4 replies to this topic

[sly]

Working on carbon, as I get the chance.
Current issue is about brush strokes showing in the print. I coat with a foam brush 1 part AD in water to 3 parts acetone. 20 mls for 7x18 sheet, which will be cut for 3 4x5 negs.

The worst example shows both streaks and small spots which look like the solution sat up in droplets.
I spread the solution with horizontal, vertical, and diagonal strokes, until it feels draggy. Do I need to keep going after that? It seems that brush marks are more likely if I keep going. I'm doing this all in quite dim light, so can't really see if I'm making brush marks. Should I be timing brush time?
I've tried dividing the solution in half, spreading the first lot and then doing the second half. thinking I might have been pushing too much around at a time, I next tried dividing in 4, and spreading 4 times over the tissue.
Don't even know if I'm making sense here. Any pointers in the right direction appreciated.

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[pschwart]

sly, on 12 May 2012 - 12:54 PM, said:

Working on carbon, as I get the chance.
Current issue is about brush strokes showing in the print. I coat with a foam brush 1 part AD in water to 3 parts acetone. 20 mls for 7x18 sheet, which will be cut for 3 4x5 negs.

The worst example shows both streaks and small spots which look like the solution sat up in droplets.
I spread the solution with horizontal, vertical, and diagonal strokes, until it feels draggy. Do I need to keep going after that? It seems that brush marks are more likely if I keep going. I'm doing this all in quite dim light, so can't really see if I'm making brush marks. Should I be timing brush time?
I've tried dividing the solution in half, spreading the first lot and then doing the second half. thinking I might have been pushing too much around at a time, I next tried dividing in 4, and spreading 4 times over the tissue.
Don't even know if I'm making sense here. Any pointers in the right direction appreciated.

Acetone works fine but does require practice to apply evenly. You might try 99% isopropyl -- it's a lot more forgiving as it's less volatile. I use a slightly different sensitizing method: I dilute the dichromate 1+1, brush for 1 minute, repeat for a total of 3 minutes, then squeegee the excess and hang to dry. I don't apply a measured amount to each tissue, and I don't let the sensitizer dry between coats. I sensitize next to a window and don't get any fog using a 1.5% sensitizer. It helps to be able to see what you are doing

[Howard Efner]

I and several others use a method similar to Phil's. Start with a measured volume of stock dichromate solution - it does not matter if it is ammonium or potassium dichromate - dilute to 1/2 of the final volume with water, and then add isopropyl alcohol to reach the final volume. It does not really matter if you use 99%, 90%, or 70% isopropyl alcohol. Using a foam brush that is loaded with the solution, brush on a full wet flowing coat and then continue to brush for X minutes. Squeegee off the excess solution and hang the tissue to dry. The brushing time (X minutes) is something you will have to determine for your tissue. It is dependent on the thickness of the gelatin layer, temperature, and gelatin composition. For B&S machine coated tissue, 2 minutes is a good time unless it is really warm in the workroom - then you might be down to 1 minute. Too warm and too long, the gelatin starts disssolving. To be able to see what you are doing, a 40 or 60 watt tungsten bulb works fine as a safelight.

If switching between ammonium dichromate and potassium dichromate, just remember that there are more dichromate ions in 1 g of ammonium dichromate than in 1 gram of potassium dichromate. 1% ammonium dichromate is equivalent to 1.15% potassium dichromate.

Howard

[pschwart]

Howard Efner, on 13 May 2012 - 10:44 AM, said:

I and several others use a method similar to Phil's. Start with a measured volume of stock dichromate solution - it does not matter if it is ammonium or potassium dichromate - dilute to 1/2 of the final volume with water, and then add isopropyl alcohol to reach the final volume. It does not really matter if you use 99%, 90%, or 70% isopropyl alcohol. Using a foam brush that is loaded with the solution, brush on a full wet flowing coat and then continue to brush for X minutes. Squeegee off the excess solution and hang the tissue to dry. The brushing time (X minutes) is something you will have to determine for your tissue. It is dependent on the thickness of the gelatin layer, temperature, and gelatin composition. For B&S machine coated tissue, 2 minutes is a good time unless it is really warm in the workroom - then you might be down to 1 minute. Too warm and too long, the gelatin starts disssolving. To be able to see what you are doing, a 40 or 60 watt tungsten bulb works fine as a safelight.

If switching between ammonium dichromate and potassium dichromate, just remember that there are more dichromate ions in 1 g of ammonium dichromate than in 1 gram of potassium dichromate. 1% ammonium dichromate is equivalent to 1.15% potassium dichromate.

Howard

Agree with all of Howard's comments. I brush for 3 minutes but my tissue is a lot thicker than B&S tissue. I also control the temperature of the sensitizing bath: I keep the sensitizer stock refrigerated, then put the diluted solution in a water bath while sensitizing.
If you use isopropyl, make sure it contains only isopropanol and water. Other additions can make your tissue misbehave. I'd be especially suspicious of products labeled "rubbing alcohol."

[banana_legs]

Hi,

I had streaking problems and using 1 part dicrhromate solution and 1 part acetone helped, althought the drying time was longer. Some tissues however were worse than others. The glop recipie and pigment used may also have something to do with it.

I use soot as a pigment and it does contain traces of oil. When I make glop with isoprop alcohol to disperse the bubbles, streaking is more of an issue. Currently I use soap as an additive in the glop to control the bubbles and it is much easier to sensitise. Could you have oil contamination of your pigment too?

Best regards,

Evan