AUTOCHROME, DARKROOM CHEMISTRY, EMULSION, EQUIPMENT, OTHER

Trying to Reverse Develop Dry Plates to Direct Positives

Reverse-developing-dry-plates-to-positive-foma-emulsion

After being quite busy in the last few months doing exams and practical work for the academy I finally have a bit more time now to experiment with more advanced alternative techniques. It’s been almost half a year since I had to push my Autochrome research aside as I just couldn’t afford the cost of production any more. I hope I will be able to proceed my research soon as I am only missing a few details! 

When I was unraveling the mystery of Autochrome piece by piece I also had to learn how to develop silver gelatine emulsion to positive. Remember Autochromes are positive colour images ready to be viewed directly with some backlight or be projected. Developing glass Dry Plates to direct positive is quite a challenge as the crucial steps have to be executed perfectly in order for the technique to work. Back then I was making my very own panchromatic silver gelatine emulsion that is sensitive to the whole visible spectrum of light. It is mixed together in a way that it allows thin coating on the glass which is key to making good direct positive images. This time I have decided to try and work with commercially available Foma emulsion that is made off small silver halide grains making it medium to high contrast which is ideal for reverse developing process.

It has been quite some time since the last time I developed to positive so the first thing I did was dive deep into my autochrome notes. There were two things I highlighted that play a crucial part in the process:

Thickness of the emulsion:

In order for the reverse development to work, coating of the emulsion has to be even and just the right thickness. If the emulsion is too thick the first developer is just not able to transform all the silver halides to metallic silver resulting. Plates not being developed all the way through causes all kinds of complications in the following steps resulting in a dense positive image. Emulsion has to be coated thin and even which is extremely hard to do by hand. One could help himself by warming up the plate, spin coating, coating with a rod… Here is one example of a thick and unevenly coated plate. You can clearly see that highlights are just too dense.

First  development:

Even though there are 11 steps to get the positive the most crucial is first development. It will determine the looks of the final image. Once you have an even coating it’s time to develop the whole thing. If your coating is perfect you can proceed with developing in a high contrast paper developer like D76. But most of the time coating is not perfect so you need to add in a silver halide solvent to help you with developing image all the way through. Most commonly used chemical is Sodium Thiosulphate also known as Hypo. Hypo is commonly used as a fixer so adding it to the developer might sound strange but it does exactly the same as in fixer. It dissolves the unexposed silver thinning the layer of emulsion and allowing the developer to do its job. If you add to much of Hypo it will eat away the whole image like you can see below.

This is not a finalised tutorial just yet as I still need to perfect a few steps of the process but I am sharing my findings and current formulas if anyone would like to experiment. 

COATING DRY PLATES FOR DIRECT POSITIVE:

Like I said before, the coating of the emulsion has to be thin and in order to do that both emulsion and glass support have to be warm. Main ingredient of a photographic emulsion is gelatine which liquifies when warmed up but the temperature shouldnt exceed 50 degrees celsius. 

Before I turn off the lights I clean off the plates, put some gloves on and get everything ready like an emulsion, spoon to take it out, smaller glass beaker to hold the emulsion, bigger beaker for hot bath, syringe, hairdryer and some paper towels.

How to clean dry plates?

Glass has to be cleaned off all the grease and dirt before coated. I often get this question and there’s no secrets to it. I am using a paste made of plain dish washing soap and calcium carbonate which is a very soft abrasive even softer than a cotton tab so it doesn’t cause any scratches on glass. I place my plates in the sink laying flat on a foam mat and add a small amount of calcium carbonate + a drop of washing soap to the centre of the plate. Then with a help of a cotton tab and circular motion I rub the whole surface. I like to do both sides. When I am done I pick up the plates by the corners, wash off the remaining paste with hot water and place it on a drying rack. Just before they are dry I finish them off with some paper towels and they are ready to be coated.

TIME TO TURN OFF THE LIGHTS

How to coat the plates thin?

