Table of Contents
This page outlines the different steps or phases in the process of fabricating a photosensitive emulsion. Since there are slight variations for each protocol, we will stick to a general description of the physical and chemical phenomena which are at play in each step for a basic formulation without any added sensitivizers.
IMPORTANT | All the emulsions described in this wikifilm are neutral and prepared according to Maddox, that is, they all contain silver bromide and iodide crystals suspended in a gelatin: we can cleverly call this a … gelatino-bromoiodide silver concoction. There are other ways of preparing emulsions as well as other ways to make a more sensitive emulsion with specific propeties. A page will be dedicated to that further on.
Preparing the Solutions
To start off, we must prepare 3 solutions, solution A and B which when mixed will form the gelatinobromoiodide crystals, and solution C which will be added afterwards to readjust the amount of gelatin.
During this step, we prepare the ingredients in solutions whose proportions will determine the characteristics of the emulsion. These proportions are given by the formulation which you'll have recovered in a “recipe” or that you'll have determined yourself by following an ensemble of complex or simplified rules if you are game (see help for formulating one's own emulsion).
The solutions are prepared at temperatures defined in the nucleation phase.
Solution A contains:
- WATER, which will play the role of the solvent, and should be demineralized.
- POTASSIUM IODIDE (Symbol KI), to form the large centers of sensitivity. We could imagine adding more and more but the quantity must remain reasonable to avoid some problems, as is the case for most of the ingredients: there are minimum and maximum amounts which will work. Remember: if you want to modify a formulation, consult the page aide à la formulation de sa propre émulsion) .
- POTASSIUM BROMIDE (Symbol KBr), to form most of the sensitive crystals. The excess of KBr which is added to solution A doesn't come into the formation of crystals, but instead plays a determining role in the form these crystals take and thus plays a role in determining the sensitivity or contrast of the emulsion…
- GELATIN, of “photo grade” quality. A fraction of this gelatin will be integrated into the grain, envelopping the crystals, allowing for the color sensitivizers to attach themselves to the surface of the grains. The gelatin acts as an intermediary, ensuring the transfer of energy from the sensitivizer excited by the light to the silver bromide, sensitive only to blue.
Solution B contains:
- WATER demineralized
- SILVER NITRATE (symbol AgNO3), which contains silver in its ionic form (Ag). This is the guy who (in simplified terms) can be said to be sensitive to light, but also, forms the latent image and, after developpment, forms the final image by becoming black. Note: silver nitrate in solution is slightly sensitive to light, thus solution B should be prepared in red light.
Solution C contains:
- WATER demineralised; in some formulations, there is no water.
- GELATIN, “photo grade”. This gives the right consistency to the emulsion, linking the crystals, all the while its gelatinous structure permitting a good diffusion of free substances inside the emulsion (for washing the emulsion, or developping it..) During the preparation, the presence of gelatin has several effects which we will precise at that time. The “solution” C is to be added at the end of physical maturation.
In this step we mix solution B into solution A, thus setting off the precipitation of silver salts sensitive to the light. This step, like all which follow, must be carried out in red light. (inactinic safelight).
If the formulation is a determinant factor for the emulsion properties, the methodology in carrying out the preparation overall, the process, will be just as influential on the final results. That is; for the same formulation, different manipulations will give the emulsion different characteristics.
For example, when we mix everything together in a short amount of time (1 to 3 minutes), we are apt to end up with a high contrast emulsion due to a homogenous crystal size. If instead we mix little by litte, by stages, we will get crystals of different forms and sizes; and theoretically, with more nuances of gray in a negative. Remarks: the two principles just described are conditional since the other phases of production also have an effect on the characteristics of the finished emulsion. The protocol should be defined beforehand so as to ensure global coherence.
The first two minutes following the mixing of the two solutions are determinant for the rest of the preparation, since they precondition the number of crystals and their forms, etc.
The chemical reaction which takes place is the following:
Potassium Bromide (or iodide ) Silver Nitrate —> Silver Bromide (or iodide) Remnants (potassium et nitrate)
KBr AgNO3 ⇒ AgBr (ou AgI) (K , NO3-)
In reality, we have :
(K , Br-) (Ag , NO3-) ⇒ (Ag , Br-) (K , NO3-)
At the start; since it's only weakly soluble in water; the silver bromide precipitates in an amorphous way:
(Ag , Br-) ⇒ AgBr
At this point, we have a salt but not a crystal properply speaking, not at all as we would like them, since crystallization implies the structured arrangement of atoms which costs energy and takes time, relatively…
So! we have to backtrack very very quickly, solubilize the amorpheous silver bromide in order to help it crystalize nicely, in an octoherdic array rather than cubic.
AgBr ⇒ (Ag , Br-)
This takes energy since we have to break and restructure the salts. This energy will come from vigorous stirring, a high temperature (70°C), the change in equilibrium in chemical species caused by the excess of potassium bromide and the presence of gelatin which adds a little bit of distance, facilitating dispersion and avoiding aggregates which would cause black specks (“peppering”). This is a simplified representation of a still more complex system, but which in practice is easily manageable.
