Here are some things I learned from this experiment:
1) Use very very ripe persimmons---I used some that were quite firm still, but they should be mushy so that if the skin is punctured the innards want to come spilling out (what a description!) and you can easily peel and mash the innards with very little effort. If they aren't ripe enough the jam will not be juicy enough and you will have half of it be gelled sugar without any fruit to speak of in it, like mine. Also the flavor of the less ripe persimmons is quite delicate and cooks right away in the 5 or so minutes it takes to get the mixture cooked enough for the pectin to set up. This is really hard because for most fruits, if they are this ripe you throw them away because they are rotting. This is why it would be nice to have previous experience with persimmons, to know how they act at different stages of maturity.
2) Add a fair amount of something acidic-like lemon or lime juice, or even citric acid powder-because the persimmons themselves aren't acidic enough to preserve correctly without it. I used lime juice and a little zest, and at the end that was all I could taste in the jam. Next time I will do citric acid, I'll just have to figure out how much.
3) Start with one batch, don't commit to more than one until you have fine-tuned the recipe, or until I fine-tune it for you :)
Today I found some extra mushy persimmons at the market and I rushed home to taste them. They are a strange fruit, they taste delicious this way, but they have an extra weird texture. Kind of slippery, like raw fish. Now I understand why they are typically turned into pudding instead of eaten raw and fresh. The texture is just wrong for eating this way. But I saw a couple of recipes briefly at the library today, one for ice cream and one for quick bread, and a reference to a fresh persimmon pie, that sounds delightful. I think I'll be experimenting with these lovely mushy ones. The one on the right is not ripe yet, and I think you can see the difference. And you can see the insides, all shiny and soft.