I am just starting to use hide glue. I added the granules and the water and let it come to a liquid and let it set for about an hour then turned it off. Today when I went to use it I turned it on and let it heat up for about a half an hour. It seemed to be all liquid and ready to use but when I went to spread it on the wood it had almost black small particles in it; like the residue when you use and eraser. Did I not let it heat long enough? Did it get something in it? Any thoughts are appreciated.