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).
The following text is an 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 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.