Manufacturing the Screen Plate
The screen plate is the most unique and recognizable part of the autochrome process, but not necessarily the most difficult to make. Three portions of ordinary potato starch, dyed orange-red, green and violet, are mixed together to form a neutral reddish-grey, and then dusted onto a glass plate. The glass has been coated in a thin, optically clear latex-based varnish, and the potato starch sticks readily to it. The starch is compressed with a roller. This compression is necessary, as the amount of light that can pass through a flattened grain is vastly more than unattended, increasing the overall speed of the plate, as well as overall brightness and saturation. Charcoal is brushed on to fill in any open space in between the grains, and the whole thing is covered in a protective coating called the "second varnish".
The final product is a very fine, random, color separation matrix, to which we would apply a panchromatic black and white emulsion. There is a ton of work that goes into each and every plate. Luckily, with a correct choice of second varnish, the screen can be reused indefinitely if an exposure doesn't work out.
Part 1 - Preparing the Starch (Optional)
Ordinary potato starch can be used here. For most of my work, I've always used "Bob's Red Mill" brand potato starch.
Original autochromes separated the smallest starch grains from the larger ones via levigation -- they mixed it with water, where the largest particles would fall out of suspension first. After a period of time, the water containing the smaller particles would be transferred to another container where they could settle out.
This separation is not strictly necessary, but will make the final resulting screen less grainy. All the plates featured in this post all use unsorted starch -- I have not yet made a screen plate with the finer stuff. I have sorted and dyed some of it, so I figured I'd include some notes here.
To the right is a picture of my simple levigation apparatus. It consists of two 5 gallon buckets. The upper bucket has a ball valve mounted a few inches up from the bottom. To begin, I add a full 1L measuring cup of potato starch to the bottom of the upper bucket, and fill the thing nearly to the top with water from a hose. I take a long spatula and thoroughly mix the starch into the water, making sure the thick deposit of starch at the bottom has all been suspended. The water is allowed to sit for 1 hour, and then the upper bucket's valve is opened, draining the contents into the lower bucket. The lower bucket is removed, and the starch allowed to settle overnight. After the starch has properly settled, the water can poured out without much loss of product, since most of it is now stuck in a thick layer on the bottom. The sorted starch can be collected and dried.
Recovery is quite low, so obtaining a reasonable amount of starch to dye will take several times. Usually I'll repeat the process with a second lower bucket using the same batch of starch in the upper bucket. Recover is even lower than the first time, and I figured additional runs will yield vastly diminished returns.
Part 2 - Dyeing The Starch
Whether you chose to sort or not, the next step here is to dye the starch a few different colors. The original dye formulas can be found in various places online, and I tried to stay as faithful as I could to them. The red and violet dyes are more or less the same, just scaled down quite a bit. I've found, at least with the dyes I sourced, that the green starch came out quite a bit more yellow than it should. I increased the amount of blue dye and decreased the amount of yellow to hopefully make it slightly more green.
Over time, I also started omitting a few other extraneous steps that were in the original Lumière operating procedure -- I do not heat the violet dye solution (my now purple equipment is a testament to the fact that it stains quite well enough as it is), and I do not add ammonia or sodium sulfate to the green starch. The ammonia never seemed to alter the colors in any way, and the sodium sulfate tended to crystalize into large white chunks that contaminated the green starch.
I first dissolve the dyes into 150mL of water in a 250mL beaker with magnetic mixing, and allow the dye to dissolve completely over 10-15 minutes or so. The green and violet solutions will be so dark that they appear black, and all of them become a bit syrupy. After I'm certain they're all dissolved, I slowly add 50 - 150g of starch into the beaker, still under magnetic stirring. After the addition, I give it a good hour of stirring before transferring the slop to my vacuum filtration setup. Early on I would use simple gravity filtration with a coffee filter, but vacuum filtration has several advantages -- Firstly, it removes the bulk of the dye much more quickly than via gravity (or a hand powered pump). Second, the quick removal of the dye from the starch tends to allow for a more consistent coloring, whereas gravity filtration causes some grains to be dyed more heavily than others. Even under a vacuum, removal of the thick dye solution can take 10 or 20 minutes -- patience is key.
I would recommend collecting and saving the dye solution after filtration, as it can be reused at least once, and most likely a few times before you need to mix up a new one.
After the starch has been collected and dried, you may find that it tends to clump up a bit. In particular, I tend to have the most trouble with the orange starch, as it forms tiny granules that stubbornly stuck together.
To unclump the starch a bit, I first add it to a mortar and pestle and mash it up. I then tip the contents through a filter strainer funnel I built. A lot of larger clumps won't pass through, but these can be forced through the mesh with the pestle. In general, this will break up the vast majority of the starch into a nice, smooth powder. In my experience, the green and violet starches don't need too much more work than this. If you experience more problems, finer filters can be bought cheaply off of eBay. I personally will strain any larger chunks through the 120 micron filter, returning what doesn't pass back to the mortar and pestle to be ground up more finely. If you're using sorted starch, I would strain everything again through a 53 micron strainer as well.
