Making, Coating and Processing a Mid-1880 Era Gelatin Emulsion
Copyright 2007 Mark Osterman, Process Historian
By the1860s, curious amateurs had invented three major variants of the
wet-plate collodion process; preserved (moist) collodion, dry collodion and
collodion emulsion. Unlike the former two, collodion emulsion relied on actually
adding silver nitrate to the halides in the collodion binder before it was
applied to the plate. The basic concept that evolved from making collodion-based
emulsions was the beginning of emulsion photographic technology for the next 150
Equipment and Materials
* You can use a sauce pan on a hot plate or the crock pot (also listed) but a laboratory hot plate w/magnetic stirrer is perfect for emulsion making and well worth the expense. You can purchase them second hand on line.
** The Terumo brand 60 cc Syringe with Catheter Tip is perfect except that the opening of the tip is too wide. Go to the hardware store and buy a tube of glue that comes with a separate tapered tip of the same size. Make a pin hole in the end of the tip with a hot needle and push this firmly over the catheter tip of the syringe.
The quantities listed below are for making one batch of approximately 350 ml emulsion. Naturally it is a better plan to buy larger amounts any time you buy these materials as they are generally less expensive when purchased in quantity.
Making the Emulsion
Take the time to read and visualize all of the following steps before you attempt to make the emulsion. You may want to practice step 5 with plain water to feel comfortable with the technique. Preparing all the materials and equipment prior to working under safelight conditions will make the procedure much easier to perform.
1. Put 3 grams photographic grade gelatin into a 500 ml Pyrex glass beaker with 85 mls distilled water. Allow at least fifteen minutes for the gelatin to become fully swollen and easily flattened (or squished) between the fingers. This is called the “first melt” gelatin.
2. Put 18 grams photographic grade gelatin into a 300 ml beaker and pour enough distilled water to cover the gelatin. Allow this gelatin to absorb as much water as possible. Different sources of gelatin absorb more water than others. You may need to add more water at a later time. This is called the “reserve” gelatin which will be drained and added to the emulsion after the first melt.
3. Dissolve the swollen first melt gelatin by placing the beaker in a hot water bath such as a small sauce pan with water on a hot plate or in a crock pot with just enough water to the level of the gelatin solution. You may also use a hot plate stirrer as long as the solution is kept in motion with the magnetic stirrer. Using a thermometer, try to keep the temperature around 120 degrees F.
4. Put 10.5 grams potassium bromide and .4 grams potassium iodide in the first melt gelatin and stir the solution until the halides are fully dissolved.
[Every operation after this should be done under red safe light. The darker the safe light and the less time you have the emulsion exposed to it, the better]!
5. Prepare the silver solution by dissolving 12 grams silver nitrate in a 300 ml Pyrex glass beaker with 85 mls distilled water. Heat this silver solution to around 120 degrees F (50 C) and draw some of this solution into the syringe. Slowly squirt the heated silver solution in a continuous stream with the tip below the surface of the gelatin halide solution while you stir it continuously (this is where a hot plate stirrer comes in handy). Refill the syringe and continue until all the silver solution has been added to the gelatin-halide solution.
[When making more sensitive emulsions you can be more accurate if you use a musician’s metronome to keep you on track as you gently and continuously push the plunger of the syringe. Try to make the plunger pass a ml marking on every click, or every other click, of the metronome.]
6. As you combine the silver with the gelatin-halide solution you will see the two clear liquids change into a milky white silver bromo-iodide emulsion.
7. After all the silver has been added, ripen the emulsion by maintaining the temperature at around 120 degrees F for 15 minutes with constant gentle stirring.
8. While the emulsion is ripening, begin draining all the excess water from the reserve gelatin. When digestion is complete, add the reserve gelatin to the emulsion and stir until the new gelatin is completely dissolved. When you first add the reserve gelatin the temperature of the emulsion will fall. Bring up the temperature of the emulsion back to 120 degrees F. and dissolve the reserve as quickly as possible. Make a notation of the time it takes for future reference.
9. Pour the hot emulsion into the glass Pyrex loaf pan and carefully slide this into the black plastic bag. Secure the opening of the bag so that no light can fog the emulsion. Place the bagged emulsion in the refrigerator for several hours or until completely set to a stiff jelly. The reason a shallow dish is used for setting the emulsion is so that it will chill faster and more evenly than if left in the beaker.
[In the following steps it is advisable to wear latex gloves; not because of potential silver stains to the hands, but to prevent contamination of the emulsion from your hands.]
10. Under red safe light remove the emulsion from the refrigerator and pull the dish from the bag. The emulsion will look white under the safe light (it is actually bright yellow). Scoop out the firm jelled emulsion with the stainless steel spoon and put it into the potato ricer.
