I've been stringing people along, telling them that I plan on adding a new autochrome blog post when I "figure things out a bit". Well, the truth of it is -- I never did! While some of the plates from earlier this year showed great promise, in general I have mostly failures ever since then. I've concluded that I was temporarily extremely lucky, the stars aligned, and I sprinted through an obstacle course of stumbling blocks more or less blindfolded. Subsequent attempts have shown to be less successful. At the very least, due to a finite amount of realistic possibilities for me to encounter, I can only assume that I will soon be running out of new problems to run in to. Knock on wood.
Updates on the Screen Plate Manufacturing Process.
Notable changes in varnish recipes.
The First Varnish
As the months grew colder, I began to have issues with the starch not adhering to the first varnish. I had run out of my first batch of "First Varnish" that I had made that summer, and it had always worked. Instead of providing a sticky, optically clear layer to the glass, the layer tended to be somewhat foggy and hard, with the starch easily brushing off. I found that heating the plate > 100F helped significantly to increase the stickiness factor, but I was frustrated that this was even a required step, since it would bottleneck the process quite a bit.
Ultimately the culprit was found -- the damar residue, leftover from the manufacture of the Second Varnish, seemed to actively inhibit stickiness! I had been led to believe that it aided in stickiness. I found that omitting the damar resin entirely from the formula resulted in a varnish of superior stickiness. The new formula is as follows:
• 14g of Elmer's Rubber Cement
• 90mL of Xylene or Toluene
This officially makes this varnish by far the easiest to whip up. Glass plates are still coated in the same manner as before. The sides are taped with 1/4" masking tape, and a quantity of First Varnish added to the middle. The plate is rocked gently, allowing the varnish to flow into contact with the unmasked area in its entirety, before draining the varnish back into the container from a corner. This method is remarkably similar to coating plate with collodion, though the varnish is a good deal more viscous.
The Second Varnish
Issues with "Clouding"
In the same manner as before, once I used up my first working batch of Second Varnish, subsequent attempts to make the stuff were apt to fail. Often in colder conditions (it was winter, after all!), the varnish would "cloud" up. After the ethyl acetate had evaporated, I would feel safe taking the plates back inside (since there remained only trace amounts of amyl acetate after the ethyl acetate had evaporated). If done promptly, the clouded areas of the plate would clear after some minutes, leaving a slight discoloration in the starch. If forgotten about, the clouded areas would heavily discolor the starch beyond use. With my original Second Varnish mixture, this was only noted on screen plates when applied below 50F. With newer formulations, this seemed to occur at a wide range of temperatures, even up to 95F!
After voicing my frustrations with a friend, they had mentioned that the "clouding" could be due to water contamination, condensing out of the air as the plate dried. This made much sense, as the highly volatile nature of ethyl acetate causes the temperature of the plate to drop by several degrees. Often, the areas of least discoloration lined up to be exactly the size and location of the bubble level rails the plate was laid on -- the bubble level was acting as a heat source, and preventing the contamination!
My workaround is as follows:
1. Level the plate with a bubble level, so that it is as level as you can possibly get it.
2. Apply heating via an infrared lamp. Attempt to heat it above ambient temperature, without going higher than 100F (to prevent solvent boiling) or below 80F (to prevent "wrinkles").
3. Apply measured amount (in my case, one tablespoon) of Second Varnish to plate. Rock to allow full coverage and allow it to dry.
4. Heating is removed when the ethyl acetate has evaporated. Allow the varnish to harden (no longer sticky to the touch).
5. Bake the plate via infrared lamp until all traces of amyl acetate are gone (the plate no longer smells like banana).
Step 4 is the most critical and requires the most babysitting. Overheating results in bubbles from boiling, and tends to cause the starch grains to drift apart, thus allowing a significant amount of white light to pass through. Underheating results in wrinkles and, in critical cases, moisture contamination. The wrinkles actually end up looking a lot worse than they are. They are very easy to see, except via transmitted light through the plate, where they are hardly noticeable at all. This may not be ideal for the perfectionist, but for practical purposes they seem to be fine.
Second Varnish Recipe
The recipe remains largely unchanged, but for convenience I'll re-print it here.
1. Add the gum damar to the ethyl acetate and stir magnetically for several hours (or, if cold, a day) until there only remains a fluffy white insoluble solid. Protect the container from evaporation and moisture contamination by keeping the top sealed with saran wrap and tape.
2. Cool the container (leave it outside if during the winter, otherwise let it chill a bit in the freezer)
3. Filter out the white solid with a few stacked coffee filters. The cold solution will likely still be a bit cloudy, and should clear as it warms
4. Add the nitrocellulose lacquer with some mixing, and bottle the varnish.
With this formulation, one full tablespoon is exactly how much is required to cover a 4x5 plate. YMMV. All work when dealing with open containers should be done either outside or a fume hood.
