When pressing rosin we've noticed there's a lot of variables that affect consistency, quantity, and quality. One of those variables is moisture content of the starting material. Herbs with a very low moisture content tend to have an unfavorable yield if any yield at all. On the flip side, a material with a higher moisture content has a more favorable yield. We are not 100% sure why moisture plays a big part but there are a few theories out there.
Some people think the moisture helps the oil flow out the herbs better. That may be true but how does that explain the flow you get from properly dried extracts? Our theory is with really dry herbs the plant material acts like a sponge. The dryer the material is, the more absorbent it will be. This theory holds true when pressing fresh herbs too. When pressing fresh herbs we noticed far too much moisture in the oil, so much so we had to let it dry for a couple days before we could collect it. If we apply our theory here the belief is that herbs retaining 50-65% of their moisture will be just absorbent enough to hold in all that undesirable moisture while letting the oil flow freely.
If the material is very dry we try rehydrating it before pressing. This is done by adding humidity packs (commonly used for storing cigars) to our curing jars/buckets. There's other ways we've tried to rehydrate our dried herbs such as adding a couple fresh leaves of the plant to our jars/buckets. Over time, we've taught ourselves how to dry the material just enough to maintain a proper moisture level. This was made easier by getting small hygrometers to place in our jars/buckets so we could get familiar with when our herbs are ready by feel. After a while of using hygrometers in our jars/buckets, you get really good at telling the moisture content just by touching the herbs. As we master the feel for moisture there's rarely have a need for the hygrometers or humidity packs unless we're testing new material.
When testing material we have yet to work with we may use the hygrometers to see what moisture content gives the most desirable returns. Where we find 50-65% work well for most, depending on the starting material we may try 70-75%. No matter what, we never press herbs under 50% moisture content. Herbs under 50% will yield poorly, and tend to be wasted from a cost standpoint. We keep a journal of all the variables as we test, the more detailed we can be with our notes the more we notice patterns that will help us get the best quality and yield possible.