Jon Hilty
The Lippmann Guide Part 4 - Mounting and Viewing
Once the plate has dried, you will be able to see the colors when angling the plate into a light source, the more diffuse the better. If you want to stop here, that's absolutely fine! But Lippmann plates, in their absolute top form, also have a "Wiener" prism attached to them. This solves a few issues - firstly, it separates the "surface reflection" (the glare of the light source off the plate) from the "metallic reflection" (the colors produced from the fringe pattern recording in the emulsion, aka the part that you want). Secondly, the prism stops the plate from redshifting or blueshifting in response to changes in ambient humidity. Although this is admittedly one of the coolest aspects of the process, the novelty has well worn off for me to the point where it's "kind of annoying to deal with". And lastly, it protects the image. The dry emulsion is incredibly fragile, and prone to scratches. Getting any oil on the surface (especially from fingers) will temporarily totally obscure the colors in that area, and it can be a bit of pain to clean off.
It is said that the prism should be about 10 degrees. I haven't played around with different angles all that much, but this seems "about right". Early on when I barely had the money for it, I accidentally commissioned a bunch of 4x5 prisms with a 6 degree angle instead of 10, and these still worked fine - they were just a bit more annoying to view, since the angle of the prism's surface reflection is a lot closer to the metallic reflection.
Sourcing The Prisms
Just Buying Them
I've been purchasing prisms from Nanyang Jingying Trade Co., Ltd. on and off for a few years through Alibaba, with generally positive results. After factoring in shipping, 6x6 prisms come out to about $5/piece, and quarter-plate about $6. Every once in a while there is one with a scratch or whatever, but it's pretty hard to argue with the price. You do have to order in bulk, though, which may not be ideal for someone who just kind of wants to play around with this process.
Making Them
You can actually pretty easily make your own prisms, by using two piece of glass and filling in the volume between them with a polyester resin. These tend to have a very slight amount of chromatic aberration compared to their glass counterparts, but it's very subtle. Since I usually coat the emulsion on 8"x10" plates and cut them down, I have a lot of extra 6cmx6cm plates that I don't really know what to do with, so I will clean off the failures and use them as glass stock for acrylic prisms.
Originally I used Eager EP4101, which was recommended in the original documentation by Hans and Darran. This appears to be no longer available. After some research, I picked out "Fiberglass Warehouse" brand "Clear Casting Resin", which as far as I can tell, is exactly the same product.
You can use some 10 degree triangular spacers to tape up some glass roughly into prism-shape, and then pour in the resin to fill them up. Smaller prisms don't really have issues with bubbles, as they have plenty of time to float to the top before the resin starts to harden. You might need to look into a vacuum chamber or pressure pot if you have trouble with bubbles getting stuck in larger prisms. Apparently I made these 3D printed spacers in like 2019 but I have zero memory as to how I used them - maybe you will find this helpful.
Eventually I got sick of taping these things up all the time, and made a silicone mold of a few existing prisms. This worked extremely well, and is probably worth the investment if you plan on making a lot of them.
These prisms do seem to fail spontaneously sometimes. I do not know exactly what is the cause, but I believe it might be due to rapid changes in temperature. Prisms that have been perfectly fine hanging out in my house for years suddenly would experience a separation of the polyester from the glass if I would, say, mail the item to someone. Polyester might not be the most robust choice for this application, but it should work fine if you're just experimenting with the process.
Casting polyester prisms in a silicone mold.
Testing the Image
A lot of times, a plate that looks great without a prism, doesn't look so great with one on, particularly with underexposed plates. The issue is, when the surface reflection and the metallic reflection are combining, sometimes the surface reflection does a bit more heavy lifting than it would seem. There have been more than a few times where I cemented a prism on with Canada balsam, only to immediately regret this, as the image suddenly turned dim and weak. Additionally, particularly with mica plates, the image appears overly yellow when viewed out in the air, and much more balanced and colorful with a prism on -- this can make it difficult to discern when you've actually made a really nice plate. To avoid this, I like to test out the plate first before committing to Canada balsam.
Early on, I would just dump the plate in a tray of some sort of organic solvent - isopropyl alcohol, ethyl acetate, acetone, toluene, pretty much any of these work. However, having containers of this stuff just kind of hanging out in your house is a recipe for an accident. Additionally, the tray of solvent would eventually start to accumulate dust and whatever else is on the plates when you plop them in.
Now, I prefer to just take some solvent and put a few drops of it on the plate, and then lay a prism down on top of it. I use lab-grade toluene for this, because it matches the final representation of the image versus Canada balsam pretty well, and it leaves a minimal amount of residue once it dries. Don't worry about the residue too much either - it goes away if you do decide to fully mount it.
