Black and white print processing.
The fixing Bath
This stage of the processing and those which will follow will condition the stability of the photographs during filing.
Two types of fixers are usually employed: that with sodium thiosulfate (soda hyposulphite), that with ammonium thiosulfate. This last acts much more quickly and reduces the duration of the fixation considerably. These thiosulfates have the property to solubilize silver bromide salts not having reacted at the time of the action of the developping bath.
The fixing bath proceeds
in two phases: complexation of residual salts, then migrating to the aqua
solution. A worn bath will deposit part of its salts as well as derived complexes
( silverthiosulfates) in the emulsion and the support (fiber papers). These
derivatives being far from water soluble, we will not be able to eliminate
them, even after a prolonged washing. Thereafter, they will break up to form
silver sulphide which will result in a particularly visible yellow colouring
in the low densities of the print and, for the fiber papers, will appear on
the back of the test.
Other deteriorations (yellow spots) will be able to appear at the moment of
a tonning process. The quality of the fixing bath is thus of first importance.
The time of fix bath depends on many factors (nature of the cation, concentration, agitation, temperature, wear of the bath...); it will have to be enough long to dissolve all unutilised silver bromide. The total duration of the fixation must be double time of clarification time (Clarification time is the time which spends the film to become transparent), but in no way the photographs will have to remain unnecessarily in this bath, in particular for fiber papers where the residual chemicals are absorbed in fibers of the paper.
An even prolonged washing will never eliminate totality from the salts impregnated at the time of an abuse of fixing I call " overfixing ".
The positioning, the way the photograph is set in the tray can influence the speed of the fixation. If the emulsion is turned downwards, the complexes silverthiosulfides will migrate by gravity in the solution. If the use of a tanning fixer reinforces the mechanical properties of the gelatine, it has the disadvantage of making more difficult the elimination of salts remaining during washing.
Twice fix bath system
The twice fix bath system fixation is a technique highly advised for the processing of the photographs intended for filing.
The print having been fixed in a worn bath is then immersed in a new solution thus eliminating totality from residual silver salts. After a certain number of passages, the first bath is replaced by the second, this last being replaced in its turn by a new bath.
To prevent the micro spots on the negative (Standard NF S 20.011 -1972) it is necessary to use a fixer whose content potassium iodide lies between 0, 1 and 0, 5 g/litre.
One can carry out a control of the exhaustion of the baths of the fixer.
- In the case of a fixer
for film, one will apply the following rule:
If the time of clarification: (disappearance of milky appearance) lasts more
of the double of initial time, the bath is regarded as worn. Time of clarification:
it is time necessary to an alkaline emulsion to lose its milky aspect when
it is plunged in a bath of fixation.
This is not visible with paper media, to therefore know the time of clarification
of a fixer paper, one will test the solution with a piece of negative strip
emulsion.
- In the case of a fixer for paper, one will make a test with the formula Kodak FT 1 below following the protocols:
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Fixation in 1 bath:
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Washing
The washing of the photographs eliminates the residual thiosulfate which can deteriorate the silver image at the time of the conservation.
In a phototype well fixed but badly washed, the thiosulfate breaks up and changes slowly into sulphuric acid, sulphur and sodium sulphide. These two last can react with the silver metal to form silver sulphide. One then observes a yellowing of the image in the zones of strong and average density.The duration of washing depends on the nature of the phototype, the temperature, the hardness water (pH)... Residual salts are eliminated more quickly when the support is waterproof (film or resin coated paper).
Generally, a washing time of 30 minutes under running water is one duration sufficient for films. The fiber base paper requires a longer washing, in order to eliminate all the products absorbed in fibres. To this end, several techniques are proposed:
the test is placed in a basin where water is changed every 5 minutes (one counts approximately 20 liters of water by meter square). This operation is repeated during one hour.
Another method consists in regulating the water flow so that the basin is filled in 5 minutes. If one washes several prints at the same time, it is imperative to make sure that they do not adhere the ones to the others and that water reaches all the parts of the image.
Other installations make it possible to improve the processing of the prints on fiberbase paper. In the tanks of the Kostiner type (like those of the ENP), the photographs are arranged vertically and separated by trays. An agitation is produced by the simultaneous arrival of water and air. Although not making it possible to wash many prints at the same time, this type of tank seems very powerful. Another system of the rotary type (French mark Deville) is composed of a round tank where the prints are subjected to a circular motion created by the side arrival of water. The disadvantage of this process lies in the fact that the prints can stick the ones to the others or to break on the edges. It is thus advised to treat some little at the same time.
Temperature of water
The
more the temperature is raised, the more washing is fast, but it is necessary
to take care not to damage the gelatine which tends to weaken when the temperature
increases. Therefore it is recommended not to exceed 25�C (minimal temperature
of 16�C).
Water hardness
It is not desirable
to use de-ionized or distilled water, because the effectiveness of washing
grows with the water hardness. To avoid any rock salt deposit on the surface
of the phototype, one will take care to rinse it with distilled water before
drying.
Auxiliary of washing
We previously mentioned
that the presence of rock salt could influence over the time of washing by
facilitating the elimination of residual salts of the fixation process. It
is thus possible to shorten this operation by the simple passage of the phototype
in a salted solution . That solution is available under the name of hypo clearing
agent (Kodak) or can be prepared (formula follows).
hypo clearing agent formula
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After fixing the print, photograph is washed a few minutes with the running running, plunged 5 to10 min in this solution, and finally washed 10 to 20 min under running water. |
Hyposulphite eliminator
With a hyposulphite eliminator takes place a true chemical reaction which
transforms, in the phototype, residual thiosulfate out of sulphate soluble,
easy to eliminate.
