How I Coat Silver Gelatin Emulsion Affordably
By George L Smyth
I am not the expert, that would be Denise Ross, but I have come upon a means of coating silver gelatin emulsion in an affordable way that works quite well.
Although I might wish to lay the blame on my mother, it really goes back to the Great Depression, where she learned to waste nothing and value financial resources. This is the reason why although I can spend a fair amount of money to resolve the issue of coating paper with silver gelatin, I was not able to go that route and was determined to find an affordable way to do it.
I used liquid emulsion many years ago in the experimentations to find my niche. I spread the emulsion on paper with a brush, which occasionally worked fine, but more often than not offered brush strokes and thickness inconsistencies that ran contrary to what I was trying to do. While others are able to find this artistic choice, it did not work for me.
I have begun to revisit the issue of making my own photographic paper, seeing the papers available for the Bromoil process leave the market (there are some available and I keep track of them at https://bromoilpapers.wordpress.com/). Although I do have a well-stocked freezer of paper available to me, I know that eventually that paper will be used. My thought is that making my own silver gelatin paper gives me a chance to maintain the consistency that is so important when using the Bromoil process. The largest barrier to this is to have a means of evenly applying the emulsion.
The best way to do this is to use what is known as a Mowrey coating blade. This is a stainless steel rectangle that has a well in the middle and one long end has a gap at the bottom that allows emulsion within the well to escape. As this blade is drawn across the paper it leaves the emulsion on the paper even with the clearance at the bottom. I have used one and it works wonderfully.
I know of only one place where this may be purchased (http://stores.photoformulary.com/emulsion-coating-blade/) but the cost is $275. When I think about how much paper I can purchase for that amount I just cannot bring myself to buying one.
In Denise Ross’s The Light Farm (http://www.blurb.com/b/6465389-the-light-farm), which is an absolute must for anyone thinking about making their own emulsion, Denise offers some suggestions in this regard and I did attempt to make my own coating blade with thick glass but just was not able to get the gap correct.
Perhaps the most popular way to spread emulsion evenly is to use a puddle pusher (http://bhpho.to/2f5LUKE). These are great with coating paper with sensitizer (like VanDyke or Cyanotype) but used as intended would simply push emulsion off the paper. The way to use this with emulsion is to wrap each end with tape to provide a gap in the middle, with the gap being the amount of emulsion to leave. I used Scotch 501 tape, which is 1 mil in thickness. As I wanted a 5 mil gap that meant five turns around each end. Although this worked, I ended up with occasional bubbles in the emulsion. I tried widening the gap, using a wet coating method, ensuring that I was coating with the paper grain, and a number of other things but could never resolve this issue.
I have had some success using a wire-wound metering rod and these can be purchased at a somewhat reasonable price – one resource would be https://www.gardco.com/pages/application/ap/wirewound_rods.cfm. I have not bought one and anyone considering doing so will want to read http://coursenotes.mcmaster.ca/4L04/Thin_Films/Meyer_Rod_Application_of_Polymer_Films.pdf to determine the proper wire diameter.
The reason I did not make the decision to go in this direction is the fact that unless the emulsion is poured evenly across the paper, one end will exhaust emulsion as the rod is drawn down the paper while the other end will continue the coating process. The part of the paper with excess emulsion can be used for testing, but I will be using digital negatives so for me this is just wasted paper and emulsion.
So there are a number of options that work, but not well for me. What did work for me was a combination of a couple of things above.
The key was the Mowrey blade. I did not want to deal with the financial outlay so I decided to design my own. As mentioned, I was not able to successfully construct one myself of thick glass, but I was able to design one to be built on a 3-D printer.
I have placed my design at http://glsmyth.com/articles/coating/mowrey-blade.zip, so anyone wishing for one can just download it and send it to be printed. I went to https://www.3dhubs.com/ and chose A&M Print's Hub to print the device, which cost me $21.50 included shipping. The design is to spread emulsion with a 6.5” width, but the object can be configured for any size.
There is a problem, however, and I will refer to it as a feature. An actual Mowrey blade has an adjustable gap, so it can be set to spread emulsions of different thicknesses. Although I have a singular need for the device at this moment, I may wish to use it for something else in the future. That is why I designed the blade with a 1/64” gap, which is much too high for silver emulsion.
When I use this device I follow it with a wire coating rod. The coating rod is what gives me the thickness I desire, and since it is preceded by a well that levels to the amount of hot emulsion within, I do not have the problem of one side running out of emulsion while continuing to coat the other.
Instead of purchasing a commercial coating rod, I went to Home Depot and bought the smallest threaded rod they had available, which was 3/8” diameter. The length I purchased was too long so I cut it to size using the cutting part of my vise grip. I measured this rod as leaving a 20 mil thick emulsion, which is too thick, but Denise’s book mentioned placing the newly coated paper on a curved surface so that emulsion is drawn to the sides, and that has helped. Subsequently I have found that these rods are available in a ¼” diameter so I may give that a try in the hopes that it will leave less emulsion.
Altogether this setup cost less than $25, so the goal of finding a means of evenly and consistently spreading emulsion has been met. That gives me $250 I can use to purchase paper.