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Topic: exposure time with sun  (Read 509 times)
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capitanjen
« on: February 07, 2007 06:30:06 PM »

i've searched for this, but i can't seem to find the answer...
i'm making my first shirts ( Smiley) and i was wondering since im using the sun, its not as dependable as a light, so how do i know if my screen is done? i dont want to overexpose it, but i dont want to underexpose it too! is there  a test or is there something i should be looking for?
thanks a bunchhhh
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entriq
« Reply #1 on: February 07, 2007 06:59:49 PM »

One standard and easy test is to do a stepped test. 

There are several ways to do this, but essentially, you need to expose a screen in the sun with a way to do several tests at the same time. 

I have done this with a simple film positive that I had the numbers 1 to 16 about 1 or so inches high, in a grid pattern with 4 rows and 4 columns with the numbers about 2 inches apart.  Prepare a screen with emulsion and place the film positive in the normal manner. 

Make 4 opaque showcards or heavy cardboard strips about 2 inches by however wide your screen is. 

Set the screen in the sun to expose, and use the strips to cover the numbers - starting at about 30 seconds or so, use the first strip to cover the number 1, then in 10 seconds slide that strip along the screen to cover the number 2 while keeping the number 1 covered,  then again in 10 seconds slide the strip to cover 3 while keeping the numbers 1 and 2 covered, and finally after another 10 seconds cover the number 4 while keeping numbers 1 - 3 covered. 

Then... you guessed it - you do it againg for numbers 5-8 with the next strip of cardboard, and so on until you have gotten to the last number - at which point you take the screen to your washout sink and wash it out.  Whichever numbers give the best washout and the best stencil is the range you want to work in.  Number one will have been exposed for 30 seconds, number two for 40 seconds, and so on.  If none of them work, you probably will need a longer
time - so do another test.

Remember that exposure time is greatly dependent on thickness of emulision, brand and type of emulsion, along with the exposure time.  As long as there is sufficient sunlight (like in the middle of the day and not too cloudy) things should work out well.  Working early or late in the day isn't so good.  Rain is bad.  Night is bad.


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entriq
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