Such a processing can be considered for archival process, but it is not without
danger: "gondolages", swinging of the print, a change of tone of
the image, a softening of the gelatine can occur. Also, we will not advise
it.
Quality control of washing and the fixing process
he standardized tests
are presented in the ISO standard 417-1977; these tests are destructive. The
maximum content authorized for usual films is 7 Mg per square meter of ion
thiosulfate. The effectiveness of the fixation is followed by the measurement
of the residual silver compounds and the quality of washing by the proportioning
of thiosulfates. After a certain lapse of time and climatic conditions', part
of thiosulfate remaining in the phototype breaks up into polythionates. The
latter, harmful for the image, are not taken into account by proportioning
with the methylene blue, the measured residual rates will be thus lower than
the effective rates after the processing.
Therefore is it significant to carry out this test quickly. The silver sulphide
method makes it possible simultaneously to measure thiosulfates and the polythionates.
This method is not sufficiently precise to answer the instructions imposed
by the standard, but one will be able nevertheless to use it.
Qualitative proportioning of residual thiosulfate salts formula:
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This solution is
to be preserved at the shelter in a glass bottle with stopper. It is
necessary to put a drop of this solution on a sample of comparable nature
and having undergone the same processing as the print. After a few minutes,
withdraw excess. If there is more than one light spot, the test is badly
washed. The darker the spot is, the more the content residual thiosulfate
is high. |
Toning
the Tonings are usually
used with an aesthetic aim. At the end of the processing, photographs acquires
a more or less hot different dial tone. However, another less visible consequence
is more interesting: in a toned phototype, silver metal is combined in a form
much more resistant to oxidizing gazes; The image thus gains a stability.
Therefore we recommend it in the case of documents which require a great perenniality,
to process to a gold toning (formula below) or selenium toning (idem). These
process will be applied only to photographs recently and perfectly
fixed and washed.
Kodak GP1 gold toning bath:
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Add the gold chloride
solution to water, and separately, dissolve thiocyanate in 125 ml of
water. Then slowly add the thiocyanate solution to that of gold chloride
while agitating quickly. |
Kodak GP2 gold toning bath:
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Selenium toner bath Kodak T55 formula :
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Add selenium and
let boil 30 min. Filter to eliminate the residues, let cool and add
ammonium chloride. supplement with cold water to make one liter. |
Drying
The weight of the water
retained by the gelatine after washing is 0, 5 to 2 grams by decimeter square,
this is variable according to nature and thickness of the emulsion. It is
necessary to add to it the water absorbed possibly by the paper support what
represents a notable quantity.
Drying consists in eliminating most of this water without it resulting neither
damage from it nor unspecified danger to the silver image for the gelatine
or the support.
A drying badly processed can modify the density of the image completely or
generally in a way unequal and partial in consequence of the migration of
the grains of reduced silver in the mass of the gelatine following the tensions
coming from its irregular desiccation.
The photoytpe in "half tone" are there prone more than the negative
ones with coarse grain, more than those with fine grain. The gelatine itself
can be damaged by total or partial fusion, in consequence of reticulations
or by swinging. If a phototype is suspended vertically, there is, during drying,
ponding towards its lower part or the desiccation will be of longer duration.
This inequality of drying is often enough to explain differences of density
and flatness. If washing were done in a strongly calcareous water, there is
great risk to see appearing spots of various forms caused by droplets or a
local concentration of the water of evaporation. One avoids them by adding
a small quantity of surface-active agent to the last water of washing added
with some drops of acetic or hydrochloric acid. A too fast drying can also
cause swinging supports in consequence of unequal tensions.
This loss of flatness is almost impossible to recover with plan-films; for
papers it appears sometimes under the aspect of a local embossing difficult
to correct. A good drying thus consists in balancing surface evaporation and
the gradual diffusion of moisture in the gelatine by progressive increase
starting from its deep layers towards surface. It is included/understood since
an air drying hot is more dangerous than a slow drying at ambient temperature
and moisture.
Hot air desiccating the surface layer of the gelatine before the diffusion
could not provide in moisture starting from the deep layers. These disadvantages
do not exist, one conceives it, with the automatic steam driers of good manufacture
in which the turbulence of the hot air avoids the inequalities of drying.
Drying of papers
papers on brilliant surface are hot or cold pressed. Those on matt or semi-matt surface are dried using a viscose sponge, of a nonfluffy fabric absorbing or blotting paper. They are then suspended or dried flat on tended frameworks of muslin, the sensitive layer downwards. They can be possibly dried prudently with hotpress, gelatin layer towards the tissue, by avoiding a too high temperature under penalty of damaging the characteristic structure of their surface. Resin coated Papers are dried in two minutes in the steam driers with hot air and in about fifteen minutes to the air of a room.
If the recommendations of this page were scrupulously respected, one must be able to produce archival quality black & white photographs which one will be able to be preserved for a very a long time.
Paris 2000, © ChristopheGlaudel for the text and respective owners for the formulas. Kostiner, Deville, Ilford, Kodak are trade marks
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