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
8 x 10 inches
|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)
8 x 10 inches
|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|
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.