If we use demineralized water up to the washing phase in the preparation of our emulsion; it is so that there be no competition or change in equilibrium in the important chemical reactions.
This step comes before the physical maturation because of its short duration. They are actually inseperable and are merged more or less if the mixing is staggered over a longer period of time.
To increase the global sensitivity; we give the crystals the time to grow by incorporating the silver bromide created by smaller crystals. The excess of potassium bromide, an elevated temperature of 70°C, and permanent stirring augment the solubility of silver bromide, facilitating its transfer from small to large crystals.
The gelatin plays an important role by enforcing the effects of stirring and high temperature, preventing the crystals from linking together until the end of the physical maturation. This phenomenon called coalesence; in which the little crystals come enlarge the big crystals, this uncontrolled growth can be useless since there is an upper limit to the crystal size after which no more sensitivity is gained. On the other hand; too much gelatin during physical maturation can have a slowing effect on this stage since the gelatin is not a pure substance and contains some molecules which delay this step.
We stop the physical maturation at the optimal time, that is, before fogging of the emulsion or before the moment when any increase in grain size no longer brings higher sensitivity.
But just before, we add solutoin C (gelatin) which will permit, after cooling, for an emulsion with a consistance we can manipulate during the washing phase. If there is no water in “solution” C, forsee enough time between introduction of the gelatin and the end of maturation for the gelatin to melt and give a homogenous mixture.
Once cooled but still liquid, we pour the preparation into a square or rectangular box, with a flat surface and allowing for a final thickness of about 3 to 5 mm. In our case, we use super8 box covers (two covers of 90m boxes for 170ml of emulsion).
To ensure complete gelification, the emulsion is place in a refridgerator for 20-30 minutes. Once the gel is hard enough, it is easy to cut it using a knife (stainless steel) in small 3mm cubes.
The smaller the cubes the larger the surface area in contact with water, favorising transfer and reducing the washing time. If the size of the cubes is too small though, some manipulations may be complicated.
This phase washes out the chemical elements we don't need which were introduced with the reactants:
Le potassium associé à l'iodure et au bromure.
- the potassium associated with iodide and bromide.
- the nitrate, complementary to the silver.
- the excess potassium bromide, essential in the first “physical” maturation in order to help the growth of grain but which would strongly diminish the gain in sensitivity during the following “chemical” maturation phase.
These first four steps are to be carried out in one go. The rest can be taken up the day after, or the day after the day after, as long as the emulsion is kept refrigerated.
The goal of this chemical maturation or remelting phase is to increase the number of irregularities in the crystal structure, thus creating more sensitive centers, augmenting the emulsion's sensitivity. In this phase the gain of sensitivity depends on the time, the temperature, and especially the addition of sensitizers such as sulfer, gold, etc. If no sensitizers are introduced, the chemical maturation is simply physical and no longer really justified.
Chemical maturation is done in a water-bath whose temperature is stabilized between 50-55°C. It starts off with the introduction of a sensitizer such as sulfer. The amount of sulfer added depends on the amount naturally present in the gelatin. In inudstry…
more info on sulfer inclusion coming soon
Coating is the step in which we coat the support (film, glass plate, photo paper, etc.). The support must permit strong adherance of the emulsion, both during coating but also after drying. In function of the support, an adhesive layer called the substratum may be necessary. If we recycle film, we can use the original bleached or fixed layer of emulsion as the substratum (without silver or silver bromide, if it was unused expired film).
The most difficult aspect in this phase is managing a fine control of thickness of the layer: if it is too thin, the density will be insufficiant. If it is too thick, we use up too much emulsion and it is expensive. Furthermore, a thick emulsion takes a longer time to develop and the film itself loses flexibility. We confirm that presently, without a precise and identically reproductible technique for coating film, comparing two film emulsions has little sense. If the emulsion layer is thicker, the film will be more sensitive.
In the process
- To be coated, the emulsion must be liquid and thus hot.
- The viscosity you want will depend on your method of coating; tests must be done.
- The hotter the emulsion, the less viscous it is.
- Viscosity is a factor which will determine the thickness of the emulsion layer (the more viscous the thicker the layer).
- At one given temperature, two emulsions made by different formulations will have different viscosities. This will especially be determined by the bloom of the gelatin and by the percentage of gelatin present in the emulsion at the time of coating (we can calculate this after formulation but one must also take into account the water lost by evaporation, which can be hard to master but which can also be compensated for).
Several hours are needed for partial dehydration of the emulsion …
tbd: effects of hot/cold drying, humidity, and their effect on the sensitivity
In the industry (chez Kodak for example), emulsions are stored away during a year before being sold or used. During this time, the emulsion continues to gain in sensitivity.