During the last months I looked deeper into X-ray film and dry plates. Dry plates are usually made with a photographic emulsion on thin glass plates which are left to dry afterwards. They go way back to the 1880's and were soon replaced by what we know as film.
These emulsions are orthochromatic in nature which means they can see only UV- and Blue light. In this regard they are similar to wet plate collodion. After doing some negative plates I had the whish to make something similar to wet plates ambrotypes, so white positive images on a glass plate. If you hold these plates against a dark background you will see the positive image and against a white background a fainter negative.
After some reading and joining a facebook group which is dedicated to dry plates I found some usefull information. Lee Lira had developed his own reversal developer which seems a bit cumbersome to create. I wanted something very simple I can use with my Kodak HC-110 and very soon Lee published a recipe for HC-110 and ammonium thiocyanate. In the meantime Nejc Urankar was developing dry plate tintypes which are aluminium plates which have been blacked on one side and covered with an emulsion. Nejc took Lee's recipe and adjusted it a bit and now we have a simple to use ambrotype developer for dry plates which produces good results.
I played around with these different contentrations and got some wonderful results:
The trick is to have your exposure quite spot on and learn when to pull the plate from the developer. (You can develop these plates under safelight.)
And here the best working recipe, which might need some adjustment for the emulsion you are using:
4 ml HC-110
300 ml water 20 degrees Celsius
3 g ammonium thiocyanate
Develop for round about 3 minutes and use plain water in your stop bath. Put the plate in there immediately for about 1 minute. Fix as usual for round about 5 minutes and water for 10-15 minutes either by very low running water or by changing the water in your trays.
You can try to up the amounts to 5 ml of HC-110 and 7 g of ammonium thiocyanate and bring the development time down to 1:30 minutes. You might want to pre-soak your plate in plain water for a few minutes especially if the emulsion contains hardener or your plate might be underdeveloped and/or gets a bluish, greenish tint.
P.S. If you spray paint the emulsion side with black acrylic paint, you get a permanent "ambrotype" which is scratch resistant. (See first photo.)