Making the Autochrome Emulsion
If you've made it this far, congratulations! To make completed autochrome plate ready for exposure, we now have to make the light-sensitive layer, the "emulsion". I've been tweaking this recipe for a few years now, but it's by no means the definitive answer here. In fact, I wouldn't be surprised if a lot of different recipes would work here. The emulsion used here is a fairly straightforward black and white emulsion, with a couple of spectral sensitizers added in to make it panchromatic (sensitive to all wavelengths of the visible spectrum of light).
In my experience, the most crucial characteristic is that the emulsion is thin, much thinner than what typical dry-plate practitioners might be used to. Thicker coatings will cause exposure times to climb, and tends to produce muddier colors. The emulsion coating must be extremely consistent from plate to plate as well, otherwise exposure times will vary wildly. Traditional hand coating techniques can't be used here very effectively, due to the extreme precision required for a successful coating.
Due to the nature of an autochrome requiring a reversal processing, I also prefer to use a recipe with a fairly low contrast, since more typical recipes tend to result in fairly high contrast plates.
As of the time I'm initially drafting this guide, this recipe is very slow compared to traditional autochromes. Exposure times in sunlight tend to vary anywhere between 10 seconds - 2 minutes, depending on the amount of color correction required. The speed of the emulsion can still be increased substantially, with the addition of gold+sulfur sensitization, and perhaps using an ammonia digest.
The following chemicals will be required to make 400mL of this emulsion:
250 Bloom Photographic Gelatin
Erythrosine Solution (1:1000, aqueous)
Pinacyanol Solution (1:1000 in methanol)
A few gallons of distilled water
In addition, I recommend having the following equipment on hand:
Beakers (1000mL, 500mL, 2x 250mL)
Syringe with interchangeable needle tip
Part 1 - Precipitation
Make the following solutions:
Solution A (500mL beaker)
Ammonium Bromide.............. 7.42g
Potassium Iodide 1% Sln...... 3mL
Erythrosine* 1:1000................ 15mL
Distilled Water.......................... 120mL
Solution B (250mL beaker)
Silver Nitrate............................. 10.2g
Distilled Water........................... 160mL
1. Heat Solution B to 55C, with magnetic stirring (I use 300RPM), ensuring the gelatin has melted.
From here on out, LIGHTS OUT
2. Fill a syringe with a fine-needle tip with Solution B, and slowly add it to Solution A over the course of a few minutes. The solution should cloud up as the silver halides form.
3. Hold at 55C for 15 minutes
4. Take off heat and place in a refrigerator to cool.
Part 2 - Washing and Finals
Under red safelights, remove the beaker of chilled emulsion after it has been cooling for several hours. Spoon out chunks, and shred them using a potato ricer. Shred the noodles into a large 1L beaker.
Fill the beaker with distilled water, and mix with a spoon for a few seconds. Allow 10 minutes to soak, and then strain the noodles through a fine-meshed spaghetti strainer. After the noodles have been drained, scrape them back into the beaker and repeat the previous steps until you have completed four changes of water. Drain and add the noodles back to a 500mL beaker. You should have about 300mL of emulsion.
From here, I split up the 300mL into 3 sets of 100mL batches. If you plan on using the whole batch, remember to adjust the following instructions accordingly.
(Optional) Prepare a Steigmann's Solution, which consists of 25mL 1% Ammonium Thiocyanate solution, and 3mL of a 1% Gold Chloride solution. From this, take 10mL and add it to 200mL of distilled water. The Steigmann's solution will introduce gold and sulfur defects into the silver halide grains, hopefully increasing the speed of the emulsion.
In a 250mL beaker, heat the 100mL batch of emulsion to 55C. Add 6mL of the previously diluted Steigmann's solution to the emulsion, and hold for 60 minutes.
Meanwhile, add 7.5g of gelatin to 300mL of distilled water to a 500mL beaker, and allow to swell. When the emulsion is done digesting, pour the emulsion into the gelatin solution, and heat back up to 45C to ensure all the gelatin has melted. When it reaches 45C, add 4mL* of 1:1000 pinacyanol and 2mL of 1% chrome alum solution. Pinacyanol will render the emulsion sensitive to red, so be sure to use as little light as possible. Give it a few minutes of mixing, and then take it off heat for coating.
Part 3 - Coating the Screens
Alright, this is going to be a little goofy. Due to the tight requirements for coating consistency required, normal coating technique can't really be used here very effectively. Plus, as you saw in the previous step, the emulsion is incredibly thin and runny, making it really hard to hold the plate and rock in your hand, as is traditionally done.
First, a leveling table needs to be built. I slapped mine together with some parts from the hardware store -- all you need is to support a flat surface (I use glass 1/4" glass) by three screws. Using a machinist's level, level the glass as much as you can ahead of time.
I also highly recommend using a 1-10mL variable pipette for dosing the emulsion, since it can be set ahead of time, and the built-in stops make it incredibly easy to use in the dark.
Due to the red sensitivity of the emulsion, I recommend using as subdued of light as possible, while still being able to see what you're doing.
For a 4x5 sized plate, dose 5mL of the emulsion to the center of the plate. Using your finger, spread the emulsion around until it has achieved full coverage, taking care not to spill any over the edge of the plate. With the far edge of the plate against the edge of the glass, rock the plate back and fourth for 5-10 seconds, before setting it level. It will depend on the ambient temperature, but allow the plates to set level for about 45 - 60 minutes to fully gel up, before placing them in a drying rack near some gentle air flow.