Handbook of American Daguerrotype | Page 8

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.
Exposure of the plate in the Camera, and Position.--The time of exposure necessary to
produce an image upon the Daguerreotype plate, can only be determined by experiment,
and requires a liberality of judgment to be exercised on the part of the operator. The
constant variation of the light renders it impossible to lay down any exact rule upon this
point. Light is not alone to be considered; the amount of coating exercises a deviating
influence, also the subjects to be represented are not equally photogenic, some requiring
much longer time of exposure than others. This may be easily observed by exposing the
plate at the same time to a plaster bust and a piece of black velvet, the first being a much
stronger reflector of light than the latter: the time necessary to produce a well developed
image of the velvet being about six times longer than that required to produce an equally
defined image of plaster. The manner of judging correctly of the time is by the
appearance of impression after it has been developed by the mercurial vapors. Should it
present a deep blue or black appearance it is solarized or over-timed. This sometimes is to
an extent, that a perfect negative is formed, the white being represented black, and the
dark light.
An object requiring the particular care and attention of the operator is the proper focus. It
is not unfrequently the complaint of sitters that their hands are represented as being
magnified and greatly out of proportion with the general figure. This is the case also with

the nose and eyes, but in a less degree. As this cannot be wholly remedied, it is desirous
to come as near as possible, and in order to do this, it is necessary to present the figure in
such a position as to bring it as nearly as possible upon the same plane by making all
parts nearly at equal distance from the lenses. This must be done by the sitter inclining
the head and bust formed to a