Gave up on the manual labor and broke out the hand-held mixer since the Osterizer was dirty. This worked really well to break the pulp loose from the skin of the grapes. Heated the mush and occasionally mashed til boiling. Let it boil for about 10 minutes mashing the grapes more every couple minutes. The skins start to give up a little color at this point and release it into the liquid. While the liquid is heating I got the cheesecloth over a strainer and bowl with a rubber band. I poured the mush into the cheesecloth. I let it sit for several minutes. Then I gathered the edges of the cheesecloth and put a rubber band round the top to make a bag with all the grape gunk in the middle. I didn't want to let it sit out overnight so I squeezed the bag in various ways for at least 30 minutes to press the liquid out. The result is over 4 cups of concentrate. I did apply my tongue to the cheesecloth bag before I threw it away and it was like a grape popsicle!
This morning I tasted it to see how much water to add, which was almost two cups for every cup of concentrate. Rachel loved it. It turned out great, even better than the previous batch.
Some crystallization occurs, probably due to the cooking process, and the crystals tend to settle at the bottom, requiring a final straining a day after the juice is done.