Problems when coating glass plates for cyanotype on glass

[largo]

Not strictly speaking silver gelatin based emulsion on a glass plate, but as my problems concern gelatin on a glass plate, I thought this was the appropriate forum to ask my questions...

I try to use the cyanotype process on glass, but I encounter several problems during the drying of the emulsion (even before the exposure under UV light)

I use the following chemistry to prepare the emulsion:

- Demineralized water: 200ml
- Potassium Ferricyanide: 8gr
- Green Ammoniacal Iron Citrate: 25gr
- Oxalic Acid: 1gr
- Gelatin 280 Bloom: 24gr

Then I prepare the emulsion in the following way :

1. Heat the water in the beaker to 40°C
2. Add the potassium ferricyanide gradually until it is completely dissolved.
3. Add the ammoniacal iron citrate gradually until complete dissolution
4. Add Oxalic acid
5. Then gradually add the gelatine to the hot mixture
6. Wait until everything has melted, mixing gently to avoid bubbles. This may take a while, so make sure all the gelatin is dissolved!
7. When everything is melted, pour the contents of the beaker into a light-tight plastic container
8. Close this container and tap the base on the work surface to bring up the bubbles.

Then I let the emulsion rest for a couple of days and then coat the plates following this procedure (after having cleaned thoroughly the plates, very important) :

1. Remove the emulsion from the refrigerator - it has been stored in a light-tight container - and use a plastic spoon to scoop out a an amount of cold emulsion sufficient for the intended number of plates to be coated. Return the unused emulsion to the refrigerator.
2. Place this amount in a plastic or stainless steel container with a light tight lid.
3. Close the lid and place this container with the emulsion in the water bath device set to 40°C (immersion heater, slow cooker, simple water bath, ...) with enough water to keep the emulsion hot without the container floating or tipping over.
4. Heat the emulsion to 40 ◦ C to melt it until it is very liquid. The actual temperature may depend on the quality and formulation of each batch of emulsion.
5. When the emulsion is completely liquid, pour some through a large square piece of clean, thin cotton cloth or through a plastic coffee filter into the pouring vessel. This step is to remove impurities and bubbles from the emulsion.
6. Allow the emulsion to rest so that the bubbles rise to the surface and burst (you can gently tap the bottom of the pouring vessel to speed up this rise). This pouring vessel can be kept warm in the water bath device between coats.
7. The glass must now be reheated to prevent the gelatin from solidifying instantly due to a too cold plate. This can be done either by using a hair dryer or by placing the glass plate on the surface of a warm (not hot) laboratory hot plate covered with two layers of paper towels.
8. While the plate is still hot, take it and support it from below with a gloved hand, fingertips supporting the back of the plate - like a waiter does with his tray when serving customers on a terrace - so that he can tilt it from side to side (collodionist style).
9. Quickly dust the plate with the soft bristle brush.
10. Holding the pouring container in the other hand, pour a sufficient amount onto the center of the plate (pouring too little is worse than pouring too much). Keep the plate level so that the emulsion forms a perfect circle.
11. Gently tilt the plate so that the emulsion gradually flows to the four corners without spilling over the back side, ending with the corner closest to you.
12. Once the plate is completely covered, gently allow the excess emulsion to drain from the last corner of the plate into the pouring container.
13. Gently tilt the plate for a few seconds toward the corner opposite the corner where the excess was spilled so that the emulsion is redistributed evenly over the surface.
14. Immediately place the plate on the cooling table. The emulsion should still be warm and suffiscient to even out on the surface of the plate before it begins to set to form a firm, even layer.

The coating goes on pretty well (I don't know if the gelatin thickness is ok or not, but it was pretty thin...).

It's during the drying process (in a lightproof box) that problems start to occur:

- Already the emulsion takes a long time to dry (24 hours after coating, it is still sticky to the touch and clearly not dry)... Is this normal?
- Then the emulsion blackens starting from one side (see pictures below), I have no idea where this can come from ?
- Finally, on one of the plates (see last picture attached), small blisters have formed during the drying process...

Do you have any tips to help me solve these problems?

Thank you!

 

[Nodda Duma]

I would guess you are seeing chemical fogging or oxidation. Circulate air in your drying box with a small fan or similar to get drying time down to a couple hours. Also, review the recipe to make sure you’re not missing something.

Bubbling is due to poor adhesion. Clean the glass with soap, water, rottenstone, and elbow grease. When rinse water sheets off the glass without forming droplets then you are good. Also might be getting put on a bit thick?

 

[Herzeleid]

Some things I can recommend.

I have noticed you do not mention any bacteriostatic, preservative in your emulsion. Gelatin is ideal medium for organisms to grow. I would consider organisms are somewhat partially involved in this process. Methyparaben or thymol are easy to find preservatives for gelatin. If you have access to listerine, 5ml of it would help.

You have added oxalic acid to your emulsion but the acidic emulsion will spontenaously go dark green and gray over time. The improved traditional cyanotype formula contains a restrainer (oxidizer in case of cyanotype). 0.2gr of Dichromate is essential in my opinion to prevent premature darkening and eliminating base fog, if you choose to include oxalic acid in your emulsion. You do not mention any hardener in your emulsion, so dichromate would also help with the adhesion.

To prevent and get rid of bubbles you can add ethyl or isopropyl alcohol and even vodka is fine. You can let your emulsion stand in 40C water for an hour to let it gas out. Alcohols will help to remove bubbles and it will also help coating since it is a surfactant.

My workflow for glass coating is different. I prepare %10 percent gelatin solution let it melt then add alcohols and preservatives. I use 0,5ml of cyanotype sensitizer per 5ml or gelatin solution. Then I add some chrome alum then coat the glass, 5ml per 4x5 glass.

I hope you find these useful.

 

[nmp]

What's the benefit of "curing" the emulsion for a couple of days? That could also be playing a role in commencement of the dark reaction, as Serder pointed out. Also heating and cooling and heating again - if you coat the plates right after making the emulsion, that would be eliminated too, not to mention resulting in less complicated workflow.

:Niranjan.