Set-up for exposing the screen positive

I just found this picture in iPhoto. I've posted a similar picture before, but this one explains better what actually happens when exposing a film positive and making a stencil on to light sensitive emulsion.

When doing the screenprints for the landscape series I did everything myself, naturally (who else would do it?) and made many mistakes -- but was startled and brought down to earth by my tutor, who said that in his day they had technicians to do this sort of thing. This was deflationary, for sure, but it seems worthwhile to me to have a total grasp of the process through doing it myself.

The photo shows how the lamp is rigged up at a specific height above the screen -- books can be added or taken away under the board as necessary to get the exact distance, it's a very flexible set-up. Also worth noting, there's a sheet of black paper under the screen frame to minimize reflected light, and a black cloth along the back to block light coming in under the door.

The screen is placed on the black paper, fabric side up; the film positive is placed on top of the fabric, and a sheet of glass is placed on top of that to ensure good contact; and then the light is turned on for six minutes. You can just about see the green light-sensitive emulsion on the yellow screen fabric. For the larger version of the tree I got a film positive made from my jpeg at Print on Demand on Tragarete Road.

Finding the best exposure time

Light-sensitive emulsion, also known as photo-emulsion, is wonderful for making certain kinds of stencils in screenprinting. In particular, it's useful for reproducing detailed ink drawings with fine delicate lines which would be difficult to do any other way. On the down side, timing the exposure correctly is tricky. If the time is too short the image will wash out too much. If it's too long, the image may take forever to wash out and parts may not wash out at all. So, getting the timing right is of the essence.

There's no hard and fast rule because every set-up is slightly different. The one good rule is to do a series of timed exposures to establish the best time for one's specific set of variables.

My variables are: A 150-watt flood lamp; distance from lamp to screen, 17 inches; diazo photo emulsion on the fabric side only (underside). After doing a series of test exposures (2, 4, 6, 8, 10 and 12 minutes) the optimum time turned out to be 5 minutes. This is an amazingly useful thing to know. I had been using much longer times and was having a terrible job washing the emulsion out.

Screen print, a failed attempt

My first attempt to make a screen print has failed.

The image, a detailed drawing of a tree, was ideal for a photo stencil, so yesterday I prepared the screen with photo emulsion and left it in the dark to dry. Today I set up the light -- a 150-watt flood lamp attached to a piece of 3 x 1" board -- and gave the screen an exposure of 24 minutes.


Sad to say, the unexposed parts of the screen (the drawing) failed to "open" properly afterwards and in the end I had to abandon it and wash out the whole screen, which was pretty difficult to do. It took over an hour of scrubbing and washing with chemicals.

I think the exposure time was too long. I will try again with a shorter exposure and possibly raise the lamp a couple of inches.

I can't complain, having skipped the step in the reference book* which recommends doing a series of timed test exposures.

*Screen Printing, Contemporary Methods and Materials by Frances and Norman Lassiter, 1978, Hunt Manufacturing, Philadelphia