If you have a lot of shadows that are totally clear, it might be useful to repeat the process, but use a black and solvent-resistant (not plastic) backing under the plate. I like to use an old piece of anodized aluminum leftover from my wet-plate days.
The Viewer
You can use any old diffuse light source to see these things, but a viewer makes it about 100 times easier to see with your eyes, and about a billion times easier to photograph them.
There's nothing super special or complicated about the viewer - it's a box with a cheap Fresnel lens on top, and a holder to angle the plate slightly. You can make it out of a cardboard box if you want to, there's nothing super specific about the dimensions I chose. I have a lamp positioned above the box with a bright, high CRI bulb. Positioning this close to the lens's inflection point (about 18 inches in my case) projects a very bright and even spotlight onto the plate.
Plans for a 1/8" (3mm) laser-cut version can be found here. By my own admission, after making a bunch of these with the intention to sell them, the 1/8" version is a little flimsy. I am currently redesigning it for 1/4" (6mm) wood, which will be available Soon™. Since I have also recently started making these in quarter-plate format, the old version was a bit too small to accomodate plates that size. I will also make the quarter-plate version available Soon™.
Cementing and Backing the Prism
If you're happy with how the plate looks, and you're ready to commit it to a prism, it's time to cement it with some Canada balsam. Beware, it's not difficult to do, but it is lengthy and messy.
Canada balsam is basically tree sap with a refractive index very close to that of glass. It's really sticky. Don't even think about opening that bottle unless you have some acetone and paper towels on hand. Washing your hands with soap will not work. Without acetone, the only way your fingers will stop being sticky, is when they've accumulated enough lint to fully cover all exposed sticky zones. You might get away with using gloves, but in my experience, the reduced dexterity involved with using them just causes me to accidentally touch the stuff more than I would have otherwise. Latex will straight up rip when trying to separate your hands. You have been warned.
I like to do all my work on an old piece of plywood, since it's flat, I can carry the workspace around, and I don't mind if it gets Canada goop all over it. Typically I will lay down a few layers of paper towels, with the plate flat on top. I've used a few different sources of Canada balsam, but Wood Finishing Enterprises seems to have the most competitively-priced product at the time of writing. I use the real shit, not stuff thinned with toluene or any other solvent.
For a 6x6 sized plate, I'll usually pour about a "quarter" sized blob in the middle of the plate. For larger plates, (especially if they are not square), start by pouring a little bit in the middle, and then towards a corner and back for each of the corners - this will draw a bit of an "X" shape. Slowly lay the prism down, first contacting the balsam blob at a bit of an angle, and then flatting it while applying firm pressure down. The balsam will spread across the plate and squish out the sides on to the paper towel that you hopefully laid down. While the downward pressure should be firm, don't "force" it too hard - applying too much pressure will overcompress everything, and when you finally do let off, air is liable to enter the sizes as everything relaxes a bit. Once the plate is fully covered, you can stop pressing it -- if more Canada balsam wants to flow out, it will do so on its own accord. Even if it doesn't seem like it, the plate is still pretty slipslidy, and the viscous nature of Canada balsam means the prism will be more than happy to start sliding off the plate at 1mm/minute the second you walk away. It's best to place a few objects against it all to gently brace it. After a few hours, wipe the excess goop off the sizes, and continue to brace it.
If you have a few bubbles between the plate and prism, ideally you want to squish them out when initially putting the prism on. If you still have some afterward, don't panic quite yet - very often, they will reduce in size over the next 24 hours until they are imperceptibly small. If you still have some after a day or so, and they really annoy you, you might need to remove the prism and start again.
The drying process is slow - typically after a couple days the plate/prism will be firm enough for general handling/admiration, but truly it will take a couple of weeks before the edges are truly dry. Any remaining prism cleanup can be achieved with acetone, Windex, paper towels, and a dream.
The final black backing can then be applied. I like to use black enamel for this purpose, and to put a good 3 coats of the stuff at least.
Wrap Up
It is very unlikely that your first results will look anything like the examples I've shared. In fact, if you've even been able to achieve any sort of color replay at all, that is a commendable achievement, and you should be proud of yourself.
The fact is, at the point of writing, this process is far from unsolved. I get better every year that I work with these things, and I am still learning - but for every success I share, there are probably 10 total failures, and hours of time "wasted". I've been occasionally referred to as a "master" of this process, but successive plate I make, it becomes more and more clear to me that this is unlikely to ever be true.
Take good notes, don't cross any black cat's paths, and I hope you can find enjoyment in this strange little process. I look forward to seeing what you can do.