Photo Emulsion Exposure

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 inches12 inches45 mins
12x 18 inches15 inches1 hr 14 mins
16 x 20 inches17 inches1 hr 32 mins  
18 x 20 inches17 inches1 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 inches12 inches10 mins
12x 18 inches15 inches16 mins
16 x 20 inches17 inches20 mins
18 x 20 inches17 inches20 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.

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