Spirit sensitizing prblems, streaking

[photojeff3200]

I've been involved with the Rawlins Oil process for almost a year now and have had a tumultuous relationship with my sensitizing technique. The Rawlins process involves sensitizing a piece of gelatin coated watercolor paper with sensitizer, exposing by way of direct contact with the negative, washing, swelling and inking with lithographic ink. I am coating my watercolor paper with 8% gelatin and .1 grams alum per liter. My sensitizer is a 10% stock solution of Potassium Dichromate cut in half 1:1 with acetone so I sensitize at 5%. For a surface area of 170 square inches (roughly 13x13 inch) I use 16ml of solution. I dampen my 3" foam brush in distilled water and shake out all the excess water before brushing. I pour about 8ml onto the surface and brush just until I feel the surface grab the brush than I pour the second 8ml and repeat. Toward the end of the brushing, before the paper starts to grab I barely apply pressure to the surface as if to feather out any lines that may be present. I then let the paper dry under safelights for 5 minutes before putting a fan on it to completely dry, about 15 more minutes.
About half the time the print dries without streaks and half the time it dries with streaks. It is VERY dry up here at 8000 feet in Colorado, could it be lack of humidity that is causing streaking? All the materials that I use are stored at ambient temperatures. I tried to store my PD in the fridge once but it came out of solution so I wont do that again. Thanks for the help...Jeff

[TheMissingLink]

Jeff,

as you described your procedure I would also have guessed the RH.

I am doing carbon most time at 60% or below, the amount of my PD is around 8ml (2x4) with 8x10, nearly the same (relative) amount you use. Depending on the needed contrast diluted with alcohol.

Do you have the chance to try your PD with alc instead of Acetone? would be the easiest way to exclude other reasons. Next I would try to increase the RH. For a simple test some wet towels for some hours in your room should work.

hth

horst

[Jim Noel]

I had trouble with streaks when using foam brushes with several processes. I now use a top quality artificial sable brush. I have separate ones for all processes utilizing silver, and those using Pt/Pd. Well worth their high cost in paper savings, and smoothness of coating.

[sanking]

What you are doing sounds very similar to spirit sensitizing for carbon printing. The use of acetone with very low humidity could be a problem unless you work very fast. I sensitize carbon tissue with a mix of 1 part dichromate solution + 1 part acetone with RH of 35%-55% and even at this RH I have to work very fast to spread the solution evenly before it dries and starts to stick. Try isopropyl alcohol instead of acetone as it will give you more time to spread the solution.

The temperature could also be a problem as the gealtin will melt when sensitizing if the temperature is too high, varies with type of gelatin but in my work 80º F is too high. If the ambient temperature is too high (over about 75ºF) you might cool the dichromate and alcohol/acetone solutions, or find a way to lower the temperature.

I would also suggest the use of high density foam rollers instead of the brush. I find it much easier to get an even coating in spirit sensitizing with the rollers compared to a brush.

Sandy

[photojeff3200]

Thanks for the help Gents. I took your advice and reduced the temperature in my darkroom to 65 degrees. I also put my acetone in the fridge and the colder temperatures produced a print without streaks. I have been going back and forth for a while using Iso and acetone with no difference. It will get pretty hot this summer and I wonder if I should get a little air conditioner for my darkroom? The RH is around 10% here, I guess I never considered that would be an issue, but I guess it is.