I will say here, that a plate submitted to only an ordinary polish is found to contain numberless minute particles of the powder made use of. Should the same plate be buffed for a long time, the polish will nearly all disappear, leaving the cavities in the surface free for the action of agents employed in subsequent operation. For this reason, I find that great amount of polishing powder should not be applied to the last buff, and it is obvious that three buffs can be employed to adventure;the two last should not receive any polishing materials.
I have examined a plate that was considered to possess a fine finish, and similar had produced good impressions;these same plates, when subjected to a long and light buffing, would present a surface no finer in appearance to the naked eye;but upon exposure to the solar radiation, would produce a well-defined image in one fourth less time than the plate without the extra buffing.
Coating the Plate.--For this purpose our mechanics and artists have provided a simple apparatus called a coating-box, which is so arranged as to be perfectly tight, retaining the vapor of the iodine or accelerators, and at the same time allowing, by means of a slide, the exposure of the plate to these vapors.
They can readily be obtained by application to any dealer, all of whom can furnish them.
The principal difficulty in coating the plate, is that of preserving the exact proportion between the quantity of iodine and bromine, or quick.
It is here necessary to say, that hardly any two persons see alike the same degree of color, so as to be enabled to judge correctly the exact tint, i. e. what one might describe as light rose red, might appear to another as bright or cherry red; consequently, the only rule for the student in Daguerreotype, is to study what appears to him to be the particular tint or shade required to aid him to produce the desired result.
Practise has proved that but a slight variation in the chemical coating, of the Daguerreotype plate will very materially affect the final result.
The operator will proportion the coating of iodine and bromine or accelerators according to the strength and composition of the latter.
Experience proves that the common impressions, iodized to a rather light yellow gold tint, and brought by the bromine to a very light, rose color, have their whites very intense, and their deep shades very black.
It is also known that if you employ a thicker coating of iodine and apply upon it a proportionate tint of bromine, so as to obtain a deep rose tint, delineations will be less marked, and the image have a softer tone.
This effect has been obvious to everyone who has practised the art.
Thus I may observe that the light coatings produce strong contrast of light and shade, and that this contrast grows gradually less, until in the very heavy coating it almost wholly disappears.
From this it will readily be perceived that the middle shades are the ones to be desired for representing the harmonious blending of the lights and shades.
Then, if we examine, with respect to strength, or depth of tone, and sharpness of impression, we see that the light coating, produces a very sharp but shallow impression; while the other extreme gives a deep but very dull one.
Here, then, are still better reasons for avoiding either extreme.
The changes through which the plate passes in coating may be considered a yellow straw color or dark orange yellow, a rose color more or less dark in tint, or red violet, steel blue or indigo, and lastly green.
After attaining this latter color, the plate resumes a light yellow tint, and continues to pass successively a second time, with very few exceptions, through all the shades above mentioned.
I will here present some excellent remarks upon this subject by Mr. Finley.
This gentleman says:
"It is well known to all who have given much attention to the subject, that an excess of iodine gives the light portions of objects with peculiar strength and clearness, while the darker parts are retarded, as it were, and not brought out by that length of exposure which suffices for the former. Hence, statuary, monuments, and all objects of like character, were remarkably well delineated by the original process of Daguerre; the plate being coated with iodine alone.
An excess of bromine, to a certain degree, has the opposite effect;the white portions of the impression appearing of a dull, leaden hue, while those which should be black, or dark, appear quite light.
This being the case, I conclude there must be a point between the two extremes where light and dark objects will be in photogenic equilibrium.
The great object, therefore, is to maintain, as nearly as possible, a perfect balance between the two elements entering into union to form the sensitive coating of the plate, in order that the lights and shades be truly and faithfully represented, and that all objects, whether light or dark, be made to appear so far conformable to nature, as is consistent with the difference in the photogenic energy of the different colored rays of light. It is this nicely-balanced combination which ensures, in the highest degree, a union of the essential qualities of a fine Daguerreotype, viz., clearness and strength, with softness and purity of tone.
"So far as I know, it is the universal practice of operators to judge of the proportion of iodine and bromine in coating the plate, by two standards of color the one fixed upon for the iodine, the other for the additional coating of bromine.
Now I maintain that these alone form a very fallacious standard.