11. Place the sheer white nylon fabric in the stainless steel wire mesh drainer in the mixing bowl and squeeze the ricer to create emulsion noodles that will fall into the center of the fabric. When the emulsion is completely noodled into the fabric, gather the edges of the cloth and secure them with a rubber band. Fill the bowl with cold tap water (add a few ice cubes) and move the noodle filled fabric around for about five minutes then let soak for five more without agitation. Change the water two more times and wash the noodles as before.
12. Drain the washed emulsion noodles thoroughly for at least 15 minutes and then place them into a clean Pyrex beaker.* Re-melt the beaker of emulsion in the electric crock pot at around 120 degrees F and add “finals.” The finals listed below are: chrome alum, added to make the emulsion set to a stronger film to withstand processing, alcohol to aid in coating and thymol, to prevent bacteria growth.
[No chrome alum in the emulsion may result in a very fragile film that can melt off the plate if processed in chemicals or washed in water that is too warm. Too much chrome alum will prevent the film from absorbing the chemicals effectively. Because the characteristics of each sample of gelatin are going to be different, the quantity of chrome alum may need to be decreased or increased as needed.]
* An old stainless steel developing tank is great for re-melting emulsions. The light trap in the lid allows you to keep the white lights on in the darkroom
4 ml of a 5% solution chrome alum (5 grams chrome alum into 100 ml distilled water) Add the chrome alum drop by drop
5 ml 95% grain alcohol
1 grain thymol
When the finals are added and fully incorporated into the emulsion pour the entire contents into a brown ceramic cheese crock, cover the opening with a piece of black opaque plastic and secure the ceramic lid with the wire spring. Place the emulsion filled crock in the refrigerator for future use. Remove only what’s needed when coating a batch of plates by scooping out the chilled emulsion with a stainless steel spoon. The stock emulsion will last many months if kept cool and protected from white light.
These plates are blue, violet and ultra violet sensitive and fairly slow by modern standards. Assume an ISO rating of between 5-10 as a starting point. They are developed under red safe light, by inspection, which is a great advantage.
The Technique of Coating Glass Plates with Gelatin Emulsions
Coating glass plates with gelatin emulsions is a little different than working with a solvent based binder such as collodion, which relies on evaporation for the coating to set to a firm film. Gelatin emulsions must be heated to a liquid form, and once applied to the glass support, be able to set back into a firm jelly at an average room temperature. This so called “set time” or “setting time,” is governed by the ambient temperature of the room, the bloom and percent of gelatin and the quantity of alum added as a “final” to the emulsion.
Shortening the setting time was almost always done by quickly lowering the temperature of the emulsion. The most common approach was to level a piece of marble or thick glass and place the coated plates upon the surface until the emulsion cooled and became firm. The setting of emulsions on paper supports was generally accomplished by chilled air.
The earliest commercially made gelatin dry plates were coated by pouring the emulsion by hand. An 1884 account of the operation at the Cramer Dry Plate Works in St. Louis was described as “eight busy men, with pitchers of emulsion on one side, a pile of glass on the other and in front of them, a peculiar leveling stand.” *
* Philadelphia Photographer, Jan 1884, p 11
The following instructions are based on the techniques of the early 1880s, before the invention of cascade or transfer coating and continuous belt chilling chambers. This system is not difficult to master and enables one person to make dozens of plates in one sitting. The only limitation is the capacity of the drying box.
Equipment and Materials Needed
Cutting & Cleaning Glass Plates
Window glass is good enough for hand coated photographic plates. Make sure you purchase these from a framing supply house in unopened boxes. The plates should be interleaved with paper to prevent surface scratches. If you have never cut glass before, pay a visit to a stained glass shop, purchase the best glass cutter you can afford and ask for a demonstration.
Cut the glass to the desired size and never lay the plate surface down on anything or it will become scratched. Place the plate upright against a wall or in a wood rack. It is extremely important to remove the razor sharp burr on all the edges on both sides with a small hand sharpening stone. Dust off the powdered glass dust with a stiff natural brush. This is also a good time to check if the plates fit your holders before you coat them.
Apply a drop of detergent to each side of the glass and wash the plate under warm running water rubbing the surface thoroughly with a small square of cotton cloth. If the plates are particularly dirty you may add a dusting of fine rottenstone or calcium carbonate to each side with the detergent. Keep washing in running warm water without the cloth until the water sheets off evenly. Handling by the edges only, place the plate upright in a rack on a piece of blotting paper to dry. Once dry, breath on the surface of the plate and rub the condensation with a clean piece of the larger cotton cloth squares until you see no streaks. I have never needed to sub plates for gelatin emulsions as long as they were washed thoroughly.