I take the emulsion out of the container and start warming it using a hot water bath. Remember you should never heat up the emulsion over 50 degrees celsius. While the emulsion is slowly melting I take a glass plate and heat it up using a hairdryer. It cools down as quickly as it warms so from here on I have to move fast. I pull around 4ml of the emulsion in my syringe and cover the whole plate as quickly as I can tilting the plate by its corners. Once the plate is coated I drain as much of the emulsion off as I possibly can thru one of the corners. Once I am done I also wipe the corner using a paper towel as the straining causes emulsion build up. Once this is done I place the plate on a cold marble slab and let it dry for at least 10 hours. 

EXPOSURE

Once the plate is dried I load it into the plate holder and it’s ready for exposure. I have been thinking about the motive a lot. Because this is an experiment with differences in coating and development I have decided to take a shot of my Darkroom Sink as the light conditions are always constant. Speed of foma emulsion is somewhere between 1-3 depending on the light situation. Because I was shooting indoors I went for iso 1 and ended up with 20s exposure at f/8. I used an Intrepid 4×5 camera and Schneider-Kreuznach Super Angulon 90mm f8. Some also recommend overexposing the plate for a bit but with a thin coating I didn’t find it necessary.

DEVELOPING DRY PLATES TO DIRECT POSITIVE

I tried many different variations of the process but the one I am presenting here brought the best result using a thin coating. If you are gonna do this please don’t forget to turn off the lights and please protect yourself by using gloves!

Step 1: FIRST DEVELOPMENT

Once again this is a crucial part of the process that is gonna determine final looks of the image. Here all the exposed silver halides are transformed into metallic silver. I developed for 5 min with Kodak HC-110 + a tiny amount of sodium thiosulphate. I agitated constantly for the first 30s and then for 5s every half a minute.

300ml of Kodak HC-110 dilution B

+0,25g of Sodium Thiosulphate 

Step 2: WASH

Wash the plate in water for 1min.

Step 3: BLEACH

In bleach the metallic silver gets removed and all I am left with are unexposed silver halides. Bleach contains very cancerogenic Potassium Dichromate so handle it with care and don’t pour it into the drain. I collect all this chemical and take it to the dumpyard where they know how to handle this stuff. Back to bleaching. I bleach for 3min agitating constantly until there are just white areas left. 

300ml of water

+1,5g of Potassium Dichromate

+ 2g of Sulfuric acid 98%

Step 4: WASH

Wash the plate in water for 1min.

Step 5: CLEARING SOLUTION

After washing I slide the plate into a clearing solution for 2 minutes agitating continuously. Clearing solution will remove the yellow stains that were formed in the bleach. 

500ml of water

+25g of Sodium Sulphite anhydrous

I TURN THE LIGHTS BACK ON!Yes you heard me right. Now it’s time to re expose all the silver that is still left on the plate. You can not really overdo this step.

Step 6: WASH

Wash the plate in water for 1min.

Step 7: SECOND DEVELOPMENT

After I am done reexposing I keep the lights on and slide the plate in the second developer. This is the most satisfying part. I develop for roughly 3min in Kodak HC-110 and enjoy the magic of a positive image appearing on the glass. Again you can really overdo this process. 

300ml of Kodak HC-110 Dilution B

Step 8: WASH

Wash the plate in water for 1min.

Step 9: FIXING

I fix the plate in my preferred fixer for 5min

Step 10: FINAL WASHING

And wash the plate for one last time in flowing tap water for 10min. 

I have developed over 25 plates and following the process I just explained this is my best plate so far 🙂

FINAL THOUGHTS 

To conclude everything I would say that the coating of the emulsion has to be very thin and even which is extremely hard to do by hand. I think that the thicker side of my thinnest plate is my goal. It is nice and translucent with just the right amount of contrast meaning I should not use more than 1,5ml of the emulsion to coat a 4×5 plate without adding any hardening agent. Also tilting the plate to strain all the excess emulsion is not ideal as it results in an uneven coating. 

I can maybe try heating up a marble slab in an oven, bring it to the darkroom, level it up, place the glass on top and coat the plate flat. This way I can spread the emulsion evenly without rush. When finished I would just drag the plate to the cold marble slab and let it dry. I would definitely like to try spin coating as well.

I would definitely like to get to the bottom of this and standardise the procedures so you can just follow the steps to a perfect positive image. I have to experiment a lot more and when I am done I will share the results with you.

For an even easier understanding of this technique I also made a VIDEO TUTORIAL

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