Recently I’ve been trying to make digital negatives to contact print on B&W FB papers. A few months back I made some initial gray scale prints (to make correction curves), and was satisfied with the results, except the sharpness, due to the lack of a proper contact printing frame. So I decided to build a proper vacuum frame modeled after the B&S design.

When I went back into the darkroom to print new gray scales (for making correction curves) I ran into some problems. The vacuum frame was doing its job, and provided better sharpness (contact), but I was getting blotchy areas in the mid-tones of the gray scale. The blotchy areas were somewhat random areas of lightness in the mid-tones, that were suppose to be solid mid-tones.

I tried to narrow down the variable that was causing this. I tried different buffer layers (plastic mesh, felt, rubber foam), but all produced the same blotchy mid-tones. I tried making a new digital negative, but this didn’t help. In addition I tried several different FB papers, and still had blotchy areas. I cleaned the glass numerous times too. I haven’t changed any other variables except for the addition of the vacuum frame.

Does anyone have any suggestions about what might be causing this, or how to alleviate the problem?

Thanks,
Martin

http://img46.imageshack.us/img46/986/img001grayscale21bu.jpg (http://imageshack.us)

This is a scan of with some of the blotchy areas noted. Notice that there are none in the pure blacks of the print, only in the mid-tones.

Someone had suggested it could be Newton Rings, because it was plastic in contact with glass. If this is the case, how would I go about dealing with this problem. Obviously others use vacuum frames on a regular basis, so there must be some solution, right?

Too regular to be newton rings. Looks like a grid of some sort. Is your vacuum drawn through a grid? Or it could be from your printer. I would check the printer variable first. Do you know someone who also does digital negatives? Print one of their negatives to see if that could be the problem.

Two of the vacuum buffers did have a type of grid (plastic mesh, and rubber webbing), but the felt I tried was fairly uniform, and also produced the same result.

I don't think its an issue with the negatives, because I have tried making several different ones (yes all on the same printer). But in the past when I made negatives and contact printed them without the vacuum frame, they turned out good (with the exception of less sharpness), and they were uniform in tone.

So I'm pretty sure the issue is with the vacuum frame. When I view the negative in the vacuum frame (while its on), I can see this uniform pattern. It looks like some areas are in very close (tight) contact with the glass, and other areas are in a little less contact with the glass. I believe this is what could possibly be causing the blotchy areas.

So I'm not really sure what I can do to regulate this contact. I was thinking of putting another buffer on top of the negative (between the negative and glass). Maybe using a sheet of blank Pictoricco White Film. The other idea is try to find another material for the buffer. But I don't know if that would help much, because the contact problem seems to be between the negative and glass.

When I lift the glass to remove the paper, most the time the negative is stuck slightly to the glass. I was thinking maybe this was a result of the humidity, and there were some moist areas creating more contact then others.

So I could possibly try wetting the negative before putting it into the frame (possibly a spay bottle), but I'm not sure of the long term effects this would have on the negative. Another option is to try keeping everything really dry, perhaps using a powder. Lastly, is the idea of the buffer between the negative and glass. It would have to be something that will allow the light to pass through uniformly, and not create the unevenness. Maybe a sheet of white film, OHP, or perhaps a piece of normal paper.

Thanks for your help so far.

-Martin

You have issues with the negative and the vacuum frame. The experiment should isolate each of these two variables to allow you to judge the source of the problem. You need to print another negative (digital or not) with your vacuum frame so you can judge if the blotches show up again. If you get blotches no matter which negative you use, the problem is with the vacuum frame. If not, the problem is with the printer.

One other area to check. Does your vacuum frame really suck? :D Do you have a vacuum gauge that measures the vacuum that is pulled on the frame? If you do have a gauge, what is it reading. You might be pulling such a great vacuum that the paper is being sucked throught the holes in the frame. My frame pulls about 25" of Hg.

Figure out these basic things first before you start messing (literally) with wetting negatives and such.