Heating and Pouring the Emulsion (under red safe light)
The whole coating procedure requires very little time; less than ten seconds for a 5x7 plate from the initial pour to placing the plate on the chilling table. It is similar to coating collodion plates, though not exactly. Naturally all of the following is performed under deep red safe light conditions. Before you turn off the white lights, level your chilling tables using the spirit level.
1. Remove the emulsion from the refrigerator. Scoop out enough cold gelatin emulsion for several plates and place this in a stainless steel container (developing tank) with a lid. Put the tank containing the emulsion in a heated crock pot and with enough water to keep the emulsion warm without having the container float or flip over. Heat the emulsion until it is very liquid. The actual temperature will depend on the pouring qualities of each batch of emulsion.
2. When the emulsion is thoroughly liquefied, pour some through a large square piece of clean cotton fabric into the pouring cup (I prefer the antique invalid cup). Allow the emulsion to settle so that the bubbles rise to the top and pop. The pouring cup can be kept warm in the crock pot between pouring plates.
3. Slightly heat the plate of glass by placing it on the surface of a warm (not hot) laboratory hot plate covered with two layers of paper towels. While the plate is still warm hold it in the left hand with the fingertips supporting the back of the plate. Give the plate a quick dusting with the make-up brush.
4. Holding the pouring cup in the right hand, pour a sufficient quantity onto the center on the plate (pouring too little is worse than pouring too much). Keep the plate level so that the creamy emulsion forms a perfect circle. If the emulsion is oblong shaped, the plate is not level.
5. Gently tilt the plate so that the emulsion flows to all four corners without going over to the back side. Once the plate is completely covered, gently let some of the emulsion flow off one corner of the plate back into the pouring cup and immediately afterwards pour some of the excess off from the opposite corner into the pouring cup. Each corner from which you poured the excess will drip a small amount of emulsion that rolls to the back of the plate. Do not worry about this, it is typical of hand coated gelatin plates and this artifact can be seen on historic examples.
6. Gently roll the plate for a couple of seconds so that the emulsion redistributes evenly on the surface and immediately place the plate on the leveled chilling slab. The emulsion should still be warm and still fluid enough to level itself on the surface of the plate before it begins to set to a firm even coating.
7. As the gelatin begins to set up you will probably see some dimples on the surface and possibly some dust. Move the plate to the next chilling table until the emulsion is firm enough to place upright on a rack in the drying box. You can test the firmness of the emulsion by touching on corner with your finger, though with experience you’ll eventually have a sense when they’re ready to be removed from the slab.
The slower gelatin emulsion plates are dried the better. Fast drying can cause ridges in the surface of the emulsion. Make sure the drying box is absolutely light tight but fitted with adequate ventilation. Collect the dry coated plates the next day and place them in a light proof box interleaved with clean paper until needed.
Processing Gelatin Emulsion Plates
The earliest developers used for gelatin emulsion plates were based on either ferrous oxalate or pyrogallic acid, known simply as pyro. Unlike the calotype, albumen negative or collodion processes, all of the developing agents for processing gelatin emulsions were used in an alkaline state. Ferrous oxalate was one of the first developers used for gelatin plates, though it fell from favor by the end of the century. Pyro was introduced in the 1850s to develop collodion negatives and has continued to attract devotees well into the 21st century for development of silver based film stock. Pyro development produces warm brown silver deposits with a slight yellow stain.
By the 1890s, hydroquinone and metol based developers were being offered by photographic suppliers. Metol developers produced cool, blue-black silver deposits and developed very quickly though with little density. Hydroquinone development resulted in warm black silver deposits that developed slowly with greater density potential than metol. By the late 19th century, most photographers chose either pyrogallic acid or a combination of metol and hydroquinone, simply called “MQ.”
A typical developer formula for processing gelatin emulsions has the following elements; the reduction agent (pyro or metol/hydroquinone), an alkaline accelerator (ammonia or sodium carbonate), a restrainer (usually potassium bromide) and a preservative such as sodium sulfite. By knowing the function of these components, a photographer could tweak the formula to suit specific needs and correct exposure problems to some degree.
Development of emulsion plates is most easily done in either a white enameled or glazed ceramic tray so that the progress of development can be easily viewed by safe light. All other chemical operations can be done in any type of tray, though Pyrex glass is always the best choice as it is easily cleaned. Development is done under either a red or deep amber safe light. The effect of over or under exposure can be seen during development and the knowledgeable photographer has the opportunity to adjust the developer as needed to produce the best possible results. It was typical in the 19th century for the photographer to have small bottles of accelerator and restrainer liquids at the ready near the processing sink.
On occasion, photographers had problems with the emulsion lifting from the edges of the plate; an effect called “frilling.” Assuming the glass support was properly cleaned, this usually happened when the developer was either too alkaline or the temperature of the developer was too hot. A simple 2% alum hardening bath before or after development, or a little alum added to the fixing solution was usually enough to prevent frilling.
