The first question would be Have you calibrated your thermometer? If it's off, that would explain it.

The second question would be Are you at altitude?

The jell point is 8 degrees above boiling, whatever that might be at your altitude. So at sea level it's 212+8=220. In other words, if you're at altitude, the jell point is lower. At 3000 feet, 8 degrees above boiling would be about 214 and that's the jell point.

Even then, the jell point will fall within a range depending upon a number of other factors. To jell the preserve needs to be 60% sugar or more. At sea level that could occur anywhere from 217-221, though usually 217-218 are more the syrup end and 221 is a firmer set.

The next question would be Are you cooking the preserve at a full rolling boil? If you're not cooking at a full rolling boil it's going to take longer to evaporate the water off to hit that magical 60% point. The fruit is also a factor, depending upon its juiciness (i.e. water content) and the level of natural sugars present.

Finally, How wide is your pan? Ideally a jam pan would be wider than it is tall for speedier evaporation. Take a look at the Maslin pan at the link. That's serious jam-making.

In other words, nothing is fixed with these natural processes. There are lots of variables.

In your case it's possible the preserve never really "jelled" because peaches are very low in pectin. But in 45+ minutes you evaporated off enough water to thicken it.

Carol

Here is a link that might be useful: Maslin Pan

Carol thanks for all the good points to ponder.

I'm at 994 ft, which means boiling point is around 211 degrees. 211 + 8 = 219.

I did use approx 60% sugar. I followed a recipe by NCHFP which called for 6 cups of peaches and 4 cups of sugar. I doubled the recipe.

I definitely had a rolling boil and as a matter of fact I have that exact Maslin Pan. I love it!

So who knows. The jam came out really good, that's all I know, lol.
It must be that my thermometer is off.

Carol thanks for all the good points to ponder.

I'm at 994 ft, which means boiling point is around 211 degrees. 211 + 8 = 219.

I did use approx 60% sugar. I followed a recipe by NCHFP which called for 6 cups of peaches and 4 cups of sugar. I doubled the recipe.

I definitely had a rolling boil and as a matter of fact I have that exact Maslin Pan. I love it!

So who knows. The jam came out really good, that's all I know, lol.
It must be that my thermometer is off.

Probably because you doubled the recipe. Because there's so much more product in the pot that needs to be cooked down, it can as much as double the amount of time you need to spend cooking the jam.

I've stopped trying to double batches of no added pectin jams - I either overcook it or it doesn't set.

The preserve wasn't at the 60% point when you placed the ingredients in the pan.

60% sugar means 60% by weight in the cooked mixture. The weight of a cup of peaches is not equivalent to a cup of sugar. Also, the peaches contain a good amount of water, so when I say 60% sugar that means in the final cooked preserve by weight it's 60% sugar and 40% fruit. The water content by that point is minimal.

(Actually 60% is a minimum. It can be 65-68% sugar, depending upon the preserve.)

Just for clarification.

Carol

It is because you doubled the recipe. That should never be done unless you are using Pomona's pectin.

Thanks everyone for your input : )

I'm starting to think it's just that the thermometer is off because the jam set completely and is just fine.