Discover the consequences of poor vacuum drawdown, and learn how to test the completeness and consistency of vacuum on your exposure unit.
To minimize the risk of this situation, always run a bleeder cord from the vacuum inlet to the center of the frame. This cord facilitates the complete drawdown of the blanket and prevents localized high-vacuum areas. One of the easiest, cheapest, and most effective materials to use for the bleeder cord is the common plastic cord used to seal window screens into their frame. It has very fine ribs that grip the window screen when it is forced into the groove of the window frame. These ribs make excellent channels for the air to escape from under the vacuum blanket of your exposure unit.
If your vacuum frame does not have a bleeder cord, you can simply add one yourself. Fashion a loop at the end of the cord approximately 1/2-in. in diameter. You can do this by taping the cord back on itself. Position this looped end directly under the vacuum inlet on your frame. This will keep the inlet from sealing to the glass while at the same time providing a channel for the air to escape. If you have more than one inlet, make one cord for each.
The cord should be at least long enough to reach the center of the frame. I usually make mine long enough to encircle the inside of each frame that I am exposing. It is important to keep the cord out of the image area, as this would leave a mark in the image upon washout.
Checking for consistent vacuum pressure
With the bleeder cord(s) installed, the next question is whether you are achieving a high enough vacuum for a good exposure. This is a bit more difficult to determine. The easiest test I have found is to look for Newton's Rings in the area of the film positive. This effect looks very similar to the rainbow-like shimmer you see when oil floats on water. An example would be a puddle in the street after a rainstorm. The Newton's Rings are very tight and reticulated. They almost look granular. The object is to have them be uniform and tight. If they are uneven or nonexistent, you have an insufficient vacuum.
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