Heat oven to 225˚F. Rinse the raspberries and drain them well. Put them into a heat-proof dish. I found that my 2 quarts fit nicely in a 13×9 Pyrex pan. Bake the pan of berries, uncovered, for about 30 minutes, because heating the berries gently will make it easier to sieve the seeds out.
Put a fine-mesh sieve over a large bowl or pot. Take the pan of hot raspberries out of the oven and sieve the berries in batches. Scoop some of the softened berries into the sieve, and press the pulp and juice through with a big spoon. It’s surprisingly hard work to press the berries through the sieve. You want to end up with only seeds in the sieve, as dry as you can manage. When you’ve pressed out as much fruit as you can, put the seeds in a clean quart-sized jar (* see Notes) and set aside. Repeat until all the berries have been sieved. Don’t forget to scrape the underside of the sieve with a clean spoon, to get all the pulp off it!
Combine the raspberry pulp with three-quarters its weight in sugar, or just estimate by volume, adding 6 c sugar to the lot, tasting it, and adding a bit more if it’s too tart. Add lemon juice. Heat at a very low setting, and stir until sugar is dissolved. Taste again for balance of sweet and tart; adjust as you wish.
Now add the pat of butter (it helps reduce foaming) and turn heat to high. Bring the jam to a boil, stirring frequently, until the jam thickens and will pass a wrinkle test. Ladle jam into prepared jars, wipe rims, and apply lids and bands. If your last jar is only partly filled, just stick a lid on it and refrigerate.
Ladle jam into prepared jars, wipe rims, and apply lids and bands. Refrigerate any partially filled jar, and seal the rest in a boiling water bath for 10 minutes.
About the seeds: you could throw them out, but I purely hate waste in the kitchen. I add about a half-cup of sugar to the quart-jar of seeds, and fill the jar with vodka, then cap it, shake to combine, and store in a cool dark cupboard until New Years Day. Then strain the seeds out, and it’s raspberry cordial!If you prefer vinegar, omit the sugar and booze, and just top off the jar with white vinegar, which is a quicker process. Let it stand 3 weeks or so, strain, and you’ve got raspberry vinegar.