A popular method of making stencils for screen printing is to use the Diazo Photo Emulsion. The screen is prepared with the photo emulsion and a negative printed on acetate is placed on the screen before it is exposed to a bright light source (find out more about Diazo Photo Emulsion).
Here is a useful excerpt about emulsion exposure from Ruth Brown’s book Digital Imagery on Fabric, page 74. Ruth’s book explores a number of techniques including cyanotypes, Bubble Jet Set 2000 and photo transfers.
You can use various light sources to expose a photo emulsion screen ranging from Photoflood bulbs to a sunbed. Received wisdom is that you use a 250w BBA photoflood bulb suspended above the surface on which you will have a screen. The guideline is that the distance from the light source should be something like the diagonal of the screen, if you want somewhere to start. If you are preparing a big screen and you have the light source too close the edges may not get enough light and wont expose correctly.
Timing of the Emulsion Exposure is vital. Speedball give a chart of suggested timings for two different bulbs in relation to the screen size and the light distance:
150 Watt Bulb, Clear Incandescent
screen size 8 x 10 inches | bulb height 12 inches | exposure time 45 mins |
10 x 14 inches | 12 inches | 45 mins |
12x 18 inches | 15 inches | 1 hr 14 mins |
16 x 20 inches | 17 inches | 1 hr 32 mins |
18 x 20 inches | 17 inches | 1 hr 32 mins |
BBA No. 1 Photoflood (250 Watt)
screen size 8 x 10 inches | bulb height 12 inches | exposure time 10 mins |
10 x 14 inches | 12 inches | 10 mins |
12x 18 inches | 15 inches | 16 mins |
16 x 20 inches | 17 inches | 20 mins |
18 x 20 inches | 17 inches | 20 mins |
Note:
These timings assume you are using a foil pie plate above your bulb to act as a reflector.
The photoflood bulb can get pretty hot a normal lamp fitting may not be suitable.
Please note that once the emulsion is mixed with the sensitiser it must be applied and exposed within a few days. The mix will cure at room temperature. This process will be complete within a few weeks, even if kept in the dark.