Thoughts on Alternatives to the Second Varnish
With the varnish in its current state being as picky as it is to make and use, I think it would be beneficial to find a more modern alternative. Initial tests with standard polyurethane from the hardware store have worked out very well. To coat with polyurethane, apply an undiluted amount and rock the plate in hand, allowing for full plate coverage. When finished, tip the corner and drain excess back into the container until the corner starts to drip. Lay flat and allow to dry for a few hours. It may be best to wait a full day or so for it to gas out completely.
Polyurethane was more or less indiscernible from the cellulose based varnish -- no issues with dyes bleeding during application, nor any issues with the emulsion not sticking (as what happens when using pure Behlen cellulose varnish). It should be noted that the only existing test with polyurethane ultimately failed when scrubbing the emulsion off. The emulsion had already hardened considerably though, so I can't say with certainty if the cellulose based varnish would have performed better.
I'm currently waiting for a solvent based acrylic to arrive, which is typically used to seal driveways. It's UV resistant and should not yellow, and unlike most acrylic lacquers it's not aqueous (which would put the starches at risk of bleeding). Hopefully that pans out!
Updates in Pressing
My newest plates are all compressed via a modified CNC router. The machine applies as much force to the plate as possible before the belt skips. Using a window roller (with sharp corners sanded smooth), the machine rolls back and fourth several times, before lifting the roller, shifting 0.5mm to the right, and performing the roll operation again. This continues until the machine reaches the end of the starch area of the plate. With the Z axis springs removed, 10 passes per row seems to compress the plate effectively. The whole program takes about 40 minutes to run, which is on par for hand compressing with the 1" roller bearings I had been using previously, which normally take about a half hour. I worry that the ground gained from saving one's hands from cramping may be lost by wear and tear on the CNC's stepper motors, since the movements are all extremely repetitive. I am currently in the early steps of designing a dedicated starch press, hopefully to eliminate a massive bottleneck in screen plate creation.
Since the roller has two points of contact, eliminating the second point should allow for double the pressure applied (since P = F/A). A few weeks ago I shaved down one side of the roller with a dremel, but this ultimately was a failed endeavor -- the single contact point caused the whole bearing to tip, damaging itself and cutting into the plate. Perhaps a bearing with the flat ring in the middle would work much better -- however I can't find such a thing. For the meantime, less pressure + additional passes achieves the required level of compression.
Clearing the Screen Plates
This is just a small note, but it's somewhat significant and I'm not sure if I actually mentioned it anywhere online. After compression, residual graphite and talc tends to get pressed in as well, causing the plate to overall look a bit spotty and dirty. Before pressing, anything more than a gentle brush will easily remove starch grains. However, after pressing, the grains are thoroughly stuck to the plate. The plate can be wiped very aggressively with a paper towel to remove excess graphite and talc, causing an overall increase in transmitted light, as well as a MASSIVE increase in consistency across the plate. Many imperfections, especially those created when hand pressing the starch with the roller bearing, are nearly entirely removed.
To Fill, Or Not To Fill?
I performed some comparisons to see how the screen plates looked when using or omitting graphite and talc completely. Classically, the graphite was used to fill in interstitial spaces between the grains, and the talc wash used to remove excess graphite. For me personally, with the present large grained starch, I'll be omitting the graphite and talc steps entirely. It seems to increase transmittance by about a half stop, which is fairly significant. The overall brightness consistency across the plate is the best I've seen yet with this process. However, I will be re-running this test when I begin to use smaller grains, as those may have a greater rate of interstitial space relative to the grain size.
Some Notes on Degradation
I have noticed that some of my earlier plates have been subject to a few different types of degradation over the last several months. I believe the Second Varnish layer was significantly softer on older screens, before I added the "baking" step to the process. Once the outer layer of the Second Varnish hardens, the amyl acetate doesn't seem to be able to continue to evaporate, locking it inside and keeping the layer somewhat malleable. Even when holding the plate with one hand, your thumb is apt to leave behind a thumbprint. Significant cracking was noted weeks after my initial autochromes were made, likely from the gelatin layer stressing the rest of the plate. One interesting thing to note that, in many instances, the cracks are completely white and the plate doesn't loose color -- meaning the gelatin is dragging the starch with it! In many cases, there is little or no color loss observed.
Baking the plates after Second Varnish doesn't totally eliminate "wrinkling", but it does seem to slow it's effect considerably. Similar to the cracking, I believe these wrinkles to be the result of stress from the gelatin layer. From what I can tell, the only way to eliminate this from occurring is to apply a final layer of varnish to the plate after processing is complete. A plate made with an original, unbaked screen made in May has yet to show any sides of degradation at all when a second coating of Second Varnish was applied a few days after processing.
The most saddening and disturbing degradation I have noticed is desaturation - where the gelatin slips across the second varnish, causing the silver grains to block random starch colors instead of an ordered one. This results in some or all of the image switching to greyscale. This seems to slowly spread across the plate, so if this is observed it would be best to apply a final coating of some sort, which seems to slow or stop it from happening.