1 pound, 6 ounces (4 heaping cups fresh berries) raspberries, Marionberries, blackberries or hulled strawberries (washed)
1 pound, 7 ounces granulated sugar (3ï»¿1/2 cups)
1/3 cup strained fresh lemon juice
1 teaspoon butter
If using strawberries, coarsely chop them first (if strawberries are frozen, let them sit at room temperature for a few minutes, which will make them easier to chop). Otherwise, leave the berries whole.
In a large bowl, combine berries with the sugar and lemon juice. Gently stir mixture using a rubber spatula until sugar is evenly distributed and juices have begun to flow; let mixture stand, stirring gently every 20 minutes or so, for at least 1 hour; if longer than that, then refrigerate (for up to 24 hours)
IF PREPARING FOR THE FREEZER: Scrape mixture into an appropriate-sized freezer container, label carefully (noting amount, when frozen and any other instructions for finishing jam) and freeze. When ready to make into jam (up to 12 months later), simply thaw and proceed as directed below.
Wash 4 half-pint jars. Keep hot until needed. Prepare lids as manufacturer directs.
Scrape mixture into a 12-inch, heavy-bottomed skillet or cast-iron pan. Add the butter (this controls production of foam). Bring mixture to a boil over medium-high heat, stirring continuously with a straight-ended wooden or nylon spatula. Adjust heat downward to keep it from boiling over and cook at a hard boil.
After about 7 minutes of cooking, you have to use a bit of judgement to determine if jam is reaching the "gelling point," where it turns from fruit in sugar to a substance that will thicken when cooled and stay thickened. For this particular jam, I've found the gelling point to be about 218 F (at sea level). So once surface begins to look very "glisteny" and bubbles seem larger, thicker and shiny, stick a candy thermometer into mixture and start monitoring temperature. You're looking at another 1 to 3 minutes of cooking, depending on how juicy a mixture you started with, and how rich in natural pectin the fruit is.
When jam reaches 218 F, remove pot from burner and let preserves sit for about 1 minute. If any foam remains on surface, most of it will be absorbed back into jam. With a spoon, skim off any foam that has not settled back into mixture.
Ladle hot preserves into 1 hot jar at a time. Wipe rim with a clean, damp cloth. Attach lid and ring, turning firmly for a good seal (jar will be very hot, so use a pot holder or towel where your hand comes in contact with it). Repeat with remaining jars and lids.
At this point, jam may be stored in refrigerator (after cooling overnight on countertop) for up to 12 months without quality suffering.
For long-term storage at room temperature, process jars in a boiling-water canner for 10 minutes (for 15 minutes at 1,000 to 3,000 feet; for 20 minutes at 3,000 to 6,000 feet; for 25 minutes above 6,000 feet). Using a jar lifter, remove processed jars from boiling water and let cool on counter, undisturbed, overnight. Makes 4 half-pints.