I used Distilled Water added a little at a time while stirring it.
I was also under the impression to use Ammonia, but was told my a mask maker, that it could potentially mess up the chemical compound, so I didn't want to take the chance with 5 gallons!
One of the main ingredients in liquid latex is amonia. Liquid latex is about 2/3 water, the amonia helps the rubber cure by speeding up the evaporation of the water. In my experience, amonia/distilled water is a great thinner.Chemically it shouldnt change the rubber. Ive thinned gallons of latex for spraying through an airless gun and for slosh casting without issue. Just wear breathing protection as the fumes are irritating. Your real enemy is cold weather... Dont expose your liquid latex to extreme temperatures. That will effect the elasticity and shear strength of your cured parts.