The Basic Procedure [under red or deep amber safe light]
The exposed plate is placed, emulsion side up, into the white tray containing enough developer to cover the plate. The developer should be used at a temperature of around 65 F. It is necessary to rock the tray during the development so that fresh developer is always in contact with the emulsion.
When developing by inspection, the most common mistake is to stop development too soon. The maximum density of an image always looks much darker when the plate is in a white tray. It will be necessary to lift the plate from the tray and inspect the progress by looking through the plate, illuminated from behind by the safe light. An average, properly exposed landscape negative develops gradually with the sky visible first, followed by architecture and eventually well lighted foliage. Do not expect to see details in the deep shadows of foliage due the insensitivity of a blue sensitive emulsion.
Once development is judged to be complete, the plate is washed under gentle running water for two minutes or in a tray with two changes of water and gentle agitation. Do not use an acid stop bath as this may shrink the emulsion causing frilling. Fix the negative in a tray of sodium thiosulfate for five minutes with occasional agitation. Wash the plate in several changes of fresh water or running water for at least 20 minutes and then place on a rack in a dust free place to dry.
Kodak Dektol can be been used effectively for processing the emulsion described earlier in this section. Begin by using it undiluted and dilute with water if you feel the maximum highlight density is too strong. You may also make your own MQ style developer. Kodak D-49 was originally formulated for processing bromide prints though it can be used undiluted for negatives made with ordinary blue sensitive emulsions. When making alkaline developers, the ingredients should be added to the hot water in the order listed and each ingredient fully dissolved before the next is added.
500 ml distilled water (around 120 F)
3.1 grams metol
45 grams sodium sulfite
11 grams hydroquinone
45 grams sodium carbonate
2.1 grams potassium bromide
Cold distilled water added to make a total 1000 ml
As with all MQ developers, metol and hydroquinone are the active developing (reduction) agents. Potassium bromide is the restrainer. Sodium carbonate is the accelerator and sodium sulfite, the preservative. If you want more density than extended development will provide, increase the hydroquinone. You may raise the pH by adding ammonia or more sodium carbonate. This is most easily done by adding drops of household ammonia. A good starting point is about 4-6 drops in 100 ml developer. Pour the developer from the developing tray into a glass beaker, add the ammonia to the developer solution and then pour the developer back into the tray. Raising the pH with ammonia or sodium carbonate will make the gelatin soften and more permeable so that the developer can be more effective, though too much will cause the emulsion to fog, lift from the glass and cause frilling.
Decreasing the potassium bromide restrainer will also cause the developer to work faster, though by doing this there is always a chance of causing fog. Tweeking the developer formula as needed eventually becomes intuitive.
Sodium Thiosulfate Fixer (working solution)
1000 ml tap water
150 grams sodium thiosulfate
There are three things you will need to construct before coating gelatin emulsion plates; 2 plate racks, 2 wood leveling stands for the chilling tables and the plate drying box.
Wood Photographic Plate Racks
Vintage plate racks can be purchased at antique shops and on internet auctions, though availability is uncertain. You can make a rack by drilling holes into the top of a wooden board and fitting a series of wood dowels. The size plate you wish to coat will dictate the size of the materials. A grooved plate rack can also be made, though this type is more requires the use of a table saw.
Leveling Stands for Chilling Tables
two 8”x8” pieces of 3/4” birch plywood
six 1 1/2”x 5/16” bolts
six 5/16” nuts
six ¼” flat washers
six 5/16” coupler nuts
A leveling stand is essentially a short adjustable tripod, with wide, flat top. Drill three 5/16” holes through the plywood. [Fig?] Countersink the holes so that the heads of the bolts sit below the surface of the plywood when installed. Install the bolts and attach the washers and nuts to the bolts on the underside of the plywood. Thread the coupler nuts on the end of each bolt. The coupler nuts allow adjustment of the leveling stand from below. The marble (or glass) chilling plate rests upon the leveling stand to complete the chilling table.
Plate Drying Box
In time you may want to make a sturdy wood box with filtered ventilation, but for your first experiments a cardboard drying box is easy to make and will do the job. You will need one, good quality, corrugated cardboard box of either double or triple wall construction and a couple extra sheets of single weight cardboard for constructing the ventilated light trap. The size of the box is dictated by the size and quantity of the plates you wish to coat in one session. While any tape will work, water soluble gummed paper backed tape (available at art supply stores) never fails over time.
Simply put, the box must allow adequate ventilation without exposure to white light. Holes must be cut into either end of the box and then fitted with a light trap as illustrated. Tape the lid closed with black tape after you fill the box with plates and for extra protection pace a piece of dark cloth over the top.
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