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.
