This section is from the American Hand Book of the Daguerreotype, by Samuel D. Humphrey. Published S. D. Humphrey, 37 Lispenard Street 1858.
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. first, because the color appears to the eye either lighter or darker, according as there is more or less light by which we inspect the coating; and secondly, because if it occur that we are deceived in obtaining the exact tint for the first coating, we are worse misled in obtaining the second, for if the iodine coating be too light, then an undue proportion of bromine is used in order to bring it to the second standard, and vice versa."
The iodine box should be kept clean and dry. The plate immediately after the last buffing, should be placed over the iodine, and the coating will depend upon the character of the tone of the impression desired. Coating over dry iodine to an orange color, then over the accelerator, to a light rose, and back over iodine one sixth as long as first coating, will produce a fine, soft tone, and is the coating generally used for most accelerators. The plate iodized to a dark orange yellow, or tinged slightly with incipient rose color, coated over the accelerator to a deep rose red, then back over iodine one-tenth as long as at first coating, gives a clear, strong, bold, deep impression.
I will here state a singular fact, which is not generally known to the operator. If a plate, coated over the iodine to a rose red, and then exposed to strong dry quick or weak bromine water, so that a change of color can be seen, then recoated over the iodine twice as long as at first coating, it will be found far more sensitive when exposed to the light than when it has been recoated over the iodine one-fourth of the time of the first coating.
Probably the best accelerating combination is the American compound formerly known as "Gurney's American compound," or some of the combinations of bromide of lime. The first is thought to possess perhaps more uniformity in its action than any other combination I have ever used.
The plate once coated should be kept excluded from the light by means of the plate holder for the camera box.
I will notice one of the principal causes having a tendency to prevent the perfect uniformity of chemical action, between the iodine and silver; hydrogen, or the moisture in the atmosphere, makes a very perceptible barrier. This moisture may arise as the result of the cold, from a want of friction in the buffing of the plate, which, coming in contact with the warmer air, as a writer on this subject says:
"It is well known that as often as bodies, when cold, are exposed to a warmer air, the humidity contained in them is condensed. It is to this effect that we must attribute the difficulty experienced in operating in most cases." This is corroborated by the results experienced by our operators. So it is seen that the plate should be of a temperature above that of the atmosphere. Mr. Gurney submits his plates to a gentle heat from a spirit lamp just before exposing them to the vapor of iodine. Experience has convinced me that a plate heated to about 80 deg. before being exposed to iodine will present a far better defined image than aplate at a temperature of 50 deg. I account for this by noticing that, at a higher temperature, the plate throws off any larger crystals that might otherwise be deposited, receiving only the finer, thus producing a more perfect chemical combination of iodide of silver. I would call the attention of the operator to this point, as presenting something of interest, and which may direct in a way of accelerating the future operations.
That the presence of a film of moisture over the plate is a preventive of uniform chemical action, may be readily understood from the fact that iodine is almost insoluble in water, requiring seven thousand parts of water to dissolve one of iodine, or one grain to a gallon of water. Yet its affinities for silver and other substances are so powerful as to prevent its existing in an insulated state, hence we can account for the frequent occurrence of a plate presenting parts of an image over its surface. It is quite evident that those parts of plate's surface covered with moisture are nothing like as sensitive to the iodine as those parts perfectly free.