Emulsion: It is the Heat and the Humidity (and Light)

The humidistat photo comes from Andy Anderson’s darkroom. You would find something similar in ours at Mirror Image, both a similar device and most days (I hope) similar readings.

As Summer is upon us in most of the country which means heat and humidity for most of us.

  • As always keep your darkroom dark or with safe lights. I see arguments on line about whether you have to do this or not. It isn’t that expensive, just do it.
  • Your screenroom ideally is at 30% to 50% humidity. You cannot measure this by what you as a human feel. You MUST use a humidistat. They don’t cost much. Get one. After drying your screens after coating, don’t put them out in the humidity or they will soak up moisture. Dry them and then keep them dry.
  • Keep the temperature down. You don’t really want to go over 80 degrees, and over 110 or 120 and you are cooked. AC units keep both temp and humidity down.

Here is my advice from my pals at Saati, known far and wide for actually giving out accurate information:


Emulsion,  as long as it is not a water resistant one, freezing is ok as long as it comes to room temperature gradually. Saati has mostly freeze/thaw stable products.


Temp over 120F will start to pre expose diazo emulsions.  Photopolymer is not as sensitive (Saati has not come up with a conclusive ceiling yet).  But rule of thumb keep below 120 degrees F.


Humidity definitely should be below 70% and above 10%


Drying cabinet (or the room if  you don’t have a cabinet) should be below 40% humidity.


Max temp is 120F.  Stay below that temp.


If kept coated and dry (best at 0% or very low) will last weeks until exposure.


Emulsion once mixed will last 4-6 weeks at room temp and life is cut approximately in 1/2 for every 10 degrees above 70 degrees. (Rick Roth guesses that refrigerating will add a week or two to the life.)


3 things that ruin a traditional diazo or dual cure emulsion… heat light and age. Photopolymers… heat and light.


Stay Cool.



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