphrey, 37 Lispenard Street 1858.

06. Developing the Daguerreotype.

After the plate has been submittedto the o peration of the light, the image is still invisible. It requires to be exposed to the vapors of heated mercury. It is not absolutely necessary to apply artificial heat to the mercury to develop the image, for fair proofs have been produced by placing a plate over the bath at the ordinary temperature of the atmosphere. This plan, however, requires a long time and cannot be adopted in practice, even if it were advisable. The time more usually required in developing the image over the mercurial vapors, is about two minutes, and the temperature is raised to a point necessary to produce the desired effect in that time. This point varies as indicated by different scales, but for the ordinary scales it is not far from 90 deg. cen.

The mercury bath is accompanied with a centigrade thermometer, by which the heat is regulated. Those furnished by the manufacturers are not always correct, and it requires some experience to find the proper degree on the scale.

I would here remark that it is advisable, when placing the spirit lamp under the bath, to so arrange it that the position of applied heat should always be on the same point, viz., should the heat be directly under the bulb containing the thermometer it would raise the mercury in the tube to the point marked, and the temperature of that in the bath would be far below what it should be; hence it is (where time is followed for developing) that many failures occur. This is observed more readily in the large baths made of thick iron, particularly upon first heating. In practice I apply the heat as nearly as possible between the centre of the bottom of the bath and the bulb containing the mercury tube. It is advisable to keep the lamp lighted under the bath from the time of commencing in the morning to the close of business at night. By this means you have a uniformity of action, that cannot be otherwise obtained.

It is well known to the experienced Daguerreotypist, that different atmospheres have a decided effect upon the mercury in developing the Daguerreotype. It will require a greater degree of heat for one atmosphere than for another. Experience alone determines this little difference.

In summer, on cloudy and stormy days, mercurial vapors rise more readily and quickly than in the temperature of autumn or winter. From 60 degrees upwards towards the boiling point (660 deg.), the vapors of mercury rise in greater abundance and collect in larger globules on cold surfaces.

For various reasons I prefer a high temperature and short exposure. It accelerates the process. It renders the lights of the picture more strong and clear, while the deep shades are more intense. It gives a finer lustre to the drapery. The solarized portions also are very seldom blue, especially after gilding. If heated too high, however, the light parts become of a dead, chalky white, and the shadows are injured by numerous little globules of mercury deposited over them. Just the right quantity of mercury leaves the impression of a transparent, pearly white tone, which improves in the highest degree in gilding. To mercurialize with exactness is a nice point. If there is reason to suspect having timed rather short in the camera, reduce the time over mercury in a corresponding proportion. A dark impression will be ruined by the quantity of mercury which would only improve a light one.

If practicable, it is most expedient that the plate be submitted to the action of mercury immediately on coming from the camera. I have frequently, however, carried plates for miles in the plate-holders and after exposing in the camera, brought them back to expose to mercury, and obtained fair proofs; but for the reason before given, it is advisable to carry along the bath, and bring out the impression on the spot.

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 beveltop 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.

2 1/4 minutes, soft; face, scarcely white; shadows, neutral; drapery, fine dark brown linen somewhat blue.

2 1/2 minutes, clear and pearly; shadows, clear and positive, of a purple tint; drapery, jet black, with the dark shades slightly frosted with mercury.

2 3/4 to 3 minutes, hard and chalky; shadows, harsh; drapery, roughened, and misty with excess of mercury.

The foregoing results will be found general.

There are numerous opinions among our operators in regard to the quantity of mercury necessary for a bath. As regards this, I need only say, similar results occur when two pounds or two ounces are used, but the quantity generally employed is about a quarter of a pound. I am of the opinion that one ounce will answer as well as a larger quantity. I know of no better proof in favor of a small quantity than that presented in the following incident. Several years since, an operator (Mr. Senter, of Auburn, N.Y.) of my acquaintance, was requested to go several miles to take a Daguerreotype portrait of a deceased person. He packed up his apparatus and proceeded over a rough road for some distance to the house where he was to take the portrait, and arranging his apparatus, with all the expedition which the occasion required, after having everything in usual order (as was supposed), he proceeded and took some ten or twelve very superior impressions. They were fine, clear, and well developed. After taking the number ordered, he proceeded to repack his apparatus, and to his surprise, when he took up the bottle he carried the mercury in, he found it still filled, and none in the bath, except only such particles as had adhered to the sides, after dusting and being jolted for several miles over the rough road. From this it will be seen that a very little mercury will suffice to develop fine proofs. I saw some of the impressions referred to above, and they were certainly well developed, and very superior specimens of our art.


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