Jon Hilty
Preparing to Expose the Autochrome Plate
The Ground Glass
Autochromes requires some slight modification to be shot on conventional large format hardware, due to the fact that the plates are exposed backwards. You will need to modify your ground glass to account for the shift in focal plane. I would recommend cutting some glass from the same stock that the autochrome plates are made from. Grinding the glass is surprisingly easy! I use just a little bit of water and a spoonful of 600 grit silicon carbide tumbling powder.
I recommend checking out this video here, for further tips and instructions.
The Plate Holder
Right now, since I personally use 3/32" (single-strength) glass, the plates don't always work 100% with older plate holders. Out of the last 3 holders I purchased, two slots were offset slightly, resulting in out-of-focus exposures. Your mileage may vary.
Personally, I prefer to use "Premo" packfilm holders, with a 3d printed insert I designed originally for Lippmann plates. The insert holds the plate nice and snug against the holder, resulting in crystal clear shots every time.
I have made my design publicly available for modification or download here.
Remember: AUTOCHROME PLATES ARE LOADED GLASS-SIDE TOWARDS THE LENS!
Color Corrective Filters
If you absolutely nail the color response from the emulsion, there's no actual need for filters during exposure. However, I've found that this is exceedingly difficult to do, and that the plates tend to have a bias towards a color. In addition to color shifts, oversensitivity to a particular color results in washed colors with low saturation.
I use a Cibachrome CMY filter kit with 9x9 sized filters to dial in the color balance for each batch of emulsion. The 3d printed filter holder I designed is also available for modification or download here.
Exposing the Autochrome Plate
There are an unbelievable amount of variables when it comes to making the emulsion, so I can't exactly tell you what your exposure time will be with any amount of accuracy. I can, however, give a few benchmarks that I use when I'm dialing in exposure parameters for a new batch of emulsion. I highly recommend the use of a spot meter here, as well as some good note taking skills.
Without any extra filtration, exposures on an EV14 (a colorful object in sunny conditions) tend to be around 10-20 seconds. Plates requiring heavy color correction (my current batch, as of writing, are heavily biased towards green and blue and require 50Y+40M filters) exposures tend to be around 1m45s.
Correct exposure here is more important than, say, a typical negative process, since the autochrome doesn't have the same amount of exposure latitude. Small to medium errors in brightness can be corrected for later with intensification or reduction, but the best results will be obtained with the correct exposure time.
Processing the Autochrome Plate
The following baths are required for developing the autochrome plate.
D-19 (Stock, dissolve compounds in the order listed)
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Distilled Water.............. 1000mL
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Metol................................ 2g
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Sodium Sulfite.............. 90g
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Hydroquinone................ 8g
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Sodium Carbonate....... 52.5g (monohydrate) or 43.5g (anhydrous)
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Potassium Bromide..... 5g
Potassium Thiocyanate (KSCN) 5% Solution
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Distilled Water........................ 500mL
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Potassium Thiocyanate....... 9.5g
Reversal Bleach
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Distilled Water........................ 1000mL
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Potassium Dichromate........ 9.5g
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Sulfuric Acid (48%)............... 25mL
Clearing Bath
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Distilled Water........................ 1000mL
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Sodium Sulfite........................ 50g
The following baths are optional for processing, but are heavily recommended to have on hand at the time of development.
Superproportional Reducer
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Distilled Water........................ 500mL
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Ammonium Persulfate........ 25g
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Sulfuric Acid (48%)............... 6mL
Chromium Intensifier
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Distilled Water........................ 250mL
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Potassium Dichromate........ 50g
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Hydrochloric Acid (38%)...... 6mL
1. First Development. The first developer can be made with 25mL D-19 Stock + 75mL distilled water + 5mL 5% KSCN solution. This usually is enough to cover one 4x5 sized plate in a 5x7 sized tray. With the lights completely off, gently develop the plate for 2m30s. The developer may be used for multiple plates, but should be disposed of when you're done for the day, or it begins to visibly turn yellow.
2. Rinse with tap water for 30s - 2 minutes. After the plate has been thoroughly rinsed, it's okay to turn on the white lights.
3. Reversal Bleach. Place the plate in a tray of the reversal bleach. If made fresh, the action of the bleach can be extremely rapid, and may only take a few seconds. An older bath may take upwards of two minutes. The solution is incredibly shelf stable, and a small bath of it can be used on dozens of plates. I prefer to keep a small amount in a glass tray with a lid, to reduce the amount I have to pour it, since potassium dichromate is somewhat hazardous. You can't overdo this step, only underdo it.
4. Rise thoroughly with tap water
5. Clearing Bath. Place the plate in a tray of fresh sodium sulfite. This will neutralize the orange chromium VI, turning it to a safer green chromium III. The bath may be used for several plates, but should be disposed of after you're done for the day. You can't overdo this step, only underdo it.
6. Rinse thoroughly with tap water. At this point, the colors can be viewed if you so choose. They should be fairly weak, but visible.
7. Second Development. Place the plate in a bath of stock D-19. The plate should darken over the course of a few minutes. You can't overdo this step, only underdo it.
8. Final rinse.
Reduction (optional). Inspect the plate via transmission with a light bulb. Is the plate too dark, with somewhat muddy colors? You might improve things with the superproportional developer I described earlier. To make the working solution, ad 30mL of the reducer stock to 70mL of water. You can eyeball this, the measurements aren't super critical. I would recommend against the use of Farmer's Reducer here, as it tends to completely nuke the highlights.
Submerse the plate in the bath with gentle agitation. After 1 minute, pull the plate and observe the image. If more reduction is required, continue to agitate, observing the plate at 30 second intervals. The reducer starts out slow, but will start to work exponentially faster the longer you reduce for, so watch out! The emulsion will start to turn from black to dark brown as you reduce. Reduce until it juuuust looks right, and then wash with tap water to remove the reducer. Let the plate sit in the clearing bath for 2 or so minutes, and then rinse again and set it to dry.
Second Development
Intensification (optional). You may find that after second development, your plate looks overexposed, with blown out colors. You may want to try intensifying the plate. I don't have as much experience intensifying plates, but I've found the best results were obtained with a chromium intensifier. A few plates that were hopelessly overexposed were rescued after several back and fourths between intensifying and redevelopment in D-19. I won't be writing the full procedure through here, as this is still fairly experimental. I have found that many plates I have intensified tended to leak at the edges, and I think the bath may be causing the second varnish to lift from the glass somewhat. Your mileage may vary.
Top Coat and Mounting. The autochromes usually seem to be stable after drying, but I still recommend applying a topcoat to the plate, or risk peeling/cracking over the next few weeks. I prefer to use "Eagle" brand acrylic gloss varnish, which is a UV-resistant solvent based acrylic driveway sealer. Apply 5-6mL in the center of your completed plate, and rock the plate around until full coverage is achieved. Set level to dry overnight. It will be tacky for a day or so, and I prefer to let the coating gas out for at least a week before mounting the plate against another piece of glass. It will depend on the thickness, but when it comes time to mount the plate against another piece of glass, I just use normal black 3/4" masking tape, running along the edges.