Ultimately with the red starch, I find that there are still clumps that just won't break down, maybe around 10% or so. Honestly, I just toss it at this point, as it's more trouble to deal with than it's worth.
Allow the three sets of dyes to dry completely for a day or two. Next, we are going to want to remove excess dye from the surfaces of the particles. If you skip this, dyes from each color may transfer to other colored particles, muddying the colors. Through experimentation, I've found that normal denatured alcohol from the hardware store does not penetrate into the starch grain. In a similar manner as before, add the colored starch to a 500mL beaker and fill it with the denatured alcohol. I prefer to magnetically mix for 30 - 60 minutes, before vacuum filtering the alcohol off and allowing the starch to dry. Dispose of the alcohol. Repeat a few more times for each color until the alcohol wash is mostly clear.
Part 3 - The First Varnish
We can now mix the "First Varnish". It's a thin, optically clear layer that adheres to the glass plate. It provides a tacky surface that the starch will readily stick to when gently dusted on.
Traditionally, the Lumières dissolved natural rubber in toluene, with a little bit of waxy damar beta-resene. I struggled trying to source for raw latex that would dissolve, even going as far as to try out unlubricated condoms. Nothing seemed to work out. Fortunately, we have a very effective alternative that is stocked in nearly every local hardware store -- rubber cement! I weighed out, dried, and weighed again a portion of rubber cement to determine roughly how much rubber it contained. The rubber cement is then diluted with xylene (toluene will work just fine too if you have some on hand) to match the proportions of the original Lumière varnish. You will note -- in my formula, there is no damar component. I'm not sure why this was added to the original varnish, but I found that it had a tendency to reduce the tackiness of the plate, rather than enhancing it. The formula I still use is as follows:
The First Varnish
Elmer's Rubber Cement, 32g
Measure out the xylene into a 250mL beaker, and place on a magnetic mixer. Weigh out the rubber cement in a disposable weigh boat. While stirring, slowly tip the weigh boat and allow the rubber cement to flow into the beaker. Mix for another 5 minute or so, until the solution is clear and homogeneous. Gravity filter the solution through a coffee filter and bottle it. This stuff will last a real long time.
Note: You're not going to get insta-cancer just by getting a whiff of it, but xylene/toluene fumes aren't great for you health either. Whenever working with the varnish, whether it's mixing it up or coating glass plates, you should probably be doing it either in a fume hood or outside.
Part 4 - Mixing and Dusting the Starch
We have our three portions of starch, now we need to mix them together correctly to form a neutral-colored screen! One might think that this is as simple as weighing out the same amount of each color, but unfortunately it's not quite that straightforward. The same mass of starch of each color may not correlate to the number of grains anymore, since the different dyes have different densities. Also, different colors may transmit more or less light than the other two. For this reason, the only practical way to mix the starch is by trial and error, and heavy emphasis on taking notes.
Firstly, let's coat a glass plate with the first varnish. This will give us a substrate on which to dust the starch we're testing. With the glass plate in one hand and the bottle in the other, pour an amount of the varnish onto the center of the glass. Gently rock the plate around, so that the varnish covers the whole surface. When done, tip the corner of the plate and let the excess run back into the bottle. Set the plate down on a level surface and allow an hour or so for it to be completely dry -- if you can still smell the xylene, it's not dry yet!
While you're waiting on this, measure out 2g of each color of starch into a beaker (or whatever other container), and mix it around with a small spoon or spatula. Eventually the individual colors will disappear, and the mix should take on a dark red appearance. You can't overmix, so be thorough here!
When the plate has dried, scoop a dime-sized amount of starch onto the plate, and spread it around a bit with a soft paint brush to about the size of a quarter. Brush the extra back into your starch container. There will be a lot of interstitial space between the grains, so to get a proper idea of the color balance we're going to need to crush it. This can be done a few ways:
1. Take a spoon and just kind of roll it around for a few seconds. You should notice the smushed starch visually darken a bit, and become a bit more shiny.
2. My preferred method is to take a 1" castor transfer ball (like these lil duders here) and roll it around in a few small circles while applying a downward pressure. You might want to try and wipe some of the oil off of the ball first, as it'll soak into the starch a bit (it's not a huge deal though).
After a small section has been crushed, observe it against a nice cool light, phone screen, or diffuse sun on a white object. You should notice a color cast. From here, you will have to determine what color to add and start the process over. Try adding 0.5g of whatever corrective color needs to be added, remix, and flatten again. This can take several tries until the screen becomes close to neutral. Don't get impatient, but don't panic if it's not perfectly neutral either. Each batch of screens I've made always have a slight color bias, and this can be corrected for later. Keep track of the additions, and once you're happy with your color balance, scale it up to make about 50-100g of starch mix.
With your full starch mix, I recommend washing them all together in alcohol one or two more times. As scary as this sounds, I've found that this gives you a nearly completely homogenous mixer, and the colors absolutely will not bleed into each other during the process. Mix as before, vacuum filtering the alcohol from the starch, and repeating until the runoff is fairly clear.