Air bubbles when coating glass plates with MACO black magic VC

I have been trying to coat glass plates for the first time.

I prepared the glass plates first by:
1. Scrubbing with isopropyl alcohol
2. Scrubbing with powdered detergent dissolved in water
3. Rinse well
4. Wipe dry with paper towels
5. Sub by dunking in gelatin (1 teaspoon to 450mL water) and allow to dry overnight

When it came to coating the glass plates with the warm MACO black magic, I tried a few different methods but found the easiest way was to brush on a thin layer of emulsion with a jiaban brush, then pour half a film canister full of emulsion onto the plate and allowed the emulsion to flow to the edges by tilting the plate at various angles before allowing the excess to drip off. Without brushing on a thin layer first, the pour didn't flow properly.

Using this method, I got a lot of air bubbles in the emulsion. Also, when trying to do a second layer, I found I couldn't use the pour and tilt method so resorted to just brushing on a second layer.

Have others had these experiences before, particularly the problem with air bubbles? What is your coating method to avoid this?


Thanks for your help in advance.

Ged

Ged, try with distilled water.
These days You can find steam distiller for reasonable price (~200US$) and You will have constant results in the dark room, without air bubbles surprises or water impurity.

G

Air bubbles can be avoided by using a container with a hole in the bottom; then, the bubbly part comes last and you can discard it.

For example, I have used a 35mm film container with a hole on one side just at the bottom. This holds about 25 ml of emulsion, which is probably enough for 4x5" plate, or even bigger, I think. Pour the emulsion in the container while keeping the hole shut with your finger (use gloves), wait for a minute or so that the bubbles go up, then coat.... This has worked for me.

Some people have reported squirting a little bit of ethanol on the top of the container to remove bubbles. Denatured might not be good for this.

Also, I don't think you need to sub glass plates with gelatin beforehand. Try to coat as is. It should spread on a clean glass.

If the adhesion is a problem (do not process over 20 deg C or so, anyway), you might need to add hardener to the emulsion and stir well just before coating. Chrome alum, glyoxal or formalin may do the job. Search this forum for more info.

Thanks for the replies!

Hi georg16nik, I am using a commercial liquid emulsion and so using this straight without dilution with any water. Do you normally dilute your emulsion?

Hi hrst, I'll give the film container with a hole a try! Are you just pouring 25mL onto the plate and letting it flow to the edges? I found if I used more that 1/2 a film canisters worth, there would be too much emulsion and it just dripped directly off the plate. I subbed the glass as people reported adhesion problems but I'll give it a go without but add the hardener to the emulsion. When you said not to process over 20degC, I presume you are talking about dev/stop/fix after exposing the plates? Are you mixing your own emulsion or using a commercial brand?


Regards,

Ged

Your plate may be too cool. A warm plate helps alleviate this problem. Also, use a drop or two of Ethyl Alcohol in the melt to thin it out. Make sure that everything is between 100 and 115 F.

A paintbrush can cause bubbles.

PE

ged said:

Thanks for the replies!

Hi georg16nik, I am using a commercial liquid emulsion and so using this straight without dilution with any water. Do you normally dilute your emulsion?..

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I meant distilled water for steps 2 and 3 of Your process.

Hi PE, thanks for the suggestion of using Ethyl Alcohol. Is the 1-2 drops the amount to add to a film canister's worth of emulsion or to the main bottle that has a volume of 300mL? I tried pre-warming the plates but I think they were below 100F. I'll try without the paintbrush next time.

Hi georg16nik, thanks for the clarification!


Regards,

Ged

I cannot answer any further because I don't have any experience with that emulsion. It may be high in gelatin content. I would guess warming it and the plate might help more in that case.

PE

ged said:

Hi hrst, I'll give the film container with a hole a try! Are you just pouring 25mL onto the plate and letting it flow to the edges? I found if I used more that 1/2 a film canisters worth, there would be too much emulsion and it just dripped directly off the plate.

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I haven't poured glass plates, but only coated film (and some glass) with a coating blade, with just enough emulsion to create a quality coating up to the point needed; the rest of the film will have bubbly emulsion and cut off after drying. So, when you have to coat from edge to edge (glass plates), I would use more emulsion than needed and discard (or save for immediate reuse) the excess flowing out from the plate edges, and stop the flow by placing my finger on the hole well before the bubbly part starts coming out.

I think many people who pour glass plates use more emulsion than absolutely necessary and let it flow to the edges and then back to the container to be immediately reused for the next plates (or discarded, if you don't need to save on money).

When you said not to process over 20degC, I presume you are talking about dev/stop/fix after exposing the plates?

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Yes. Unhardened emulsions are very fragile, so be careful:
- not to exceed 20deg C
- not to touch the emulsion
- not to agitate vigorously, but gently
- not to use the combination of carbonate developer and too acidic stop bath, possibly creating CO2 bubbles?
- to dry it carefully, etc.

And, a hardening fix (possibly combined with a hardening (staining) developer) is recommended.

Emulsions hardened with easily available glyoxal, formalin or chrome alum should be much better (I'm using glyoxal), but still not the same hardness as today's modern commercial emulsions.

And note that adhesion and hardening are two totally different things! Good adhesion is a MUST and good hardening is a plus. For example, I think I once had hardening make adhesion worse, because the curling of the emulsion during drying was combined with stronger, harder emulsion that could pull the emulsion off the base more easily by developing a curl . On the other hand, it's said here that chrome alum also helps with adhesion with glass plates, but many people use no hardener at all.

Also, have you tried adding a very small amount of wetting agent (such as PhotoFlo) to the emulsion before coating? It could help the emulsion to run evenly if the heating of the plate is not enough to solve the problem.

Are you mixing your own emulsion or using a commercial brand?

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I've never used commercial ones; just made some on my own. It's actually quite fun and I had great results without much work. Most of my tests have been just with food gelatin. Try the "Real formula" posted by PE here as a sticky topic, for starters... (Or get PE's book! I haven't had money to place the order yet, but soon it will become possible.) Also check http://www.thelightfarm.com/ , Denise Ross' site with a great deal of recipes and many different, home-kitchen like approaches.

Thanks everyone for your great suggestions! I'll try them out in my next coating session.


Regards,

Ged