Don't worry, I've gotten the bad jokes out of my system with that title. Recently I had a bad case of diarrhea of the mouth, or at least what counts for it over text, in regards to gallium as a mirror. It doesn't take long for the mind to wander to find replacements for mercury in the process, particularly since it is used for its physical properties rather than its chemical ones. For the uninitiated, a Lippmann plate requires a mirror in direct contact with the emulsion. The mirror reflects light back into the emulsion, ultimately causing an interference pattern. The pattern is recorded in the emulsion, allowing colors to be reconstructed when angled into reflected light. Mercury, as a liquid, is free to flow directly in contact with the emulsion. Back in Gabriel Lippmann's day, they used a specialized plate holders that would allow the photographer to pour in the mercury immediately before exposure and drain it afterward (continued presence of mercury would destroy the latent image). Even back then, Hermann Krone knew that the mercury could be omitted from the process entirely, instead relying on the reflection caused by the air-gelatin boundary of the emulsion. What this adds in safety, it sacrifices in quality -- the interference pattern is significantly weaker, resulting in greater exposure times and weaker colors. Having lived with myself my entire life, I know how unreasonably clumsy and forgetful am. Handling mercury, at best, will result in my early death and, as both a Lippmann and Daguerreotype enthusiast, this is a problem. What reasonable alternatives to the mercury Lippmann mirror are there? Gallium Enter gallium. melting at roughly 85F, it's a shiny, silver metallic compound that sure as hell looks a lot like mercury. I'm not the first to think of this and I surely won't be the last -- but as far as I can tell no one has documented their trials and (almost exclusively) tribulations. Here's what I have seen, so far:
Gallium can be heated and applied to a plate. It can be 'sandwiched between two glass plates. If it's sufficiently thin, it can be peeled like a foil -- thicker and it will snap, thinner and it will flake.
I was unable to get a uniformed distribution when sandwiching by hand, due to the outside of the liquid gallium forming a 'skin' -- it would burst at the weakest point and run straight to a corner of the plate without spreading. Heating the plate and coating area above melting point may remedy this, but I'd be wary of fogging the plate.
Gallium is incredibly messy. It will leave a trail of metallic destruction on you and everything it touches. Fine metallic smudges will mark all your favorite darkroom equipment. Left-handed people forced to write with pencil know what I'm talking about.
Gallium cannot be melted off the plate. Unlike mercury, gallium clings to everything. Gently scraping the liquid off still leaves behind metallic smudges (and massive scratches on your emulsion). Thicker coatings could not be removed without fracturing the glass.
My experiments with gallium as a mirror ended in 2015, and yet to this day I'm still finding chunks of gallium falling out of my wet plate holder. Gallium, not even once.
I think gallium warrants further investigation as a mirror. Possibly an environment where the plates are warmed and spaced apart a millimeter or so -- where the gallium could be poured between them and allowed to solidify. Still, I don't envy the unlucky soul to take a bunch of gallium plates out on a 95F day. One final, crazy thought: gallium-oxide is used to coat the insides of gallium thermometers to make it behave more like mercury -- more flow, less stickiness. Could a small amount be mixed into an emulsion to allow it to behave accordingly? Other Liquid Mirrors Wikipedia has a pretty good list of alloys with low melting points. Here's a couple interesting ones:
Ceerolow 117: Melts at 117F. Could be melted onto a plate, hopefully without fogging it. Contains cadmium and lead, so it will be somewhat toxic.
Galinstan: Melts at -2F. Contains gallium, but non-toxic. Appears to have the similar properties to gallium.
NaK alloys are explosions waiting to happen.
Caesium and Rubidium: also explosive.
Solid Alternatives
Adamant to find a mirror that won't poison me slowly or blow me up quickly, here are a few other options to look into:
Silver film: See p. 345 of "An Experimental Study of the Lippmann Color Photograph" by Eugene Ives. It details how silver can be applied to a flexible varnish and applied to the plates when they are still wet. The plates are slow to dry, apparently causing the best red sensitizers to fail (particularly pinacyanol). Ethyl violet may work, but in general I don't much like it in Lippmann photography.
Nickel mirror: See p.8, The British Journal of Photography, Jan. 1910. Unfortunately it doesn't really go into much more detail than that.
Aluminum Mirror Film: Hans Bjelkhagen has stated many times that this is not an option, due to the rapid formation of aluminum-oxide on the surface of the film as soon as it comes in contact with air. I have no reason to contest this by any means, as he is incredibly knowledgeable in the subject. It's just that I have a 3'x25' roll of the stuff now and I have to do SOMETHING with it.
A little bit of experimentation has shown that a 'breathing' on a dried plate and applying aluminum film allowed it to stick pretty convincingly. I wonder if a plate could be humidified to a state to where it could receive the mirror in direct contact, but not so much as to be 'wet' (thus reducing drying time and, hopefully, pinacyanol's red sensitizing abilities). Hopefully by the summer I'll be making my own flexible mirrors and starting back up my mirrored plate experiments. If you've made it this far, I commend your ability to sift through my ramblings. Thanks for reading!