When I started using hide glue, I found that leaving it stand unused
for a long period it would develop mold on the top surface as Bill
Mackin describes.  My practice has been to mix only what I intended to
use at the moment.  If I ran short, it was no problem to add more to
the pot.  When that job was done I would fill the pot with water and
let it heat up until I could easily remove the remaining glue before
putting it away.

I feel that keeping the glue pot clean and free of contaminants is the
best way to insure that the next batch of glue will perform as
expected.

Bob Moore
Orlando, FL